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Section 8 : Entomology in Public Health Practice
154   Introduction to Entomology                                          Rina Tilak    903
155   Principles of Vector Control                                        Rina Tilak    906
156   Housefly                                                            Rina Tilak    927
157   Mosquitoes                                                          Rina Tilak    931
158   Fleas                                                               Rina Tilak    940
159   Human Lice                                                          Rina Tilak    944
160   Sand Flies                                                          Rina Tilak    946
161   Some Annoying Pests                                                 Rina Tilak    948
162   Envenomizing Pests                                                  Rina Tilak    952
163   Ticks and Mites                                                     Rina Tilak    955
164   Rodents                                                             Rina Tilak    960
166   Snakes                                                              Rina Tilak    966
(iv) Order Hemiptera : It contains the bedbugs and some other
   154         Introduction to Entomology                            species like kissing or ‘assassin’ bugs, which act as vectors of
                                                                     ‘Chagas’ disease.
                                                      Rina Tilak     (b) Class Arachnida : It includes arthropods like ticks, mites,
                                                                     spiders and scorpions. The class is characterized by the presence
                                                                     of eight legs, body divided into two parts viz. cephalothorax
The word ‘Entomology’ is derived from the Greek words
                                                                     (head and thorax are fused together) and abdomen and
‘ENTOMON’ meaning an insect and ‘LOGOS’ meaning science,
                                                                     absence of antennae and wings. The cephalothorax bears six
thus ideally making Entomology ‘the branch of science which
                                                                     pairs of appendages, the first two pairs function as mouthparts
deals with the study of insects’; however, the scope of the
                                                                     (chelicerae and pedipalps) and last four pairs as walking legs.
subject has been broadened to include study of all Arthropods.
Phylum Arthropoda constitutes all invertebrates with jointed         (c) Class Crustacea : It includes lobsters, crabs, water fleas
appendages and presence of chitinous exoskeleton besides             and cyclops; some species of these are intermediate hosts of
other features. The word Arthropoda is derived from two words        certain human helminths, e.g. Cyclops as intermediate host of
‘ARTHRON’ meaning jointed and ‘PODA which means legs or
                                          ’,                         guinea worm infestation.
appendages. The Phylum has many important classes of which           (d) Class Myriapoda : It includes centipedes and millipedes.
Class Insecta is the largest, constituting more than four million
                                                                     Modes of Disease Transmission
insect species. The other important classes are Arachnida,
Crustacea and Myriapoda.                                             Arthropods transmit diseases to man through specialized modes
                                                                     of disease transmission. The modes of disease transmission by
Vector borne diseases are one of the leading causes of morbidity
                                                                     arthropods can be classified as under :
and mortality the world over and pose a major public health
challenge especially to the third world or developing countries.     Direct Contact : When two hosts are in direct contact with
One of the important measures to combat these diseases is            other, the arthropod vector itself gets transferred from one host
through control of vectors. To ensure effective vector control,      to the other, e.g. pediculosis and scabies.
the knowledge about their lifecycle, habits, habitat and             Mechanical transmission : In this mode of transmission,
diseases transmitted is essential. The chapters in this section      the disease causing organism is transmitted on the outside
will guide the reader on these aspects so that sound vector          or inside the bodies of arthropods without undergoing any
control strategies can be formulated, wherever and whenever,         development, propagation or any changes in the pathogenicity
vector control is desired.                                           e.g. diseases transmitted by houseflies - diarrhoea, dysentery,
Classification of Arthropods                                         cholera, hepatitis A & E etc.

Diversity of structures amongst arthropods necessitates the          Biological Transmission : The disease causing organism
sub-division of the phylum Arthropoda into a number of               undergoes certain biological changes inside the body of the
classes, orders, families, genera and species. The following         vector. Depending on the type of biological changes, biological
classes include species of medical importance.                       transmission has been further classified as :

(a) Class Insecta : It comprises about 70% of all the known          Cyclo-developmental : In this mode of transmission, the
species in the animal kingdom. The insects are characterized         disease causing organism undergoes a part of its cycle in the
by the presence of six legs, body divided into head, thorax and      vector and simply develops or grows inside the body of the
abdomen and presence of antennae besides other features.             vector, e.g. Wuchereria bancrofti (Filariasis) transmitted by
The head bears the mouthparts, eyes in the form of compound          Culex female.
or simple eyes or at times may have no eyes and a pair of            Cyclo-developmental-propagative or Cyclo-propagative :
antennae. The thorax is subdivided into three segments with a        In this mode of transmission, the disease causing organism
pair of legs in each called the pro, meso and metathoracic legs.     undergoes a part of its life cycle in the vector and also grows/
The class is further subdivided into 29 orders of which only 4       develops and multiplies inside the body of the vector. Example
contain species of medical importance.                               is Plasmodium sp (Malaria) transmitted by Anopheles female.
(i) Order Diptera : It possesses one pair of wings and a pair        Propagative : In this transmission mode, the disease organism
of ‘halters’, which are vestigial wings. This is the largest order   simply grows and multiplies in the body of the vector, e.g.
comprising about seventy thousand described species with             Yersinia pestis (Plague) transmitted by rat flea and all
world wide distribution. It contains such insects of medical         bacterial, viral, spirochaetal or rickettsial diseases transmitted
importance as mosquitoes, sandflies, simulium flies and              by arthropods.
house-flies.                                                         Other Specialized Modes of Disease Transmission : These
(ii) Order Anoplura : It comprises true or sucking lice, which       specialized modes of disease transmission are generally
are parasitic on mammals and the chewing lice, which infest          encountered in ticks and mites.
birds as well as mammals. Lice of medical importance to man          Trans-ovarian : The disease organism is transmitted to eggs
are human lice.                                                      through ovary of infected female, e.g. Orientia tsutsugamushi,
(iii) Order Siphonaptera : It contains many species of fleas         the agent for Scrub typhus transmitted by trombiculid mite-
including the rat flea, which is a vector of plague and endemic      Leptotrombidium deliense.
typhus.



                                                               • 903 •
Table - 1 : Important arthropod borne diseases
                Disease                         Vector                              Causal organism                          Reservoir
                                                        I Mosquito borne diseases
Malaria                         Anopheles species                         Plasmodium species                    Man
Filariasis                      Culex quinquefasciatus                    W bancrofti (nocturnal, periodic)     Man
                                Aedes niveus group                        W. bancrofti (diurnal sub-periodic)   Man
                                Mansonoides species                       Brugia malayi                         Man, Primate
Chikungunya                     Aedes species                             Arbovirus group A                     Man
Dengue fever & DHF              Aedes species                             Arbovirus group B                     Man
Yellow fever                    Aedes species                             Arbovirus group B                     Man/ Monkeys
Japanese Encephalitis           Culex vishnui group                       Arbovirus group B                     Mammals/ Birds
                                (C tritaeniorhynchus)

                                                           II Sandfly borne diseases
Leishmaniasis
Visceral (Kala azar)            Phlebotomus argentipes                    Leishmania donovani                   Man/Mammals
Cutaneous (Oriental sore)       P. papatasi                               L. tropica                            Man/Mammals
Espundia                        P. sergenti                               L. braziliensis                       Man/Mammals
Sandfly fever                   P. sergenti, P. papatasi                  Virus                                 Man

                                                            III Fly borne diseases
Bacillary dysentery             M domestica                               Shigella                              Man
Amoebic dysentery               M domestica                               E. histolytica                        Man
Gastroenteritis                 M domestica                               Specific/Non specific organisms       Man/animals
Typhoid                         M domestica                               Salmonella typhi                      Man
Paratyphoid                     M domestica                               Paratyphoid A&B                       Man
Cholera                         M domestica                               Vibrio cholera                        Man
Poliomyelitis                   M domestica                               Virus                                 Man
Viral hepatitis (Type A)        M domestica                               HAV                                   Man
Trachoma                        M domestica                               C trachomatis                         Man
Yaws                            M domestica                               T pertenue                            Man

                                                            IV Flea borne diseases
Plague (Bubonic)                Xenopsylla species                        Yersinia pestis                       Rodents
Endemic/Murine Typhus           Xenopsylla species                        R. typhi                              Rodents/
                                                                                                                Domestic animal
Chiggerosis (Jigger)                                                      -                                     -
                                Tunga penetrans (chigoe)                                                        Dogs, cats, wild
Dipylidium caninum              Ctenocephalides. felis/canis              Dipylidium caninum
Hymenolepis diminuta                                                      Hymenolepis diminuta                  Carnivores
                                X cheopis / N fasciatus
H nana                          X cheopis / C canis/ Pulex irritans       H nana                                Rats, mice

                                                            V Louse borne diseases
Epidemic typhus                 Pediculus humanus                         R. prowazeki                          Man
Epidemic relapsing fever        Pediculus humanus                         Borrelia recurrentis                  Man
Trench fever                    Pediculus humanus                         Bartonella quintana                   Man/animals
Dermatitis                      Pediculus humanus/ capitis                Secondary organisms                   Man

                                                            VI Tick borne diseases
Kyasanur Forest Disease (KFD)   Hard ticks species                        Arbovirus group B                     Monkeys/Birds
Tick typhus                     Hard ticks species                        R conorii                             Dogs
Tularaemia                      Hard ticks species                        P tularensis                          Rabbits/ Rodents/ cattle
Relapsing fever                 Soft Tick                                 B duttoni                             Rats

                                                            VII Mite borne diseases
Scrub typhus                    L. deliense                               Orientia tsutsugamushi                Rodents
Rickettsial pox                 Allodermanyssus sanguineus1               R akari                               Rodents
Scabies                         S scabei                                  -                                     Man

                                                   VIII Cyclops transmitted diseases
Dracontiasis                    Cyclops species                           D medinensis                          Man
Fish tape worm                  Cyclops species                           D. latum                              Fish

                                                               IX Reduviid bugs
Chagas disease                  Reduviid/Cone-nosed                       T cruzi                               Domestic animals/ man

                                                                 X Tsetse flies
Trypanosomiasis                 Glossina species                          T gambiense and T rhodesiense         Wild animals/ cattle/ man




                                                                      • 904 •
Trans-stadial : The disease causing organism is transmitted         Study Exercises
from one stage to another e.g. Tick typhus organism - Rickettsia
                                                                    Short Notes : Modes of disease transmission.
conorii transmitted from infected larva to nymph to adult.
                                                                    MCQs & Exercises
Arthropod Borne Diseases
                                                                    1)	 Class Insecta is characterised by presence of how many
Arthropods are responsible for transmission of innumerable               legs? : (a) 4 (b) 5 (c) 6 (d) 8
diseases. Some of the important arthropod borne diseases is         2) 	 Mosquito belongs to the order a) Diptera (b) Anoplura
listed in Table - 1 along with their vectors, causative organisms        (c) Siphonoptera (d) Hemiptera.
and reservoir hosts.                                                3) 	 Mode of transmission of malaria by Anopheles is :
                                                                         (a) Cyclo - developmental (b) Cyclo - propogative
Summary
                                                                         (c) Propagative (d) Cyclo - developmental propagative
Entomology is the branch of science which deals with the            4) 	 Orientia tsugtsugamushi is the causal organism for the
study of insects, however, the scope of the subject has been             diseases (a) Scrub typhus (b) Epidemic typhus (c) Endemic
broadened to include the study of all Arthropods. Amongst                typhus (d) Tick typhus.
the many important classes of the phylum, Insecta is the            5) 	 Which of the following is not a mosquito borne disease :
largest constituting more than 4 million species. Vector borne           (a) Dengue (b) Filaria (c) Leishmaniasis (d) Yellow fever
diseases are one of the leading causes of morbidity & mortality     6) 	 Aedes species cannot transmit : (a) Chickungunya
especially in the developing countries. These diseases can               (b) Dengue haemorrhagic fever (c) Japanese Encephalitis
be largely combated by effective vector control for which a              (d) Yellow fever
sound knowledge of the bionomics of the vector is needed. The       7) 	 Causal organism for bubonic plague : (a) R typhi
important classes of Phylum Arthropoda are Insecta, Arachnida,           (b) Y pestis (c) H nana (d) H diminuta
Crustacea, Myriapoda. The class Insecta is characterized by         8) 	 Murine Typhus is transmitted by : (a) Hard tick species
presence of six legs, body divided into head, thorax & abdomen.          (b) Soft tick species (c) Xenopsylla species (d) Trombiculid
The class has 4 orders of medical importance, Diptera, Anoplura,         Mite species
Siphonaptera & Hemiptera. The order Diptera has 1 pair of           9) 	 Causal organism for epidemic Typhus : (a) B quintana
wings and contains insects such as mosquitoes, sandflies,                (b) R prowazeki (c) Borrelia recurrentis (d) R typhi
simulium flies and houseflies. The order Anoplura comprises         10) 	KFD is transmitted by : (a) Soft Tick (b) Mite (c) Hard Tick
true or sucking lice. Order siphonoptera includes the rat flea;          (d) Lice
the order hemiptera contains bed bugs. The class Arachnida          11) 	___________ is the largest class of Phylum Arthropoda
includes ticks, mites, spiders and scorpions; body is divided            constituting more than __________ insect species
into cephalothorax and abdomen and presence of eight legs.          12) Direct mode of transmission is seen in _______ & ______
The class crustacea includes lobsters, crabs, water fleas and       13) Entomology is derived from the Greek word ENTOMON
Cyclops. Myriapoda includes millipedes and centipedes.                   meaning _________ & LOGOS meaning __________
Arthropods transmit diseases to man by different modes of           14) Arthropoda is derived from two words ARTHRON meaning
transmission, mainly direct, mechanical & biological. In direct          __________ & PODA meaning __________
transmission, two hosts come in direct contact with each other      15) Leptotrombidium deliense is vector for __________
e.g. pediculosis & scabies, mechanical transmission where           Answers : (1) c; (2) a; (3) b; (4) a; (5) c; (6) c; (7) b; (8) c;
disease causing organism is transmitted without undergoing          (9) b; (10) c; (11) Insecta ; 4 Million; (12) Pediculosis;
any change in pathogenecity or development e.g. diseases            Scabies; (13) Insect, Science; (14) Jointed, Appendages;
transmitted by houseflies. Biological transmission is further       (15) Scrub Typhus.
divided into cyclodevelopmental - the disease causing organism
undergoes part of its life cycle in the vector e.g. Wuchereria      Further Suggested Reading
bancrofti; cyclopropogative - the disease causing agent             1.	 Gordon C cook, Alimuddin Zumla, Manson’s Tropical diseases, 21st Ed,
                                                                        2003, Chapter 50, Rickettsial infections, p 900.
undergoes part of the lifecycle in the vector & also multiplies     2.	 Service Mike. Medical Entomology for students. 3rd ed. Cambridge :
inside the vector e.g. malaria transmitted by Anopheles female.         Cambridge University Press, 2004.
The other mode is propogative where disease causing organism        3.	 Kettle DS. Medical and Veterinary Entomology. 2nd ed. CAB International,
                                                                        1995.
simply grows & multiplies in the body of vector e.g. Yersinia
                                                                    4.	 Hati AK. Medical Entomology. Allied Book Agency, Kolkata, 2001.
pestis (plague) transmitted by rat flea, other specialized modes
are transstadial & transovarian.




                                                              • 905 •
stream flushing and regulation of the water level in reservoirs,
   155        Principles of Vector Control                          vegetation removal, shading and exposure to sunlight.
                                                                    (c) Modification or Manipulation of Human Habitation or
                                                     Rina Tilak     Behaviour : This means “A form of environmental management
                                                                    that reduces man-vector - pathogen contact”. Examples of this
                                                                    approach are siting of settlements away from vector sources,
Control of arthropods is one of the key strategies in the
                                                                    mosquito/rodent proofing, personal protection and hygienic
management of vector borne diseases. A strategist should
                                                                    measures against vectors and provision of mechanical barriers,
have sound knowledge of the bionomics, distribution, seasonal
                                                                    providing facilities for water supply, disposal of waste water
prevalence, vectorial capacity, insecticide susceptibility status
                                                                    and excreta, laundry, bathing and recreation to prevent or
and role of arthropods in diseases transmission coupled with
                                                                    discourage human contact with infested water.
the knowledge of identification features of the incriminated
vectors for formulating effective control strategies. Once armed    Chemical Control
with this knowledge, the choice of effective vector management
                                                                    The new era of control of vector borne disease began with the
tools may be exercised; the range and sophistication of control
                                                                    discovery of the insecticidal value of Dichloro Diphenyl Trichloro
methods is impressive. The various control options available
                                                                    ethane (DDT). DDT was first synthesized by Othmar Zeidler in
are as follows :
                                                                    1874 at Strasbourg, Germany. In 1939, Paul Muller of the Geigy
●● Environmental Control                                            Company in Basle, Switzerland, discovered its remarkably long
●● Chemical Control                                                 residual insecticidal property, earning him the Nobel Prize in
●● Biological Control                                               Medicine. The availability of several effective, safe and low
●● Personal Protective measures                                     cost pesticides, coupled with improvements in the techniques
●● Mechanical control                                               of their application, made it possible for many governments in
●● Physical control                                                 the developed as well as developing countries to embark upon
●● Genetic control                                                  extensive countrywide programmes for the control or eradication
●● Legislative control                                              of vector borne diseases. However, development of resistance
Environmental Control                                               amongst vectors to insecticides has necessitated reassessment
                                                                    of the place of pesticides in vector control programmes. Besides
The important environmental control measures which are
                                                                    the technical and financial difficulties, there is a growing
increasingly being used are described below :
                                                                    concern about the environmental contamination resulting from
Environmental Management                                            the persistent use of insecticides.
This has been defined as “The planning, organization, carrying      Classification of Insecticides
out and monitoring of activities for the modification and/or
                                                                    Pesticides may be classified in many ways based on mode of
manipulation of environmental factors or their interaction with
                                                                    entry, target stage, chemical composition and mode of action.
man with a view to prevent or minimize vector propagation and
                                                                    However, the most common classification used is based on
reducing man-vector-pathogen contact.” This is a naturalistic
                                                                    chemical composition. According to this classification, the
approach which attempts to extend and intensify natural
                                                                    insecticides are classified in the following categories as
factors which limit vector breeding, survival and contact with
                                                                    presented in Fig. - 1.
man.
(a) Environmental Modification : It is defined as “A form
of environmental management consisting of any physical
transformation that is permanent or long-
lasting of land, water and vegetation
aimed at preventing, eliminating or Fig. - 1 : Classification of insecticides as per chemical composition
reducing the habitats of vectors without                                      Insecticides
causing unduly adverse effects on the
quality of the human environment”.                         Natural                                   Synthetic
Environmental modification includes
drainage, filling, velocity alteration,       Plant Based
                                                                                           Inorganic              Organic
land levelling and transformation of          Pyrethrum            Mineral Oils
                                                                                         Paris Green
impoundment margins.                         Azadirachtin
(b) Environmental Manipulation : It
is defined as “A form of environmental                         Organophosphates                                  Synthetic
management consisting of any planned                                Malathion                                   Pyrethroids
                                                                                         Carbamates
recurrent activity aimed at producing                               Temephos                                   Deltamethrin
                                            Organochlorines                                Propoxur
temporary conditions unfavourable to                                 Fenthion                                    Cyfluthrin
                                                  DDT                                    Bendiocarb
the breeding of vectors in their habitats”.                         Dichlorvos                                   Bifenthrin
                                                                                            Carbaryl
Examples of environmental manipulation                             Fenitrothion                            Lambda-Cyhalothrin
activities are water salinity changes,                         Pirimiphos methyl                                Permethrin



                                                              • 906 •
Natural Insecticides                                                 Chapter on Mosquitoes for further details on use). As per Govt.
Plant based                                                          of India Gazette notification number S.O. 378(E) dt 26th May
                                                                     1989, the use of DDT in Agriculture has been withdrawn and
Pyrethrum : Pyrethrum extract is obtained from the dried
                                                                     restricted to 10000 MT/ annum for Public Health programme
heads of the flower Chrysanthemum cinerariafolium and
                                                                     except in case of North East as Insecticidal Residual Spray
contains the active ingredients pyrethrins I and II, constituting
                                                                     (IRS).
1 to 2% of the total weight of the raw pyrethrum. Pyrethrum is
characterized by rapid knockdown action on arthropods even           Organophosphorus Compounds : These insecticides are
when used in very low dilution. It is very unstable in light and     derivatives of phosphoric acid and act by inhibiting the
air and has practically no residual effect. This makes repeated      activity of cholinesterase. Many of the insects, which have
applications necessary. Pyrethrum is available as 2% extract,        become resistant to Organochlorines are still susceptible to
which needs 20 times dilution to make it 0.1% solution, which        the members of this group. However, due to their extensive
is actually used for spraying. Using a 0.4 mm or lower calibre       use in agricultural as well as public health field, more and
nozzle, 50 to 100 ml of pyrethrum solution in kerosene oil is        more insects are developing resistance to Organophosphorus
sprayed per 100 m3 of space. Addition of an Organophosphorus         compounds. Some of the common compounds are Malathion,
insecticide to pyrethrum formulation is a common commercial          Temephos, Fenthion, Dichlorovos (DDVP) and Fenitrothion.
practice for obtaining a better effect. It is one of the main        Malathion : It is one of the least toxic Organophosphorus
insecticidal constituents in aerosol dispensers and also an          compounds. Malathion is a broad spectrum insecticide, with
insecticide of choice for ULV sprays. Pyrethrum is perhaps the       efficacy against a large number of pests ranging from mosquitoes,
most acceptable insecticide for use in cook houses, dining halls     houseflies, cockroaches, bedbugs, lice etc. It is available as
and other food preparation areas.                                    Malathion Technical (95%) for use as space spray, 50% Water
Azadirachtin : The active ingredient Azadirachtin is obtained        Dispersible Powder (WDP) and Emulsifiable Concentrate (EC)
from the seed kernels of neem plant Azadirachta indica.              for residual control and 90% dust for use against fleas and lice.
Azadirachtin has insecticidal, fungicidal, bactericidal, viricidal   Malathion under the National Vector Borne Diseases Control
properties including insect growth regulating qualities besides      Programme of India is being used as Indoor Residual Spray
deterrent, anti-ovipositional, anti-feedant, fecundity and           against mosquitoes in areas where the vectors have become
fitness-reducing properties on insects. It has been variously        resistant to DDT. The dosage of its application is 2 g/m2 (Refer
formulated for mosquito larval and adult control in the form         to Chapter on Mosquitoes for further details). As ULV spray it
of liquid and cream formulations. Neem products contain up           has been very widely used during outbreaks of Dengue and JE
to 3% Azadiractin.                                                   as an anti adult mosquito measure. However, development of
                                                                     resistance has been reported in a large number of vectors to
Mineral Oils : Kerosene oil, diesel oil, petrol and crude engine
                                                                     Malathion.
oil have been successfully used as mosquito larvicides. The
oil film cuts off the air supply, enters and blocks the trachea,     Temephos : It is available as 50% EC. It is the only insecticide
may act as a stomach poison, and also lowers surface tension         approved for use in potable water. Because of its low toxicity,
which prevents larvae from floating. Malariol is the best and        it has been successfully used for the control of Anopheles
easiest larvicidal oil to use for Anopheline and Culicine larvae.    stephensi breeding in wells and domestic containers at a
Malariol is used as it is @ 10 litres per 500 linear meters.         dosage of 1ppm (Refer to Chapter on Mosquitoes for details).
                                                                     Sand impregnated with Temephos in 1% concentration has been
Synthetic Insecticides : These can be organic or inorganic.
                                                                     used in some countries against Aedes aegypti which breeds in
The only inorganic compound used in vector control (mosquito
                                                                     containers of clean and potable water. It has proved to be very
larvicide) was Paris green (Copper-aceto-arsenite). It acted as
                                                                     successful in Guinea worm eradication programme in India.
stomach poison when ingested by mosquito larvae. However, it
is not used any more for mosquito control.                           Fenthion : It is formulated as 82.5% EC and as granules
                                                                     containing 2% toxicant. It is a good mosquito larvicide but can
The Organic Insecticides fall into four major groups viz.
                                                                     not be used in potable water bodies. It is highly effective as a
Organochlorines, Organophosphates, Carbamates and Synthetic
                                                                     larvicide against Culex quinquefasciatus or any other vector
pyrethroids.
                                                                     found breeding in non potable water bodies at a dosage of 1
Organochlorine Compounds : These compounds are contact               ppm (Refer to Chapter on Mosquitoes for further details). It
poisons and act on the nervous system. These or their                can also be used for housefly control as a larvicide (Refer to
degradation products are more or less stored in body fat and         Chapter on Houseflies for details).
may be demonstrated in milk, urine or sweat. They all have
                                                                     Pirimiphos methyl : This insecticide is being considered as an
variable residual action for variable periods and are toxic to
                                                                     alternative insecticide for Indoor residual spray. It is available
man and animals. The most important and the only member of
                                                                     as 25% WP; 2 kg is mixed in 10 litres of water and sprayed @ 10
this group used in Public Health is DDT.
                                                                     litres/ 250 sq m area to give a deposit of 2g/sqm. Three rounds
Dichloro-diphenyl-trichloroethane (DDT) : Currently DDT is           of spray are recommended as is followed in case of Malathion.
being used for indoor residual spray in the North Eastern states
                                                                     Dichloro-dimethyl-dichlorvinyl-phosphate        (DDVP      or
of India. A deposit of 1 g of DDT/m2 of surface area of walls
                                                                     Dichlorvos) : It differs from other organophosphorus
and ceiling upto a height of 3.5 m in all dwellings applied at 8
                                                                     compounds in that it possesses a much greater vapour pressure
weeks’ interval, effectively controls majority of the mosquitoes
                                                                     at ordinary temperature which produces fatal insecticidal
and also other arthropods resting on the treated wall (Refer to


                                                               • 907 •
vapour. It is available as 72.6% EC. It can be combined with solid    are available as Pretreated or Long Lasting Nets (LLNs).
substances like wax and used as tablets or bricks thus allowing       Other Synthetic pyrethroids used in Public health : There
it to evaporate slowly. It is one of the common insecticides used     is a large range of molecules used in the field of Public health
for disinsecting aircraft. It is an effective housefly larvicide.     besides the ones listed above. These molecules are Allethrin,
Fenitrothion : It is available as Fenitrothion 40% water              Resmethrin, Phenothrin, Cypermethrin, Imiprothrin, Bifenthrin,
dispersible powder (WDP). The insecticide has shown promise           Cyhalothrin, Cyphenothrin etc. These are all available as WP EC
                                                                                                                                  ,
as an effective insecticide for control of bedbugs; however           or Aerosol formulations for use against pests like cockroaches,
toxicity constraints have limited its widespread use.                 houseflies and mosquitoes.
Carbamates : These compounds are derivatives of carbamic              Newer Group of Insecticides
acid and resemble Organophosphorus compounds in their                 Phenyl pyrazoles : Fipronil is the only member of this class of
mode of action. Some of the preparations produce a rapid              insecticide. Fipronil acts by antagonizing the effect of GABA.
knockdown effect like that of pyrethrum. The inhibition of            It is available as 0.3% Gel for use against cockroach as a crack
Acetylcholine esterase is reversible with Carbamates and hence        and crevice treatment. It is a systemic material with contact
these compounds are less toxic. Some of the compounds in              and stomach activity. It has a unique action called ‘cascade
common use are Propoxur, Carbaryl and Bendiocarb.                     effect’ which is evident due to necrophagy seen in cockroaches.
Propoxur : It is formulated as WDP as well as EC. It is considered    When cockroaches consume the insecticide bait, they are killed;
as the least toxic Carbamate compound for man and domestic            these dead cockroaches when consumed by other cockroaches
animals. It has a flushing out effect and therefore is commonly       bring about the death of these cockroaches and this goes on for
used for cockroach and bedbug control. It is also used in bait        about two months or so, thus obviating the need to retreat the
formulations against houseflies and cockroaches.                      area at lesser intervals.
Bendiocarb : Bendiocarb is an alternative insecticide for Indoor      Neo Nicotinoids : Imidacloprid is the sole member from this
residual Spraying. It is available as 80% WP For indoor residual
                                            .                         class. It acts by causing irreversible blockage of postsynaptic
spraying, it is recommended @ 200 mg/sqm. Two rounds of               acetylcholine receptors. Imidacloprid is a systemic insecticide,
spray are recommended for effective control against malaria.          having notable contact and stomach action. Imidacloprid is
Synthetic Pyrethroids : These are synthetic derivatives or            available as 2.15% Gel for use against cockroaches and as Bait
analogues of natural Pyrethrum. These are broad spectrum,             for use against houseflies, where it is formulated with housefly
highly potent with quick knock down action and long residual          pheromone - Muscalure.
life. Synthetic pyrethroids are many times more effective than        Biorational Insecticides : ‘Biorational’ means any substance
the previously available insecticides. Their relative safety          of natural origin that has a detrimental or lethal effect on
to man and higher animals, their efficient biodegradability           specific target pest, e.g. insects. These insecticides are non-
together with their higher target specific toxicity makes             toxic to man, plants and animals and have little or no adverse
them very attractive materials for integrated vector control.         effects on the environment. An overview of the biorational
The commonly available products are Permethrin, Allethrin,            insecticides is presented in Fig. - 2.
Phenothrin, Cypermethrin, Cyfluthrin, Deltamethrin and
Bifenthrin. The Synthetic pyrethroids are formulated as WDP    ,       Fig. - 2 : Biorational Insecticides Used in Vector Control
EC, SC, Flow, EW and ULV formulations. Being broad spectrum,
these insecticides are being used for vector control as residual                          Biorational Insecticides
spray, space spray and topical application as well as for
treatment of clothing.                                                                           Pheromones                Biocides
Deltamethrin : It is one of the most widely used Synthetic                Insect Growth
pyrethroid molecule in the field of vector control. It is available                          Muscalure                 Bacillus
                                                                            Regulators
in many formulations for various vector control strategies viz.                              Oviposition attractant    thuringiensis
SC 2.5% (Flow) formulation for treatment of bednet and routine                                                          var israelensis
household pest control activity; 2.5% WP formulation for Indoor                                                         Bacillus
Residual spray in Malathion resistant areas and 1.25 ULV for                                                            sphaericus
space spraying. The target dose (for Indoor Residual Spray) is            Chitin Synthesis       Juvenile Hormone
generally 20 mg of a.i. (active ingredient) per sq m of surface              Inhibitors                Mimics
area.                                                                      Diflubenzuron         Methoprene
                                                                           Lufenuron             Pyriproxyfen
Cyfluthrin : Besides Deltamethrin, this is the next most widely
                                                                           Novaluron             Fenoxycarb
used molecule. It is available as 0.5% EW formulation for
                                                                                                  Triflumuron
treatment of bednets; 5% EC for household use and 10% WP for
use as indoor residual spray in Malathion resistant areas.
Permethrin : Widely used for control of lice, scabies and for         Insect Growth Regulators : A new approach to vector control
treatment of clothing and bednets. The product is formulated in       is the use of substances that adversely affect insect growth
varying concentration as Shampoo formulation for use as anti-         and development. The enzymes and hormones that regulate
lice treatment and 5% cream for use in scabies treatment. Bed         developmental processes within an insect’s body can sometimes
nets treated with Permethrin at the manufacturing stage itself        be exploited as chemical control weapons. These compounds,


                                                                • 908 •
often known as Insect Growth Regulators (IGRs) can be used          houseflies) and oviposition attractant (Aedes control) have
to stimulate development at inappropriate times or inhibit it       been exploited in the field of vector management.
at other times. They are quite selective in their mode of action    Biocides : The development of insecticide resistance amongst
and potentially act only on target species. Most of the IGRs that   the major pests and vectors coupled with the non target toxicity,
have shown effectiveness against insect pests, cause the rapid      necessitated development of safer alternatives to insecticides.
death of the insect through failure of a key regulatory process     This led to the screening, promotion and use of a large number
to operate or function. IGRs generally control insects either       of biorational products of which biocides are one of the most
through inhibition of chitin synthesis or interference with         important control options. The two biocides used in the field
metamorphosis by mimicking the action of juvenile hormone.          of vector control are Bacillus thuringiensis var israelensis and
The major groups of IGR compounds include :                         Bacillus sphaericus. Both these products are widely used as
Chitin Synthesis Inhibitors : These chemicals inhibit the           larvicides in Mosquito control programmes and act as stomach
moulting process by blocking the activity of chitin synthetase,     poison.
an enzyme needed by epidermal cells when constructing               Bacillus thuringiensis var israelensis (Bti) : It was discovered
a new exoskeleton. Because of this mode of action, Chitin           in 1976 and has been found to be effective as mosquito larvicide.
Synthesis Inhibitors (CSIs) are highly specific to arthropods.      It is a gram positive spore forming bacteria. Bti produces toxins
They act rather slowly (2-5 days), but eventually disrupt any       which are present in parasporal body called the ‘protein crystal’.
process that involves construction of new cuticle (e.g., molting,   It primarily kills by the action of delta-endotoxin. When the
hatching, pupation). They are most effective when used against      mosquito larva ingests the protein crystal (inactive protoxin),
the immature stages of a vector. Diflubenzuron, is used for         it is activated inside its midgut by the action of proteases into
controlling mosquitoes, houseflies etc. It is available as 25%      active protoxin; these bind to the cell receptors present on
EC, WP & 0.5% Granules and is used @ 1.0 g/ acre of surface         midgut epithelium and cause disturbance in osmoregulatory
water as mosquito larvicide. Lufenuron, is a systemic CSI and       mechanism which leads to swelling and eventual bursting of
is especially effective for flea and tick infestation control on    the epithelium and finally death of the larvae. The product is
animals. Novaluron is a recent addition to the list, which has      available as WP Granules, AS & Briquette. Bti 12 AS is used @
                                                                                     ,
been found effective against the mosquitoes. It is a contact        20ml/m2 and has been found to be effective up to 15 days (for
larvicide and is available as 10% EC. It is used @ 20 μg a.i./ l    details refer chapter on mosquito). Bti however, suffers from
and the efficacy lasts up to 3 months.                              the disadvantage that it can not be used in polluted waters
Juvenile Hormone Analogues or Mimics : Juvenile hormone             or where particulate matter is more; it also cannot recycle in
analogues or Juvenile Hormone Mimics (JHM’s) act by inhibiting      nature. It is used in non potable water bodies.
the developmental changes associated with embryogenesis,            Bacillus sphaericus : A naturally occurring bacterium used
morphogenesis, and reproduction. During normal development,         against mosquito larvae. It is more effective in polluted water
JH levels are elevated in larvae (or nymphs) and decrease           and can recycle and persist in nature. It is available in various
prior to pupation (or adult eclosion). Contact exposure to JH       formulations like Bti viz. pellets, briquettes, granules & WP It
                                                                                                                                   .
analogues during the egg stage or after the last larval molt        is used @ 20ml/m2 and has been found to be effective up to
can inhibit development, delay maturation, and eventually           three weeks.
result in death. Since the onset of mortality is usually quite
                                                                    Fumigants : Some of the fumigants used as pesticides are
slow (days to weeks), these chemicals are not used during
                                                                    carbon tetrachloride, methyl bromide, ethylene dibromide,
epidemics; however, these chemicals are much in demand for
                                                                    chloropicrin, carbon disulphide and dichlorvos (DDVP).
routine vector control due to their specificity and safety to
non-target organisms. Several compounds (e.g., Methoprene,          Application Techniques
Pyriproxyfen, Fenoxycarb, Triflumuron) have been successfully
                                                                    Control of arthropods in different habitats necessitates the use
incorporated into vector management programmes especially
                                                                    of different types of spraying equipment as well as a variety of
Dengue and Malaria and in products used for controlling ants,
                                                                    formulations such as liquids, granules and dusts. For example
fleas, and other household pests. Pyriproxyfen can cause
                                                                    control may involve treatment of small domestic or peridomestic
sterilization and inhibition of growth of adult insects; it has a
                                                                    water collections which are ideal breeding places for Aedes
residual effect up to 3 to 6 month indoors and 30 days outdoors.
                                                                    mosquitoes; applications to stagnant waters in cesspools,
It is widely used against mosquitoes @ 2gm a.i./ sq m.
                                                                    ditches and drains where Culex mosquitoes breed, large bodies
Pheromones : Pheromones are semiochemicals (chemicals               of standing water where certain Anopheles mosquito species
which mediate interactions between organisms) secreted by           may be breeding; or aerosol spraying of extensive areas to halt
an organism which provokes specific reaction in receiving           epidemics. To meet with diverse situations, significant progress
organisms of the same species. These chemicals may further          has been made in improving the spraying equipment. The Ultra
be classified based on the type of interaction mediated e.g. sex    Low Volume (ULV) equipment for ground and aerial spray to
pheromone (muscalure secreted by houseflies), oviposition           control mosquitoes and other haematophagous arthropods has
attractant (mosquito larvae), aggregation pheromone                 resulted in not only the elimination of several impediments
(cockroach, bedbugs), alarm pheromones, trail pheromones            like frequent mixing and reloading but helped in increasing
etc. The scope of pheromones in vector control is promising,        the speed of application and reducing the dosages and costs. It
however so far only two pheromones viz. Muscalure (in               is specially recommended for control of an outbreak of vector
combination with Imidacloprid insecticide as baits against          borne disease.


                                                              • 909 •
Formulations                                                         from 1 to 5%. These can be used in irrigation channels,
                                                                     irrigated or flooded lands, paddy fields and particularly where
Manufacturers combine pesticides with other materials
                                                                     there is vegetation on the water surface. After sinking, these
to make usable concentrations called formulations. These
                                                                     formulations disintegrate slowly releasing small particles of
formulations are designed to kill insects readily without
                                                                     insecticides. These can be effectively used also in small water
causing undue hazards to non-target organisms when diluted
                                                                     collections such as ornamental tanks and earthen pots, tree
and applied correctly. Factors influencing application and
                                                                     holes and other domestic or peridomestic breeding places of
efficacy of an insecticide are its toxicity, size and shape of
                                                                     Aedes mosquitoes.
its particles, concentration in formulation; type of solvent
used, type of surface to be sprayed, atmospheric temperature         Wettable Powder : This formulation consists of the technical
and humidity, type of sprayer and its nozzle, training of the        grade pesticide, an inert carrier and a wetting agent (usually a
spraying personnel, the bionomics, morphology and physiology         synthetic detergent) that helps it mix with water. These usually
of the particular arthropod. No single preparation can meet the      contain 50 to 75% of the toxicant. Most of these can be put
requirement of vector control in all spheres of human ecology.       directly into water and require only slight agitation to make
Solutions, emulsions, suspensions, water dispersible powders,        suspension; others may require mixing with a small amount of
dusts and granules to suit different conditions and problems         water to form a paste or slurry. The required volume of water
are therefore, prepared and used.                                    is then added to paste or slurry followed by thorough agitation
                                                                     of the mixture.
Technical Grade Pesticide
                                                                     When water is added to a wettable powder it makes a suspension;
This is the basic toxic agent in its purest commercial form.
                                                                     this enables the pesticide to stay on porous surfaces like
Some technical grade pesticides are liquids; others occur in
                                                                     concrete, plaster or unpainted wood. Water penetrates these
solid form. Technical grade Malathion is used in ultra low
                                                                     surfaces, leaving the carrier and the maximum amount of the
volume space applications.
                                                                     pesticide on the surface available to kill pests. Suspensions have
Types of Formulations                                                other advantages, too. They have no solvent odour, and they
Formulations essentially are of three types : Solid or dry, liquid   don’t tend to irritate or penetrate skin. However, they generally
and gaseous formulations.                                            need agitation to keep pesticidal particles from settling out.
Solid or Dry formulations                                            Also, they tend to clog sprayer nozzles and strainers, especially
                                                                     when the wettable powder is stored for long periods in humid
Dusts : Dusts are normally ready-to-use formulations with a          areas or when a high concentration is used.
low percentage of active ingredient (usually 1 - 10%) plus a
                                                                     Liquid formulations
very fine inert carrier such as talc, chalk, diatomaceous earth,
clay or volcanic ash. These materials are usually low in cost,       Emulsifiable Concentrates : Emulsifiable concentrates consist
easy to apply, non-staining and non-toxic to vegetation. Dusts       of the technical grade pesticide (typically 45% to 75%), a solvent,
are always used dry and can easily drift into non-target areas       and an emulsifying agent, usually a synthetic detergent. This
if they are not applied carefully. For this reason, outdoor          agent is used to allow the concentrate to be diluted in water,
applications should be made only when the wind is calm. A            resulting in an emulsion.
common use for dusts is in crack and crevice or spot treatments      Emulsifiable concentrates are usually clear but emulsions look
indoors in out-of-sight areas (behind equipment, in wall voids       similar to milk. Finished sprays are emulsions or solutions
and so on) which remain dry. The residual pesticidal activity        diluted to field strength. Unlike solutions, most emulsions
of dust is normally fairly long, provided the dust stays dry, but    need a little periodic agitation to keep the concentrate from
quickly loses it toxicity in the presence of moisture. They don’t    separating out of the water. Emulsions are used for residual
adhere well to vertical surfaces.                                    treatments. Pests that contact these surfaces are killed by
Dusts are used on people during mass delousing operations to         the pesticidal residue. Emulsions may damage aluminium,
control outbreaks of lice borne diseases. Dusts are also used        varnish, and painted surfaces due to the action of solvents such
for flea control during plague outbreak. Dusts aren’t generally      as Xylene. Emulsions may also be corrosive to metal sprayers
absorbed through the skin, but may be dangerous if inhaled           and their fittings and hence sprayers made of stainless steel,
into the respiratory tract.                                          aluminium or other non-corrosive materials should be used.
Granules : These are basically the same as dust formulation,         Oil Solutions : These formulations consist of a technical grade
except the carrier particles are larger and thus don’t stick to      pesticide dissolved in a solvent such as kerosene or diesel
leaves allowing penetration in dense foliage. This is a real         oil. Solutions are available as ready-to-use formulations (for
advantage when the pesticide must reach the water surface for        example ordinary household fly and mosquito sprays with a
mosquito control in vegetated swamps, or if it must get to the       low percentage of pesticide) and as solution concentrates. These
ground surface through trees and shrubs for chigger control.         concentrates contain a high percentage of insecticide and must
Granules are also available in timed-release formulations that       ordinarily be diluted in oil or another suitable solvent. Some
release a dosage of the pesticide over an extended period of         concentrates are used without dilution in Ultra Low Volume
time. Other advantages of using granules are that they provide       (ULV) applications. Oil solutions applied as finished sprays
longer lasting effects and their use results in less drift than      often kill insects on contact, since the oil helps the pesticide
generally occurs with liquids or dusts.                              penetrate the insect’s waxy body wall.
The percentage of insecticide in granules and pellets varies         Ultra-Low Volume (ULV) : While most items of ULV pesticide



                                                               • 910 •
dispersal equipment use the readily available solutions or           Ticks depending on the formulation. Care should be taken
technical grade formulations, there are special ULV formulations     while handling aerosol cans since they can explode if punctured
available for e.g. Deltamethrin 1.25 ULV etc.                        or overheated, even after the pesticide has been dispensed.
Gaseous formulations : Gases are primarily used in                   Common insecticides used as aerosols are Pyrethroids,
fumigation operations. They may be prepared as liquefied             Malathion, DDVP and repellents like DEET and DEPA. These
gases and packaged in pressure containers or in a material           are used for disinfesting aircrafts, tents, rooms, other small
form that reacts with the moisture in the air to form a gas.         enclosures, uniforms and for topical application. An emission
The gas molecules can penetrate cracks, crevices and tightly         of nearly 15 seconds is enough for a 100 m3 space.
packed material. Gases are the most dangerous pesticides used        Equipment
and hence special safety equipment and training are necessary
when using gases and must never be attempted except by               Equipment used for vector control can be broadly classified as
trained pest management personnel operating in pairs. One            ground equipment and equipment used for aerial applications.
of the common gaseous formulations viz. Calcium cyanide              (a) Ground Equipment
(powder) and Aluminium phosphide (tablet) are used for rodent        (i)	 Sprays for production of fine or coarse spray which may be
control.                                                                   either manually operated or power operated.
Special formulations                                                 (ii)	 Sprayers for the production of mist which may be either
Resin Strips : Pesticide-impregnated resin strips release                  with gaseous energy nozzles (manual operated or power
vapours as they are heated or exposed to normal room                       operated) or with centrifugal nozzles.
temperatures. The use of resin strips in rooms occupied by           (iii)	Devises for the production of aerosols which may
the young, the elderly or in food preparation and food serving             be mechanical, thermal or gaseous energy aerosol
areas is strictly prohibited.                                              generators.
                                                                     (iv)	 Dusting equipment which may be manually operated or
Baits : Baits are commonly used to manage scavenging pests
                                                                           power operated.
such as rodents, ants, flies, and cockroaches, which are
                                                                     (v)	 Applicators for granules and pellets, manually or power
particularly difficult to manage with standard techniques.
                                                                           operated.
Baits consist of the toxicant mixed with a food attractive to the
target pest or with water. For this reason, baits made with local    (b)	Aerial Equipment
foods are normally more effective than premixed formulations.        Equipment for aerial sprays is essentially the same but with
Recent development is the use of pheromone Muscalure with            certain modifications. The equipment in common use is the
Imidacloprid as bait for houseflies.                                 boom and nozzle system.
Gel : One of the special formulations developed for use against      Sprayers
cockroaches is Gel formulation. Gels comprise some food
attractant mixed with the toxicant and some stabilizing agents.      The equipment commonly used for spraying various insecticidal
Examples are Fipronil and Imidacloprid Gels marketed against         formulations are the hand operated sprayers, power operated
cockroaches.                                                         sprayers, aerosol dispensers, fog generators and dusters.

Shampoo : This formulation has been specially developed              (a) Hand Operated
for use against head lice infestation. Permethrin is the most        These are hand sprayers, knapsack sprayer and compression
common ingredient of the commercially available anti-lice            sprayer.
Shampoo formulations worldwide.                                      (i) Hand Sprayer : The hand sprayer is used for space spraying
Beads / Pellets / Briquettes : Small floating beads, pellets or      of small apartments. It is provided with a small can for holding
briquettes incorporating biocides - Bti and B sphaericus have        ½ to 1½ litres of spray fluid and a cylindrical plunger type air
been developed against Anopheline larvae. These formulations         pump. The nozzle size is less than 0.4 mm in order to produce a
can be made as controlled release formulations as well.              fine spray. The simplest form is the familiar ‘flit gun’ producing
Paints and Lacquers : These can be used for incorporation            intermittent spray. A number of other light hand sprayers have
of insecticides especially for control of pests on ships. These      been designed, which can be pressurized in the manner of
preparations remain effective for long periods. The new              compression sprayers and are used to produce a mist or fine
insecticide, Imidacloprid is also available as a paint formulation   droplet spray.
against houseflies.                                                  (ii) Knapsack Sprayer : This is designed to fit on to the back
Mats / Coils : These are special formulations which have been        of the operator and usually has a capacity of 15 to 20 litres.
developed as controlled release formulation for indoor use           It incorporates a light but powerful diaphragm pump actuated
against mosquitoes. These have synthetic pyrethroids such as         by a lever carried forward to the operator’s hand where it is
Allethrin, which acts as toxicant to knock down the mosquitoes       worked by an up-and-down movement. These sprayers are used
when used indoors.                                                   both for larviciding and residual spraying. The nozzle size used
                                                                     for residual spraying varies between 0.78 to 1.0 mm so as to
Aerosols : Aerosols are pressurized cans containing a small
                                                                     produce a coarse spray.
amount of pesticide driven through a small nozzle. They’re
commonly used as space sprays for flying insects viz.                (iii) Compression Pneumatic Sprayer : This is the commonest
mosquitoes and houseflies and as residual sprays for Mites/          type of equipment used in National Vector Borne Diseases
                                                                     Control Programme for the application of insecticides. It has


                                                               • 911 •
a hand operated pump incorporated to build up adequate               (d)	 Wash your overalls and other protective clothing at the
pressure. When the pressure is released by a trigger on the               end of each working day in soap and water and keep them
lance, the liquid is forced out from the tank to the nozzle by            separate from the rest of the family’s clothes.
the compressed air and a continuous spray of the insecticide         (e)	 If the insecticide gets on your skin, wash off immediately
formulation is produced. It is slung over the shoulder with               with soap and water.
one strap or may be carried on the back with two straps. It is       (f)	 Change your clothes immediately if they become
operated by one person.                                                   contaminated with insecticides.
(b) Power Operated Sprayers                                          (g)	 Inform your supervisor immediately, if you do not feel
                                                                          well.
These are useful for application of insecticides over large areas.   (h)	 Wear protective clothing (Fig. - 3) :
These are hydraulic sprayers in which the spray liquid is expelled   	    - Broad rim hat (protects head, face and neck from spray
to the nozzle by positive displacement by the plunger pump.               droplets).
Insecticide tanks built into a truck or mounted over a hand          	    - Goggles or face shield (protects face and eyes against
trolley are connected directly to a power operated compressor.            spray fall-out).
By means of a long hose the spraying fluid is conveyed under         	    - Face mask (protects nose and mouth from airborne
pressure through the lance to the nozzle.                                 particles of the spray fall-out).
(c) Insecticidal Fog Generators                                      	    - Long sleeved overalls (Keep overalls outside of boots).
Several types of fog generators are now available for the            	    - Rubber gloves.
production of insecticidal fogs in the open on a large scale.        	    - Boots.
In these fogging machines the oily solution of the insecticide       Preparations
is finely atomized by the powerful blast of hot exhaust gases        The Household : Inform the householder of the spraying
from a petrol engine.                                                schedule and the purpose of spraying, giving them time to
(d) Aerosol Dispensers                                               prepare and vacate the house. Occupants MUST leave houses
These are used for disinfestations of aircrafts, tents, rooms        before spraying. Rooms occupied by sick people who cannot
and similar small enclosures. It contains insecticide and a          be moved must NOT be sprayed. Remove all household items,
propellant. Common aerosols contain Synthetic pyrethroids            including water, food, cooking utensils and toys from the house.
or their combination, which are routinely used for mosquito,         Move and cover, or take out the furniture to allow easy access
cockroaches and fly control.                                         for spraying walls. Items that can not be removed should be
                                                                     well covered.
(e) Dust Gun
                                                                     Equipment : Indoor residual spraying of insecticides is
Insecticidal dusts are applied against lice and fleas in rat         normally done using hand-operated compression sprayers.
burrows or on water surfaces as dry powders diluted with inert       Before starting a spray operation, the equipment must be
dusts. Small light weight guns are used for mass delousing of        checked. Faulty sprayers may result in poor control or over-
infested people.                                                     treatment. Examine the sprayer visually to ensure that all parts
Residual Spraying                                                    are present, assembled correctly and are in good condition
                                                                     (Fig. - 4).
This is the application of insecticides to surfaces so that the
insecticide particles remain on the surface in the form, size and
                                                                      Fig. - 3 : Protective Fig. - 4 : Hand-operated
quantity suitable for insects to pick up on contact and sufficient
                                                                      clothing for spraying compression sprayer
to exert a lethal effect over a long period. Organochlorine,
Organophosphorus, Synthetic pyrethroids and Carbamate                                 A: Hat                    H.Hose
compounds can thus be applied on the inside walls of houses                           B: Goggles
                                                                                                                   G.Strainer
and also on thick bushes in forests. The type of surface to                           C: Mask
                                                                                                                        D.Pump (handle)
which an insecticide is applied influences its toxicity against
insects and its persistence. Solutions and emulsions quickly                                                            E.Pressure gauge
get soaked in the absorbent surfaces of soft bricks and mud                              D: Long                         J.Trigger
walls which take in a large portion of insecticidal material                             Sleeved
                                                                                         Overalls                        C.Lid
deposited on them; but when suspended in water it remains
over the surface after the water evaporates or gets absorbed. The                                         B.
                                                                                                                          A.Sprayer tank
nozzles of sprayers used for residual spraying must conform to                         E: Rubber    Shoulder
the need of having a droplet size which is neither too large nor                       Gloves          strap
                                                                                                                         F.Lance
too small. Similarly, safety precautions should be observed, as
follows, while spraying as per standard WHO guidelines :                               F: Boots                               K. Foot rest
(a)	 Do not eat, drink or smoke while working.                                                                          I.Nozzle
(b)	 Wash your hands and face with soap and water after                                                    Nozzle body
     spraying and before eating, smoking or drinking.
                                                                                                           Nozzle tip
(c)	 Shower or bathe at the end of every day’s work and change
                                                                                                           Nozzle cap
     into clean clothes.



                                                               • 912 •
A.	  Sprayer tank                                                 (c)	 Operate the pump using both hands and with foot on
B.	  Shoulder strap                                                    the footrest. Pump to the working pressure of 55 psi
C.	  Lid                                                               (Fig. - 7).
D.	  Pump (handle)
E.	  Pressure gauge                                                Fig. - 7
F.	  Lance
G.	  Strainer
H.	  Hose
I.	  Nozzle - check correct type of nozzle is fitted and is not
     damaged or worn (flat fan nozzle with 55º to 60º swath
     and 0.75 l/min flow rate at 700 g/ sq cm).
J.	 Trigger on/off valve. Is the strainer inside valve handle
     clean?
K.	 Footrest
Before using an insecticide, use clean water to ensure that the
equipment operates properly and does not leak. Wear protective
clothing. To check, follow the steps below :
(a)	 Pour clean water into the tank (never fill tank more than
     3/4 full) (Fig. - 5).

 Fig. - 5

                                                                  (d)	 Check tank is holding pressure. Listen for hissing sound of
                                                                       escaping air (Fig. - 8).

                                                                   Fig. - 8




(b)	 Fit the lid. Turn the handle to lock the lid in position
     (Fig. - 6).
                                                                  (e)	 Check to make sure there are no leaks along lance and
 Fig. - 6                                                              hose, especially where hose joins tank and trigger on/off
                                                                       valve (Fig. - 9).

                                                                   Fig. - 9




                                                            • 913 •
(f)	 Operate trigger on/off valve to make sure that spray is        container with water for several hours before the blockage is
     emitted from the nozzle (Fig. - 10).                           removed by a very soft toothbrush. NEVER clean nozzle with
                                                                    a hard pin or piece of wire and NEVER put a nozzle to your
 Fig. - 10                                                          mouth to blow through it.
                                                                    Mixing, Handling and Spray Techniques
                                                                    Prepare the insecticide spray according to the manufacturer’s
                                                                    instructions. The insecticide may be mixed separately in a
                                                                    bucket and poured into the sprayer. Water soluble sachets,
                                                                    tablets and insecticides granules are added directly to the
                                                                    water filled tank. These formulations mix readily with water
                                                                    and reduce the hazards associated with handling and mixing
                                                                    in a separate container. When the sprayer has been filled with
                                                                    water to the maximum level indicated on the tank, the lid of
                                                                    the tank is fitted and the sprayer pumped to a pressure of 55
                                                                    psi by pumping 55 times (700 g/sq cm). The contents of the
                                                                    tank should be thoroughly mixed by shaking the tank before
                                                                    starting to spray (Fig. - 12).

                                                                     Fig. - 12
(g)	 Check the spray pattern from the nozzle by spraying a
     dry wall surface. Look to see that the pattern is even and
     without streaks. Ensure nozzle does not drip when trigger
     on-off valve is released (Fig. - 11).

 Fig. - 11




                                                                    Spraying in a room should commence from the backside of a
                                                                    door clockwise completing the plain surfaces of walls. Then the
                                                                    crevices on the walls and inside portion of windows etc. should
                                                                    be sprayed. Thereafter the pillars, under surfaces of furniture
                                                                    and lastly the ceilings should be taken for spray.
(h)	 Calibrate the nozzle with water in the tank. Pump to 55        Spray is done from roof to floor, using downward motion, to
     psi (700 g/ sq cm). Open the trigger on-off valve for one      complete one swath; then stepping sideways and spraying
     minute, collect the discharge and measure the amount in a      upwards from floor to roof. Spray is applied in vertical swaths
     measuring jug. Empty the jug. Discharge for a further one      52-56 cm wide. Swaths should overlap by 5 cm and spraying
     minute and measure the amount. Repeat again and calculate      should be undertaken as shown in Fig. - 13. Normal swath
     the average of the three, one-minute measurements.             coverage will take 2.7 sec if height of wall is assumed to be
With the above procedure, the average discharge of an 8002          3 meters and hence in one minute 22-23 swaths will cover a
nozzle is about 750 ml per minute. If the discharge is incorrect,   wall of 10-11 metres length and 3 metres height i.e. 30 - 33 sq
check the nozzle and the screen filters to ensure they are not      m. It takes about 5 minutes to spray a house with an average
clogged. If necessary replace nozzle. Repeat the calibration.       surface area of 150 sq m.
If the nozzle is clogged : The opening in a nozzle is very small
and must not be damaged. Clogged nozzles should be put in a



                                                              • 914 •
Time your spray speed to cover one meter every 2.2 seconds,
 Fig. - 13
                                                                 i.e. 4.5 seconds for a 2 m high wall. Timing may be aided by
                                                                 mentally counting “one thousand and one - one thousand and
                                                                 two - one thousand and three -…”. Adjust the mental counting
                                                                 procedure according to the local language (Fig. - 15). If spray
                                                                 stops due to a blockage in nozzle, unscrew the nozzle cap,
                                                                 remove blocked nozzle and replace with a new one. The blocked
                                                                 nozzle should be cleaned as explained above. Do not let spray
                                                                 drip on the floor. Re-pressurize the tank when the pressure
                                                                 falls.

                                                                  Fig. - 15




To ensure the correct swath width, keep the spray tip about
45 cm from the wall. Lean forwards as you spray from top of
the wall and move back as you bring the nozzle downwards
(Fig. - 14).

 Fig. - 14
                                                                 Procedures after Spraying
                                                                 (a)	 Advise the occupants to stay outside until the spray is
                                                                      dry.
                                                                 (b)	 Instruct the householder to sweep or mop the floor before
                                                                      children or pets are allowed to re-enter.
                                                                 (c)	 Instruct the householder not to clean the sprayed
                                                                      surfaces.
                                                                 Disposal of Remains of Insecticides and Empty Packaging:
                                                                 At the end of the day’s work, put the washings from the sprayer
                                                                 into pit latrines, if available, or into pits dug especially for
                                                                 this purpose and away from sources of drinking water. Dilute
                                                                 any insecticide with more water before putting into pits. It
                                                                 is advisable to prepare only sufficient insecticidal solution
                                                                 to avoid disposal of remaining insecticidal solution. Never
                                                                 pour the remaining insecticide into rivers, pools or drinking
                                                                 water sources. All empty packaging should be returned to the
                                                                 supervisor for SAFE disposal. Never re-use empty insecticide
                                                                 containers. Empty insecticide containers should NOT be burned
                                                                 or buried.
                                                                 Maintenance of Equipment : After completing the day’s work,
                                                                 de-pressurize the tank and empty any remaining insecticide,
                                                                 following the instructions given in the previous section. Clean
                                                                 the tank as explained below :
The flow of liquid from the nozzle tip at 700 g/sq cm pressure
                                                                 ●● De-pressurize the tank.
is 750 ml/minute. Hence 30 sq m surface will be covered with
                                                                 ●● Fill the tank half-full with clean water.
750 ml of the insecticide solution.
                                                                 ●● Replace the lid.



                                                           • 915 •
●●   Shake the tank so all inside surfaces are washed               Fig. - 19
     (Fig. - 16).

 Fig. - 16




                                                                   ●●   With lid open, turn tank upside down, open the on/off valve
                                                                        and let all the water drain out of the hose and lance.
                                                                   ●●   Ensure the lance is parked to protect nozzle when not in
                                                                        use.
                                                                   ●●   When storing the sprayer for a long period, hang it upside
                                                                        down with lid open to allow air circulation. Allow lance to
                                                                        hang from D-ring on the tank with the trigger valve kept
                                                                        open (Fig. - 20).

                                                                    Fig. - 20

●●   Pump up to 700 g/ sq cm pressure. Spray water through
     nozzle (Fig. - 17).

 Fig. - 17




●●   De-pressurize the tank and pour out any remaining water
     into pit latrines or into a pit away from sources of water.
●●   Unscrew trigger on/off valve handle and check and clean
     the strainer.
●●   Reassemble the trigger on/off valve (Fig. - 18).
                                                                   Space Spraying
 Fig. - 18                                                         It is an ideal method for bringing about rapid control of vectors
                                                                   in emergency or epidemic situations and may also be used for
                                                                   seasonal control of flying insect pests or vectors. An additional
                                                                   objective may be to reduce or interrupt the transmission cycle
                                                                   of insect-borne diseases. However, it may not be ideal for all
                                                                   vectors or situations and as such may not be an economical
                                                                   method of control. Among the disease vectors affecting public
                                                                   health, the most important and widespread are mosqui­ oes,  t
●●   Remove the nozzle tip and wash                                houseflies, sandflies and other biting flies; some of these may
●●   Refit the nozzle                                              be tar­ eted for space treatment.
                                                                          g
●●   Clean the outside of tank.                                    Immediate killing of actively flying insects requires a cloud
                                                                   of insecticide droplets that they will encounter in flight. To


                                                             • 916 •
be cost-effective and obtain good biological efficacy, space          Cold Fog
spraying requires :                                                   The cold fog is produced by a special device (cold fogger) that
(a)	 Knowledge of the behaviour and biology of the target             breaks up the chemical into microscopic droplets by mechanical
     species - to understand where and when space treatments          means, basically with a high-pressure pump and an extremely
     will be effective;                                               fine nozzle. The spray droplets are generated without any
(b)	 Knowledge of insecticides and formulations most suitable         external heat. With cold fogs, the volume of spray is kept to
     for space spraying;                                              a minimum. Ultra-low-volume insecticide formulations are
(c)	 Knowledge of pesticide application technology - to know          commonly used for such applications. The cold fogger may
     which equip­ ent is needed and how to use it; and
                  m                                                   dispense formulations in a very concentrated form and generate
(d)	 Monitoring and surveillance of the target species and            the droplets (usually in the 5-30 micron diameter range) in a
     vector-borne disease problem to evaluate the efficacy of         precise manner. However, its ability to penetrate dense foliage
     the programme.                                                   or obstacles is not as good as that of the thermal fogging. Cold
A space spray - technically a fog (sometimes referred to as an        fogging is sometimes called Ultra Low Volume (ULV) treatment
aerosol) is a liquid insecticide dispersed into the air in the form   as it allows the utilization of only a very small amount of
of hundreds of millions of tiny droplets less than 50 μm in           chemical for coverage of a large area.
diameter with a view to cause by contact, immediate knock             Like thermal fogging, cold fogging also does not have lasting
down of the flying or resting insects in confined spaces. Space       residual effects. It is, therefore, essential to carry out fogging at
sprays, even when they settle on surfaces do not have much            the time when the vectors are most active to hit them directly.
residual action. It is only effective while the droplets remain
                                                                      Advantages
airborne. Therefore, they have to be repeated at frequent
intervals. Space sprays are applied mainly as thermal fogs or         ●● The amount of diluents is kept to a minimum, resulting in
cold fogs.                                                                 lower appli­ ation cost and increased acceptability. Some
                                                                                        c
                                                                           formulations are ready to use, thereby reducing operator
Thermal Fog                                                                exposure
Thermal fog is produced by special devices known as thermal           ●● Mostly water-based and water-diluted formulations
foggers that use heat to break up the chemical into very small             are used which pose a low fire hazard and are more
droplets (usually in 5-30 µm diameter range) which then                    environmental friendly
disperse in the air. When the chemical (usually diluted with          ●● Application is more efficient because a lower volume of
oil-based carrier) is heated, it is vaporized in a combustion              liquid is applied
chamber and then expelled via an outlet tube to form a dense          ●● No traffic hazard as the spray cloud is nearly invisible
fog cloud when it condenses on contact with cool ambient air.         Disadvantages
The insecticide used in thermal fogs is diluted in a carrier          ●● Dispersal of the spray cloud is difficult to observe
liquid, which is usually oil-based. Hot gas is used to heat the       ●● Higher technical skills and regular calibration are required
pesticide spray, decreasing the viscosity of the oil carrier and           for efficient operation of equipment.
vaporizing it. When it leaves the nozzle, the vapour hits colder      Space Spray Equipment
air and condenses to form a dense white cloud of fog. Most of         Selection of appropriate equipment for space spraying depends
the droplets are smaller than 20 µm. The droplet size is affected     on the size and accessibility of the target area as well as the
by the interaction between the formulation, the flow rate and         human resources and operational capacity of the programme.
the temperature at the nozzle (usually > 500°C). The volume           Sometimes smaller machines may be needed in conjunction
of spray mix­ ure applied in vector control is usually 5-10 litres
              t                                                       with vehicle-mounted equipment to treat narrow pathways and
per hectare, with an absolute maximum of 50 litres per hectare.       other areas inaccessible to vehicles or sheltered from prevailing
The hot emission gas is obtained from engine exhaust, friction        air movements. Cold fog equipment is recommended where
plate/engine exhaust or from a pulse jet engine.                      thermal fogs may cause a traffic hazard. Aerial application
Advantages                                                            of space sprays may be justified where access with ground
●● Easily visible fog, so dispersal and penetration can be            equipment is difficult and/or extensive areas need to be treated
     readily observed and monitored;                                  very quickly.
●● Good public relations in some circumstances as people can
                                                                      Equipment for Thermal Fogging
     see some­ hing being done about the problem; and
                t
●● Low concentration of active ingredient in the spray mixture        Hand-carried Thermal Foggers : These are used for treating
     and reduced operator exposure.                                   houses and certain outdoor areas of limited size or accessibility,
Disadvantages                                                         e.g. markets, hotel grounds and parks. There are two types of
                                                                      hand-carried thermal foggers; pulse jet and fric­ ion plate.
                                                                                                                      t
●● Large volumes of organic solvents are used as diluents,
     which may have bad odour and result in staining;                 Vehicle-mounted Thermal Foggers : Large thermal fog
●● High cost of diluents and spray application;                       generators use an air-cooled motor to run an air blower, fuel
●● Householders may object and obstruct penetration of fog            pump and insecticide pump. Air from the “roots type air blower”
     into houses by closing windows and doors;                        is delivered into the combustion chamber. There it is mixed with
●● Fire risk from machinery operating at very high                    gasoline vapour and ignited so that temperatures reach 426-
     temperatures with flam­ able solvents; and
                             m                                        648°C. The diluted insecticide liquid is pumped via a simple flow
●● Can cause traffic hazards in urban areas.                          delivery valve and injected into a cup in the fog head or directly


                                                                • 917 •
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
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Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
Human resources section_8-textbook_on_public_health_and_community_medicine
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Human resources section_8-textbook_on_public_health_and_community_medicine

  • 2. Section 8 : Entomology in Public Health Practice 154 Introduction to Entomology Rina Tilak 903 155 Principles of Vector Control Rina Tilak 906 156 Housefly Rina Tilak 927 157 Mosquitoes Rina Tilak 931 158 Fleas Rina Tilak 940 159 Human Lice Rina Tilak 944 160 Sand Flies Rina Tilak 946 161 Some Annoying Pests Rina Tilak 948 162 Envenomizing Pests Rina Tilak 952 163 Ticks and Mites Rina Tilak 955 164 Rodents Rina Tilak 960 166 Snakes Rina Tilak 966
  • 3. (iv) Order Hemiptera : It contains the bedbugs and some other 154 Introduction to Entomology species like kissing or ‘assassin’ bugs, which act as vectors of ‘Chagas’ disease. Rina Tilak (b) Class Arachnida : It includes arthropods like ticks, mites, spiders and scorpions. The class is characterized by the presence of eight legs, body divided into two parts viz. cephalothorax The word ‘Entomology’ is derived from the Greek words (head and thorax are fused together) and abdomen and ‘ENTOMON’ meaning an insect and ‘LOGOS’ meaning science, absence of antennae and wings. The cephalothorax bears six thus ideally making Entomology ‘the branch of science which pairs of appendages, the first two pairs function as mouthparts deals with the study of insects’; however, the scope of the (chelicerae and pedipalps) and last four pairs as walking legs. subject has been broadened to include study of all Arthropods. Phylum Arthropoda constitutes all invertebrates with jointed (c) Class Crustacea : It includes lobsters, crabs, water fleas appendages and presence of chitinous exoskeleton besides and cyclops; some species of these are intermediate hosts of other features. The word Arthropoda is derived from two words certain human helminths, e.g. Cyclops as intermediate host of ‘ARTHRON’ meaning jointed and ‘PODA which means legs or ’, guinea worm infestation. appendages. The Phylum has many important classes of which (d) Class Myriapoda : It includes centipedes and millipedes. Class Insecta is the largest, constituting more than four million Modes of Disease Transmission insect species. The other important classes are Arachnida, Crustacea and Myriapoda. Arthropods transmit diseases to man through specialized modes of disease transmission. The modes of disease transmission by Vector borne diseases are one of the leading causes of morbidity arthropods can be classified as under : and mortality the world over and pose a major public health challenge especially to the third world or developing countries. Direct Contact : When two hosts are in direct contact with One of the important measures to combat these diseases is other, the arthropod vector itself gets transferred from one host through control of vectors. To ensure effective vector control, to the other, e.g. pediculosis and scabies. the knowledge about their lifecycle, habits, habitat and Mechanical transmission : In this mode of transmission, diseases transmitted is essential. The chapters in this section the disease causing organism is transmitted on the outside will guide the reader on these aspects so that sound vector or inside the bodies of arthropods without undergoing any control strategies can be formulated, wherever and whenever, development, propagation or any changes in the pathogenicity vector control is desired. e.g. diseases transmitted by houseflies - diarrhoea, dysentery, Classification of Arthropods cholera, hepatitis A & E etc. Diversity of structures amongst arthropods necessitates the Biological Transmission : The disease causing organism sub-division of the phylum Arthropoda into a number of undergoes certain biological changes inside the body of the classes, orders, families, genera and species. The following vector. Depending on the type of biological changes, biological classes include species of medical importance. transmission has been further classified as : (a) Class Insecta : It comprises about 70% of all the known Cyclo-developmental : In this mode of transmission, the species in the animal kingdom. The insects are characterized disease causing organism undergoes a part of its cycle in the by the presence of six legs, body divided into head, thorax and vector and simply develops or grows inside the body of the abdomen and presence of antennae besides other features. vector, e.g. Wuchereria bancrofti (Filariasis) transmitted by The head bears the mouthparts, eyes in the form of compound Culex female. or simple eyes or at times may have no eyes and a pair of Cyclo-developmental-propagative or Cyclo-propagative : antennae. The thorax is subdivided into three segments with a In this mode of transmission, the disease causing organism pair of legs in each called the pro, meso and metathoracic legs. undergoes a part of its life cycle in the vector and also grows/ The class is further subdivided into 29 orders of which only 4 develops and multiplies inside the body of the vector. Example contain species of medical importance. is Plasmodium sp (Malaria) transmitted by Anopheles female. (i) Order Diptera : It possesses one pair of wings and a pair Propagative : In this transmission mode, the disease organism of ‘halters’, which are vestigial wings. This is the largest order simply grows and multiplies in the body of the vector, e.g. comprising about seventy thousand described species with Yersinia pestis (Plague) transmitted by rat flea and all world wide distribution. It contains such insects of medical bacterial, viral, spirochaetal or rickettsial diseases transmitted importance as mosquitoes, sandflies, simulium flies and by arthropods. house-flies. Other Specialized Modes of Disease Transmission : These (ii) Order Anoplura : It comprises true or sucking lice, which specialized modes of disease transmission are generally are parasitic on mammals and the chewing lice, which infest encountered in ticks and mites. birds as well as mammals. Lice of medical importance to man Trans-ovarian : The disease organism is transmitted to eggs are human lice. through ovary of infected female, e.g. Orientia tsutsugamushi, (iii) Order Siphonaptera : It contains many species of fleas the agent for Scrub typhus transmitted by trombiculid mite- including the rat flea, which is a vector of plague and endemic Leptotrombidium deliense. typhus. • 903 •
  • 4. Table - 1 : Important arthropod borne diseases Disease Vector Causal organism Reservoir I Mosquito borne diseases Malaria Anopheles species Plasmodium species Man Filariasis Culex quinquefasciatus W bancrofti (nocturnal, periodic) Man Aedes niveus group W. bancrofti (diurnal sub-periodic) Man Mansonoides species Brugia malayi Man, Primate Chikungunya Aedes species Arbovirus group A Man Dengue fever & DHF Aedes species Arbovirus group B Man Yellow fever Aedes species Arbovirus group B Man/ Monkeys Japanese Encephalitis Culex vishnui group Arbovirus group B Mammals/ Birds (C tritaeniorhynchus) II Sandfly borne diseases Leishmaniasis Visceral (Kala azar) Phlebotomus argentipes Leishmania donovani Man/Mammals Cutaneous (Oriental sore) P. papatasi L. tropica Man/Mammals Espundia P. sergenti L. braziliensis Man/Mammals Sandfly fever P. sergenti, P. papatasi Virus Man III Fly borne diseases Bacillary dysentery M domestica Shigella Man Amoebic dysentery M domestica E. histolytica Man Gastroenteritis M domestica Specific/Non specific organisms Man/animals Typhoid M domestica Salmonella typhi Man Paratyphoid M domestica Paratyphoid A&B Man Cholera M domestica Vibrio cholera Man Poliomyelitis M domestica Virus Man Viral hepatitis (Type A) M domestica HAV Man Trachoma M domestica C trachomatis Man Yaws M domestica T pertenue Man IV Flea borne diseases Plague (Bubonic) Xenopsylla species Yersinia pestis Rodents Endemic/Murine Typhus Xenopsylla species R. typhi Rodents/ Domestic animal Chiggerosis (Jigger) - - Tunga penetrans (chigoe) Dogs, cats, wild Dipylidium caninum Ctenocephalides. felis/canis Dipylidium caninum Hymenolepis diminuta Hymenolepis diminuta Carnivores X cheopis / N fasciatus H nana X cheopis / C canis/ Pulex irritans H nana Rats, mice V Louse borne diseases Epidemic typhus Pediculus humanus R. prowazeki Man Epidemic relapsing fever Pediculus humanus Borrelia recurrentis Man Trench fever Pediculus humanus Bartonella quintana Man/animals Dermatitis Pediculus humanus/ capitis Secondary organisms Man VI Tick borne diseases Kyasanur Forest Disease (KFD) Hard ticks species Arbovirus group B Monkeys/Birds Tick typhus Hard ticks species R conorii Dogs Tularaemia Hard ticks species P tularensis Rabbits/ Rodents/ cattle Relapsing fever Soft Tick B duttoni Rats VII Mite borne diseases Scrub typhus L. deliense Orientia tsutsugamushi Rodents Rickettsial pox Allodermanyssus sanguineus1 R akari Rodents Scabies S scabei - Man VIII Cyclops transmitted diseases Dracontiasis Cyclops species D medinensis Man Fish tape worm Cyclops species D. latum Fish IX Reduviid bugs Chagas disease Reduviid/Cone-nosed T cruzi Domestic animals/ man X Tsetse flies Trypanosomiasis Glossina species T gambiense and T rhodesiense Wild animals/ cattle/ man • 904 •
  • 5. Trans-stadial : The disease causing organism is transmitted Study Exercises from one stage to another e.g. Tick typhus organism - Rickettsia Short Notes : Modes of disease transmission. conorii transmitted from infected larva to nymph to adult. MCQs & Exercises Arthropod Borne Diseases 1) Class Insecta is characterised by presence of how many Arthropods are responsible for transmission of innumerable legs? : (a) 4 (b) 5 (c) 6 (d) 8 diseases. Some of the important arthropod borne diseases is 2) Mosquito belongs to the order a) Diptera (b) Anoplura listed in Table - 1 along with their vectors, causative organisms (c) Siphonoptera (d) Hemiptera. and reservoir hosts. 3) Mode of transmission of malaria by Anopheles is : (a) Cyclo - developmental (b) Cyclo - propogative Summary (c) Propagative (d) Cyclo - developmental propagative Entomology is the branch of science which deals with the 4) Orientia tsugtsugamushi is the causal organism for the study of insects, however, the scope of the subject has been diseases (a) Scrub typhus (b) Epidemic typhus (c) Endemic broadened to include the study of all Arthropods. Amongst typhus (d) Tick typhus. the many important classes of the phylum, Insecta is the 5) Which of the following is not a mosquito borne disease : largest constituting more than 4 million species. Vector borne (a) Dengue (b) Filaria (c) Leishmaniasis (d) Yellow fever diseases are one of the leading causes of morbidity & mortality 6) Aedes species cannot transmit : (a) Chickungunya especially in the developing countries. These diseases can (b) Dengue haemorrhagic fever (c) Japanese Encephalitis be largely combated by effective vector control for which a (d) Yellow fever sound knowledge of the bionomics of the vector is needed. The 7) Causal organism for bubonic plague : (a) R typhi important classes of Phylum Arthropoda are Insecta, Arachnida, (b) Y pestis (c) H nana (d) H diminuta Crustacea, Myriapoda. The class Insecta is characterized by 8) Murine Typhus is transmitted by : (a) Hard tick species presence of six legs, body divided into head, thorax & abdomen. (b) Soft tick species (c) Xenopsylla species (d) Trombiculid The class has 4 orders of medical importance, Diptera, Anoplura, Mite species Siphonaptera & Hemiptera. The order Diptera has 1 pair of 9) Causal organism for epidemic Typhus : (a) B quintana wings and contains insects such as mosquitoes, sandflies, (b) R prowazeki (c) Borrelia recurrentis (d) R typhi simulium flies and houseflies. The order Anoplura comprises 10) KFD is transmitted by : (a) Soft Tick (b) Mite (c) Hard Tick true or sucking lice. Order siphonoptera includes the rat flea; (d) Lice the order hemiptera contains bed bugs. The class Arachnida 11) ___________ is the largest class of Phylum Arthropoda includes ticks, mites, spiders and scorpions; body is divided constituting more than __________ insect species into cephalothorax and abdomen and presence of eight legs. 12) Direct mode of transmission is seen in _______ & ______ The class crustacea includes lobsters, crabs, water fleas and 13) Entomology is derived from the Greek word ENTOMON Cyclops. Myriapoda includes millipedes and centipedes. meaning _________ & LOGOS meaning __________ Arthropods transmit diseases to man by different modes of 14) Arthropoda is derived from two words ARTHRON meaning transmission, mainly direct, mechanical & biological. In direct __________ & PODA meaning __________ transmission, two hosts come in direct contact with each other 15) Leptotrombidium deliense is vector for __________ e.g. pediculosis & scabies, mechanical transmission where Answers : (1) c; (2) a; (3) b; (4) a; (5) c; (6) c; (7) b; (8) c; disease causing organism is transmitted without undergoing (9) b; (10) c; (11) Insecta ; 4 Million; (12) Pediculosis; any change in pathogenecity or development e.g. diseases Scabies; (13) Insect, Science; (14) Jointed, Appendages; transmitted by houseflies. Biological transmission is further (15) Scrub Typhus. divided into cyclodevelopmental - the disease causing organism undergoes part of its life cycle in the vector e.g. Wuchereria Further Suggested Reading bancrofti; cyclopropogative - the disease causing agent 1. Gordon C cook, Alimuddin Zumla, Manson’s Tropical diseases, 21st Ed, 2003, Chapter 50, Rickettsial infections, p 900. undergoes part of the lifecycle in the vector & also multiplies 2. Service Mike. Medical Entomology for students. 3rd ed. Cambridge : inside the vector e.g. malaria transmitted by Anopheles female. Cambridge University Press, 2004. The other mode is propogative where disease causing organism 3. Kettle DS. Medical and Veterinary Entomology. 2nd ed. CAB International, 1995. simply grows & multiplies in the body of vector e.g. Yersinia 4. Hati AK. Medical Entomology. Allied Book Agency, Kolkata, 2001. pestis (plague) transmitted by rat flea, other specialized modes are transstadial & transovarian. • 905 •
  • 6. stream flushing and regulation of the water level in reservoirs, 155 Principles of Vector Control vegetation removal, shading and exposure to sunlight. (c) Modification or Manipulation of Human Habitation or Rina Tilak Behaviour : This means “A form of environmental management that reduces man-vector - pathogen contact”. Examples of this approach are siting of settlements away from vector sources, Control of arthropods is one of the key strategies in the mosquito/rodent proofing, personal protection and hygienic management of vector borne diseases. A strategist should measures against vectors and provision of mechanical barriers, have sound knowledge of the bionomics, distribution, seasonal providing facilities for water supply, disposal of waste water prevalence, vectorial capacity, insecticide susceptibility status and excreta, laundry, bathing and recreation to prevent or and role of arthropods in diseases transmission coupled with discourage human contact with infested water. the knowledge of identification features of the incriminated vectors for formulating effective control strategies. Once armed Chemical Control with this knowledge, the choice of effective vector management The new era of control of vector borne disease began with the tools may be exercised; the range and sophistication of control discovery of the insecticidal value of Dichloro Diphenyl Trichloro methods is impressive. The various control options available ethane (DDT). DDT was first synthesized by Othmar Zeidler in are as follows : 1874 at Strasbourg, Germany. In 1939, Paul Muller of the Geigy ●● Environmental Control Company in Basle, Switzerland, discovered its remarkably long ●● Chemical Control residual insecticidal property, earning him the Nobel Prize in ●● Biological Control Medicine. The availability of several effective, safe and low ●● Personal Protective measures cost pesticides, coupled with improvements in the techniques ●● Mechanical control of their application, made it possible for many governments in ●● Physical control the developed as well as developing countries to embark upon ●● Genetic control extensive countrywide programmes for the control or eradication ●● Legislative control of vector borne diseases. However, development of resistance Environmental Control amongst vectors to insecticides has necessitated reassessment of the place of pesticides in vector control programmes. Besides The important environmental control measures which are the technical and financial difficulties, there is a growing increasingly being used are described below : concern about the environmental contamination resulting from Environmental Management the persistent use of insecticides. This has been defined as “The planning, organization, carrying Classification of Insecticides out and monitoring of activities for the modification and/or Pesticides may be classified in many ways based on mode of manipulation of environmental factors or their interaction with entry, target stage, chemical composition and mode of action. man with a view to prevent or minimize vector propagation and However, the most common classification used is based on reducing man-vector-pathogen contact.” This is a naturalistic chemical composition. According to this classification, the approach which attempts to extend and intensify natural insecticides are classified in the following categories as factors which limit vector breeding, survival and contact with presented in Fig. - 1. man. (a) Environmental Modification : It is defined as “A form of environmental management consisting of any physical transformation that is permanent or long- lasting of land, water and vegetation aimed at preventing, eliminating or Fig. - 1 : Classification of insecticides as per chemical composition reducing the habitats of vectors without Insecticides causing unduly adverse effects on the quality of the human environment”. Natural Synthetic Environmental modification includes drainage, filling, velocity alteration, Plant Based Inorganic Organic land levelling and transformation of Pyrethrum Mineral Oils Paris Green impoundment margins. Azadirachtin (b) Environmental Manipulation : It is defined as “A form of environmental Organophosphates Synthetic management consisting of any planned Malathion Pyrethroids Carbamates recurrent activity aimed at producing Temephos Deltamethrin Organochlorines Propoxur temporary conditions unfavourable to Fenthion Cyfluthrin DDT Bendiocarb the breeding of vectors in their habitats”. Dichlorvos Bifenthrin Carbaryl Examples of environmental manipulation Fenitrothion Lambda-Cyhalothrin activities are water salinity changes, Pirimiphos methyl Permethrin • 906 •
  • 7. Natural Insecticides Chapter on Mosquitoes for further details on use). As per Govt. Plant based of India Gazette notification number S.O. 378(E) dt 26th May 1989, the use of DDT in Agriculture has been withdrawn and Pyrethrum : Pyrethrum extract is obtained from the dried restricted to 10000 MT/ annum for Public Health programme heads of the flower Chrysanthemum cinerariafolium and except in case of North East as Insecticidal Residual Spray contains the active ingredients pyrethrins I and II, constituting (IRS). 1 to 2% of the total weight of the raw pyrethrum. Pyrethrum is characterized by rapid knockdown action on arthropods even Organophosphorus Compounds : These insecticides are when used in very low dilution. It is very unstable in light and derivatives of phosphoric acid and act by inhibiting the air and has practically no residual effect. This makes repeated activity of cholinesterase. Many of the insects, which have applications necessary. Pyrethrum is available as 2% extract, become resistant to Organochlorines are still susceptible to which needs 20 times dilution to make it 0.1% solution, which the members of this group. However, due to their extensive is actually used for spraying. Using a 0.4 mm or lower calibre use in agricultural as well as public health field, more and nozzle, 50 to 100 ml of pyrethrum solution in kerosene oil is more insects are developing resistance to Organophosphorus sprayed per 100 m3 of space. Addition of an Organophosphorus compounds. Some of the common compounds are Malathion, insecticide to pyrethrum formulation is a common commercial Temephos, Fenthion, Dichlorovos (DDVP) and Fenitrothion. practice for obtaining a better effect. It is one of the main Malathion : It is one of the least toxic Organophosphorus insecticidal constituents in aerosol dispensers and also an compounds. Malathion is a broad spectrum insecticide, with insecticide of choice for ULV sprays. Pyrethrum is perhaps the efficacy against a large number of pests ranging from mosquitoes, most acceptable insecticide for use in cook houses, dining halls houseflies, cockroaches, bedbugs, lice etc. It is available as and other food preparation areas. Malathion Technical (95%) for use as space spray, 50% Water Azadirachtin : The active ingredient Azadirachtin is obtained Dispersible Powder (WDP) and Emulsifiable Concentrate (EC) from the seed kernels of neem plant Azadirachta indica. for residual control and 90% dust for use against fleas and lice. Azadirachtin has insecticidal, fungicidal, bactericidal, viricidal Malathion under the National Vector Borne Diseases Control properties including insect growth regulating qualities besides Programme of India is being used as Indoor Residual Spray deterrent, anti-ovipositional, anti-feedant, fecundity and against mosquitoes in areas where the vectors have become fitness-reducing properties on insects. It has been variously resistant to DDT. The dosage of its application is 2 g/m2 (Refer formulated for mosquito larval and adult control in the form to Chapter on Mosquitoes for further details). As ULV spray it of liquid and cream formulations. Neem products contain up has been very widely used during outbreaks of Dengue and JE to 3% Azadiractin. as an anti adult mosquito measure. However, development of resistance has been reported in a large number of vectors to Mineral Oils : Kerosene oil, diesel oil, petrol and crude engine Malathion. oil have been successfully used as mosquito larvicides. The oil film cuts off the air supply, enters and blocks the trachea, Temephos : It is available as 50% EC. It is the only insecticide may act as a stomach poison, and also lowers surface tension approved for use in potable water. Because of its low toxicity, which prevents larvae from floating. Malariol is the best and it has been successfully used for the control of Anopheles easiest larvicidal oil to use for Anopheline and Culicine larvae. stephensi breeding in wells and domestic containers at a Malariol is used as it is @ 10 litres per 500 linear meters. dosage of 1ppm (Refer to Chapter on Mosquitoes for details). Sand impregnated with Temephos in 1% concentration has been Synthetic Insecticides : These can be organic or inorganic. used in some countries against Aedes aegypti which breeds in The only inorganic compound used in vector control (mosquito containers of clean and potable water. It has proved to be very larvicide) was Paris green (Copper-aceto-arsenite). It acted as successful in Guinea worm eradication programme in India. stomach poison when ingested by mosquito larvae. However, it is not used any more for mosquito control. Fenthion : It is formulated as 82.5% EC and as granules containing 2% toxicant. It is a good mosquito larvicide but can The Organic Insecticides fall into four major groups viz. not be used in potable water bodies. It is highly effective as a Organochlorines, Organophosphates, Carbamates and Synthetic larvicide against Culex quinquefasciatus or any other vector pyrethroids. found breeding in non potable water bodies at a dosage of 1 Organochlorine Compounds : These compounds are contact ppm (Refer to Chapter on Mosquitoes for further details). It poisons and act on the nervous system. These or their can also be used for housefly control as a larvicide (Refer to degradation products are more or less stored in body fat and Chapter on Houseflies for details). may be demonstrated in milk, urine or sweat. They all have Pirimiphos methyl : This insecticide is being considered as an variable residual action for variable periods and are toxic to alternative insecticide for Indoor residual spray. It is available man and animals. The most important and the only member of as 25% WP; 2 kg is mixed in 10 litres of water and sprayed @ 10 this group used in Public Health is DDT. litres/ 250 sq m area to give a deposit of 2g/sqm. Three rounds Dichloro-diphenyl-trichloroethane (DDT) : Currently DDT is of spray are recommended as is followed in case of Malathion. being used for indoor residual spray in the North Eastern states Dichloro-dimethyl-dichlorvinyl-phosphate (DDVP or of India. A deposit of 1 g of DDT/m2 of surface area of walls Dichlorvos) : It differs from other organophosphorus and ceiling upto a height of 3.5 m in all dwellings applied at 8 compounds in that it possesses a much greater vapour pressure weeks’ interval, effectively controls majority of the mosquitoes at ordinary temperature which produces fatal insecticidal and also other arthropods resting on the treated wall (Refer to • 907 •
  • 8. vapour. It is available as 72.6% EC. It can be combined with solid are available as Pretreated or Long Lasting Nets (LLNs). substances like wax and used as tablets or bricks thus allowing Other Synthetic pyrethroids used in Public health : There it to evaporate slowly. It is one of the common insecticides used is a large range of molecules used in the field of Public health for disinsecting aircraft. It is an effective housefly larvicide. besides the ones listed above. These molecules are Allethrin, Fenitrothion : It is available as Fenitrothion 40% water Resmethrin, Phenothrin, Cypermethrin, Imiprothrin, Bifenthrin, dispersible powder (WDP). The insecticide has shown promise Cyhalothrin, Cyphenothrin etc. These are all available as WP EC , as an effective insecticide for control of bedbugs; however or Aerosol formulations for use against pests like cockroaches, toxicity constraints have limited its widespread use. houseflies and mosquitoes. Carbamates : These compounds are derivatives of carbamic Newer Group of Insecticides acid and resemble Organophosphorus compounds in their Phenyl pyrazoles : Fipronil is the only member of this class of mode of action. Some of the preparations produce a rapid insecticide. Fipronil acts by antagonizing the effect of GABA. knockdown effect like that of pyrethrum. The inhibition of It is available as 0.3% Gel for use against cockroach as a crack Acetylcholine esterase is reversible with Carbamates and hence and crevice treatment. It is a systemic material with contact these compounds are less toxic. Some of the compounds in and stomach activity. It has a unique action called ‘cascade common use are Propoxur, Carbaryl and Bendiocarb. effect’ which is evident due to necrophagy seen in cockroaches. Propoxur : It is formulated as WDP as well as EC. It is considered When cockroaches consume the insecticide bait, they are killed; as the least toxic Carbamate compound for man and domestic these dead cockroaches when consumed by other cockroaches animals. It has a flushing out effect and therefore is commonly bring about the death of these cockroaches and this goes on for used for cockroach and bedbug control. It is also used in bait about two months or so, thus obviating the need to retreat the formulations against houseflies and cockroaches. area at lesser intervals. Bendiocarb : Bendiocarb is an alternative insecticide for Indoor Neo Nicotinoids : Imidacloprid is the sole member from this residual Spraying. It is available as 80% WP For indoor residual . class. It acts by causing irreversible blockage of postsynaptic spraying, it is recommended @ 200 mg/sqm. Two rounds of acetylcholine receptors. Imidacloprid is a systemic insecticide, spray are recommended for effective control against malaria. having notable contact and stomach action. Imidacloprid is Synthetic Pyrethroids : These are synthetic derivatives or available as 2.15% Gel for use against cockroaches and as Bait analogues of natural Pyrethrum. These are broad spectrum, for use against houseflies, where it is formulated with housefly highly potent with quick knock down action and long residual pheromone - Muscalure. life. Synthetic pyrethroids are many times more effective than Biorational Insecticides : ‘Biorational’ means any substance the previously available insecticides. Their relative safety of natural origin that has a detrimental or lethal effect on to man and higher animals, their efficient biodegradability specific target pest, e.g. insects. These insecticides are non- together with their higher target specific toxicity makes toxic to man, plants and animals and have little or no adverse them very attractive materials for integrated vector control. effects on the environment. An overview of the biorational The commonly available products are Permethrin, Allethrin, insecticides is presented in Fig. - 2. Phenothrin, Cypermethrin, Cyfluthrin, Deltamethrin and Bifenthrin. The Synthetic pyrethroids are formulated as WDP , Fig. - 2 : Biorational Insecticides Used in Vector Control EC, SC, Flow, EW and ULV formulations. Being broad spectrum, these insecticides are being used for vector control as residual Biorational Insecticides spray, space spray and topical application as well as for treatment of clothing. Pheromones Biocides Deltamethrin : It is one of the most widely used Synthetic Insect Growth pyrethroid molecule in the field of vector control. It is available  Muscalure  Bacillus Regulators in many formulations for various vector control strategies viz.  Oviposition attractant thuringiensis SC 2.5% (Flow) formulation for treatment of bednet and routine var israelensis household pest control activity; 2.5% WP formulation for Indoor  Bacillus Residual spray in Malathion resistant areas and 1.25 ULV for sphaericus space spraying. The target dose (for Indoor Residual Spray) is Chitin Synthesis Juvenile Hormone generally 20 mg of a.i. (active ingredient) per sq m of surface Inhibitors Mimics area.  Diflubenzuron  Methoprene  Lufenuron  Pyriproxyfen Cyfluthrin : Besides Deltamethrin, this is the next most widely  Novaluron  Fenoxycarb used molecule. It is available as 0.5% EW formulation for  Triflumuron treatment of bednets; 5% EC for household use and 10% WP for use as indoor residual spray in Malathion resistant areas. Permethrin : Widely used for control of lice, scabies and for Insect Growth Regulators : A new approach to vector control treatment of clothing and bednets. The product is formulated in is the use of substances that adversely affect insect growth varying concentration as Shampoo formulation for use as anti- and development. The enzymes and hormones that regulate lice treatment and 5% cream for use in scabies treatment. Bed developmental processes within an insect’s body can sometimes nets treated with Permethrin at the manufacturing stage itself be exploited as chemical control weapons. These compounds, • 908 •
  • 9. often known as Insect Growth Regulators (IGRs) can be used houseflies) and oviposition attractant (Aedes control) have to stimulate development at inappropriate times or inhibit it been exploited in the field of vector management. at other times. They are quite selective in their mode of action Biocides : The development of insecticide resistance amongst and potentially act only on target species. Most of the IGRs that the major pests and vectors coupled with the non target toxicity, have shown effectiveness against insect pests, cause the rapid necessitated development of safer alternatives to insecticides. death of the insect through failure of a key regulatory process This led to the screening, promotion and use of a large number to operate or function. IGRs generally control insects either of biorational products of which biocides are one of the most through inhibition of chitin synthesis or interference with important control options. The two biocides used in the field metamorphosis by mimicking the action of juvenile hormone. of vector control are Bacillus thuringiensis var israelensis and The major groups of IGR compounds include : Bacillus sphaericus. Both these products are widely used as Chitin Synthesis Inhibitors : These chemicals inhibit the larvicides in Mosquito control programmes and act as stomach moulting process by blocking the activity of chitin synthetase, poison. an enzyme needed by epidermal cells when constructing Bacillus thuringiensis var israelensis (Bti) : It was discovered a new exoskeleton. Because of this mode of action, Chitin in 1976 and has been found to be effective as mosquito larvicide. Synthesis Inhibitors (CSIs) are highly specific to arthropods. It is a gram positive spore forming bacteria. Bti produces toxins They act rather slowly (2-5 days), but eventually disrupt any which are present in parasporal body called the ‘protein crystal’. process that involves construction of new cuticle (e.g., molting, It primarily kills by the action of delta-endotoxin. When the hatching, pupation). They are most effective when used against mosquito larva ingests the protein crystal (inactive protoxin), the immature stages of a vector. Diflubenzuron, is used for it is activated inside its midgut by the action of proteases into controlling mosquitoes, houseflies etc. It is available as 25% active protoxin; these bind to the cell receptors present on EC, WP & 0.5% Granules and is used @ 1.0 g/ acre of surface midgut epithelium and cause disturbance in osmoregulatory water as mosquito larvicide. Lufenuron, is a systemic CSI and mechanism which leads to swelling and eventual bursting of is especially effective for flea and tick infestation control on the epithelium and finally death of the larvae. The product is animals. Novaluron is a recent addition to the list, which has available as WP Granules, AS & Briquette. Bti 12 AS is used @ , been found effective against the mosquitoes. It is a contact 20ml/m2 and has been found to be effective up to 15 days (for larvicide and is available as 10% EC. It is used @ 20 μg a.i./ l details refer chapter on mosquito). Bti however, suffers from and the efficacy lasts up to 3 months. the disadvantage that it can not be used in polluted waters Juvenile Hormone Analogues or Mimics : Juvenile hormone or where particulate matter is more; it also cannot recycle in analogues or Juvenile Hormone Mimics (JHM’s) act by inhibiting nature. It is used in non potable water bodies. the developmental changes associated with embryogenesis, Bacillus sphaericus : A naturally occurring bacterium used morphogenesis, and reproduction. During normal development, against mosquito larvae. It is more effective in polluted water JH levels are elevated in larvae (or nymphs) and decrease and can recycle and persist in nature. It is available in various prior to pupation (or adult eclosion). Contact exposure to JH formulations like Bti viz. pellets, briquettes, granules & WP It . analogues during the egg stage or after the last larval molt is used @ 20ml/m2 and has been found to be effective up to can inhibit development, delay maturation, and eventually three weeks. result in death. Since the onset of mortality is usually quite Fumigants : Some of the fumigants used as pesticides are slow (days to weeks), these chemicals are not used during carbon tetrachloride, methyl bromide, ethylene dibromide, epidemics; however, these chemicals are much in demand for chloropicrin, carbon disulphide and dichlorvos (DDVP). routine vector control due to their specificity and safety to non-target organisms. Several compounds (e.g., Methoprene, Application Techniques Pyriproxyfen, Fenoxycarb, Triflumuron) have been successfully Control of arthropods in different habitats necessitates the use incorporated into vector management programmes especially of different types of spraying equipment as well as a variety of Dengue and Malaria and in products used for controlling ants, formulations such as liquids, granules and dusts. For example fleas, and other household pests. Pyriproxyfen can cause control may involve treatment of small domestic or peridomestic sterilization and inhibition of growth of adult insects; it has a water collections which are ideal breeding places for Aedes residual effect up to 3 to 6 month indoors and 30 days outdoors. mosquitoes; applications to stagnant waters in cesspools, It is widely used against mosquitoes @ 2gm a.i./ sq m. ditches and drains where Culex mosquitoes breed, large bodies Pheromones : Pheromones are semiochemicals (chemicals of standing water where certain Anopheles mosquito species which mediate interactions between organisms) secreted by may be breeding; or aerosol spraying of extensive areas to halt an organism which provokes specific reaction in receiving epidemics. To meet with diverse situations, significant progress organisms of the same species. These chemicals may further has been made in improving the spraying equipment. The Ultra be classified based on the type of interaction mediated e.g. sex Low Volume (ULV) equipment for ground and aerial spray to pheromone (muscalure secreted by houseflies), oviposition control mosquitoes and other haematophagous arthropods has attractant (mosquito larvae), aggregation pheromone resulted in not only the elimination of several impediments (cockroach, bedbugs), alarm pheromones, trail pheromones like frequent mixing and reloading but helped in increasing etc. The scope of pheromones in vector control is promising, the speed of application and reducing the dosages and costs. It however so far only two pheromones viz. Muscalure (in is specially recommended for control of an outbreak of vector combination with Imidacloprid insecticide as baits against borne disease. • 909 •
  • 10. Formulations from 1 to 5%. These can be used in irrigation channels, irrigated or flooded lands, paddy fields and particularly where Manufacturers combine pesticides with other materials there is vegetation on the water surface. After sinking, these to make usable concentrations called formulations. These formulations disintegrate slowly releasing small particles of formulations are designed to kill insects readily without insecticides. These can be effectively used also in small water causing undue hazards to non-target organisms when diluted collections such as ornamental tanks and earthen pots, tree and applied correctly. Factors influencing application and holes and other domestic or peridomestic breeding places of efficacy of an insecticide are its toxicity, size and shape of Aedes mosquitoes. its particles, concentration in formulation; type of solvent used, type of surface to be sprayed, atmospheric temperature Wettable Powder : This formulation consists of the technical and humidity, type of sprayer and its nozzle, training of the grade pesticide, an inert carrier and a wetting agent (usually a spraying personnel, the bionomics, morphology and physiology synthetic detergent) that helps it mix with water. These usually of the particular arthropod. No single preparation can meet the contain 50 to 75% of the toxicant. Most of these can be put requirement of vector control in all spheres of human ecology. directly into water and require only slight agitation to make Solutions, emulsions, suspensions, water dispersible powders, suspension; others may require mixing with a small amount of dusts and granules to suit different conditions and problems water to form a paste or slurry. The required volume of water are therefore, prepared and used. is then added to paste or slurry followed by thorough agitation of the mixture. Technical Grade Pesticide When water is added to a wettable powder it makes a suspension; This is the basic toxic agent in its purest commercial form. this enables the pesticide to stay on porous surfaces like Some technical grade pesticides are liquids; others occur in concrete, plaster or unpainted wood. Water penetrates these solid form. Technical grade Malathion is used in ultra low surfaces, leaving the carrier and the maximum amount of the volume space applications. pesticide on the surface available to kill pests. Suspensions have Types of Formulations other advantages, too. They have no solvent odour, and they Formulations essentially are of three types : Solid or dry, liquid don’t tend to irritate or penetrate skin. However, they generally and gaseous formulations. need agitation to keep pesticidal particles from settling out. Solid or Dry formulations Also, they tend to clog sprayer nozzles and strainers, especially when the wettable powder is stored for long periods in humid Dusts : Dusts are normally ready-to-use formulations with a areas or when a high concentration is used. low percentage of active ingredient (usually 1 - 10%) plus a Liquid formulations very fine inert carrier such as talc, chalk, diatomaceous earth, clay or volcanic ash. These materials are usually low in cost, Emulsifiable Concentrates : Emulsifiable concentrates consist easy to apply, non-staining and non-toxic to vegetation. Dusts of the technical grade pesticide (typically 45% to 75%), a solvent, are always used dry and can easily drift into non-target areas and an emulsifying agent, usually a synthetic detergent. This if they are not applied carefully. For this reason, outdoor agent is used to allow the concentrate to be diluted in water, applications should be made only when the wind is calm. A resulting in an emulsion. common use for dusts is in crack and crevice or spot treatments Emulsifiable concentrates are usually clear but emulsions look indoors in out-of-sight areas (behind equipment, in wall voids similar to milk. Finished sprays are emulsions or solutions and so on) which remain dry. The residual pesticidal activity diluted to field strength. Unlike solutions, most emulsions of dust is normally fairly long, provided the dust stays dry, but need a little periodic agitation to keep the concentrate from quickly loses it toxicity in the presence of moisture. They don’t separating out of the water. Emulsions are used for residual adhere well to vertical surfaces. treatments. Pests that contact these surfaces are killed by Dusts are used on people during mass delousing operations to the pesticidal residue. Emulsions may damage aluminium, control outbreaks of lice borne diseases. Dusts are also used varnish, and painted surfaces due to the action of solvents such for flea control during plague outbreak. Dusts aren’t generally as Xylene. Emulsions may also be corrosive to metal sprayers absorbed through the skin, but may be dangerous if inhaled and their fittings and hence sprayers made of stainless steel, into the respiratory tract. aluminium or other non-corrosive materials should be used. Granules : These are basically the same as dust formulation, Oil Solutions : These formulations consist of a technical grade except the carrier particles are larger and thus don’t stick to pesticide dissolved in a solvent such as kerosene or diesel leaves allowing penetration in dense foliage. This is a real oil. Solutions are available as ready-to-use formulations (for advantage when the pesticide must reach the water surface for example ordinary household fly and mosquito sprays with a mosquito control in vegetated swamps, or if it must get to the low percentage of pesticide) and as solution concentrates. These ground surface through trees and shrubs for chigger control. concentrates contain a high percentage of insecticide and must Granules are also available in timed-release formulations that ordinarily be diluted in oil or another suitable solvent. Some release a dosage of the pesticide over an extended period of concentrates are used without dilution in Ultra Low Volume time. Other advantages of using granules are that they provide (ULV) applications. Oil solutions applied as finished sprays longer lasting effects and their use results in less drift than often kill insects on contact, since the oil helps the pesticide generally occurs with liquids or dusts. penetrate the insect’s waxy body wall. The percentage of insecticide in granules and pellets varies Ultra-Low Volume (ULV) : While most items of ULV pesticide • 910 •
  • 11. dispersal equipment use the readily available solutions or Ticks depending on the formulation. Care should be taken technical grade formulations, there are special ULV formulations while handling aerosol cans since they can explode if punctured available for e.g. Deltamethrin 1.25 ULV etc. or overheated, even after the pesticide has been dispensed. Gaseous formulations : Gases are primarily used in Common insecticides used as aerosols are Pyrethroids, fumigation operations. They may be prepared as liquefied Malathion, DDVP and repellents like DEET and DEPA. These gases and packaged in pressure containers or in a material are used for disinfesting aircrafts, tents, rooms, other small form that reacts with the moisture in the air to form a gas. enclosures, uniforms and for topical application. An emission The gas molecules can penetrate cracks, crevices and tightly of nearly 15 seconds is enough for a 100 m3 space. packed material. Gases are the most dangerous pesticides used Equipment and hence special safety equipment and training are necessary when using gases and must never be attempted except by Equipment used for vector control can be broadly classified as trained pest management personnel operating in pairs. One ground equipment and equipment used for aerial applications. of the common gaseous formulations viz. Calcium cyanide (a) Ground Equipment (powder) and Aluminium phosphide (tablet) are used for rodent (i) Sprays for production of fine or coarse spray which may be control. either manually operated or power operated. Special formulations (ii) Sprayers for the production of mist which may be either Resin Strips : Pesticide-impregnated resin strips release with gaseous energy nozzles (manual operated or power vapours as they are heated or exposed to normal room operated) or with centrifugal nozzles. temperatures. The use of resin strips in rooms occupied by (iii) Devises for the production of aerosols which may the young, the elderly or in food preparation and food serving be mechanical, thermal or gaseous energy aerosol areas is strictly prohibited. generators. (iv) Dusting equipment which may be manually operated or Baits : Baits are commonly used to manage scavenging pests power operated. such as rodents, ants, flies, and cockroaches, which are (v) Applicators for granules and pellets, manually or power particularly difficult to manage with standard techniques. operated. Baits consist of the toxicant mixed with a food attractive to the target pest or with water. For this reason, baits made with local (b) Aerial Equipment foods are normally more effective than premixed formulations. Equipment for aerial sprays is essentially the same but with Recent development is the use of pheromone Muscalure with certain modifications. The equipment in common use is the Imidacloprid as bait for houseflies. boom and nozzle system. Gel : One of the special formulations developed for use against Sprayers cockroaches is Gel formulation. Gels comprise some food attractant mixed with the toxicant and some stabilizing agents. The equipment commonly used for spraying various insecticidal Examples are Fipronil and Imidacloprid Gels marketed against formulations are the hand operated sprayers, power operated cockroaches. sprayers, aerosol dispensers, fog generators and dusters. Shampoo : This formulation has been specially developed (a) Hand Operated for use against head lice infestation. Permethrin is the most These are hand sprayers, knapsack sprayer and compression common ingredient of the commercially available anti-lice sprayer. Shampoo formulations worldwide. (i) Hand Sprayer : The hand sprayer is used for space spraying Beads / Pellets / Briquettes : Small floating beads, pellets or of small apartments. It is provided with a small can for holding briquettes incorporating biocides - Bti and B sphaericus have ½ to 1½ litres of spray fluid and a cylindrical plunger type air been developed against Anopheline larvae. These formulations pump. The nozzle size is less than 0.4 mm in order to produce a can be made as controlled release formulations as well. fine spray. The simplest form is the familiar ‘flit gun’ producing Paints and Lacquers : These can be used for incorporation intermittent spray. A number of other light hand sprayers have of insecticides especially for control of pests on ships. These been designed, which can be pressurized in the manner of preparations remain effective for long periods. The new compression sprayers and are used to produce a mist or fine insecticide, Imidacloprid is also available as a paint formulation droplet spray. against houseflies. (ii) Knapsack Sprayer : This is designed to fit on to the back Mats / Coils : These are special formulations which have been of the operator and usually has a capacity of 15 to 20 litres. developed as controlled release formulation for indoor use It incorporates a light but powerful diaphragm pump actuated against mosquitoes. These have synthetic pyrethroids such as by a lever carried forward to the operator’s hand where it is Allethrin, which acts as toxicant to knock down the mosquitoes worked by an up-and-down movement. These sprayers are used when used indoors. both for larviciding and residual spraying. The nozzle size used for residual spraying varies between 0.78 to 1.0 mm so as to Aerosols : Aerosols are pressurized cans containing a small produce a coarse spray. amount of pesticide driven through a small nozzle. They’re commonly used as space sprays for flying insects viz. (iii) Compression Pneumatic Sprayer : This is the commonest mosquitoes and houseflies and as residual sprays for Mites/ type of equipment used in National Vector Borne Diseases Control Programme for the application of insecticides. It has • 911 •
  • 12. a hand operated pump incorporated to build up adequate (d) Wash your overalls and other protective clothing at the pressure. When the pressure is released by a trigger on the end of each working day in soap and water and keep them lance, the liquid is forced out from the tank to the nozzle by separate from the rest of the family’s clothes. the compressed air and a continuous spray of the insecticide (e) If the insecticide gets on your skin, wash off immediately formulation is produced. It is slung over the shoulder with with soap and water. one strap or may be carried on the back with two straps. It is (f) Change your clothes immediately if they become operated by one person. contaminated with insecticides. (b) Power Operated Sprayers (g) Inform your supervisor immediately, if you do not feel well. These are useful for application of insecticides over large areas. (h) Wear protective clothing (Fig. - 3) : These are hydraulic sprayers in which the spray liquid is expelled - Broad rim hat (protects head, face and neck from spray to the nozzle by positive displacement by the plunger pump. droplets). Insecticide tanks built into a truck or mounted over a hand - Goggles or face shield (protects face and eyes against trolley are connected directly to a power operated compressor. spray fall-out). By means of a long hose the spraying fluid is conveyed under - Face mask (protects nose and mouth from airborne pressure through the lance to the nozzle. particles of the spray fall-out). (c) Insecticidal Fog Generators - Long sleeved overalls (Keep overalls outside of boots). Several types of fog generators are now available for the - Rubber gloves. production of insecticidal fogs in the open on a large scale. - Boots. In these fogging machines the oily solution of the insecticide Preparations is finely atomized by the powerful blast of hot exhaust gases The Household : Inform the householder of the spraying from a petrol engine. schedule and the purpose of spraying, giving them time to (d) Aerosol Dispensers prepare and vacate the house. Occupants MUST leave houses These are used for disinfestations of aircrafts, tents, rooms before spraying. Rooms occupied by sick people who cannot and similar small enclosures. It contains insecticide and a be moved must NOT be sprayed. Remove all household items, propellant. Common aerosols contain Synthetic pyrethroids including water, food, cooking utensils and toys from the house. or their combination, which are routinely used for mosquito, Move and cover, or take out the furniture to allow easy access cockroaches and fly control. for spraying walls. Items that can not be removed should be well covered. (e) Dust Gun Equipment : Indoor residual spraying of insecticides is Insecticidal dusts are applied against lice and fleas in rat normally done using hand-operated compression sprayers. burrows or on water surfaces as dry powders diluted with inert Before starting a spray operation, the equipment must be dusts. Small light weight guns are used for mass delousing of checked. Faulty sprayers may result in poor control or over- infested people. treatment. Examine the sprayer visually to ensure that all parts Residual Spraying are present, assembled correctly and are in good condition (Fig. - 4). This is the application of insecticides to surfaces so that the insecticide particles remain on the surface in the form, size and Fig. - 3 : Protective Fig. - 4 : Hand-operated quantity suitable for insects to pick up on contact and sufficient clothing for spraying compression sprayer to exert a lethal effect over a long period. Organochlorine, Organophosphorus, Synthetic pyrethroids and Carbamate A: Hat H.Hose compounds can thus be applied on the inside walls of houses B: Goggles G.Strainer and also on thick bushes in forests. The type of surface to C: Mask D.Pump (handle) which an insecticide is applied influences its toxicity against insects and its persistence. Solutions and emulsions quickly E.Pressure gauge get soaked in the absorbent surfaces of soft bricks and mud D: Long J.Trigger walls which take in a large portion of insecticidal material Sleeved Overalls C.Lid deposited on them; but when suspended in water it remains over the surface after the water evaporates or gets absorbed. The B. A.Sprayer tank nozzles of sprayers used for residual spraying must conform to E: Rubber Shoulder the need of having a droplet size which is neither too large nor Gloves strap F.Lance too small. Similarly, safety precautions should be observed, as follows, while spraying as per standard WHO guidelines : F: Boots K. Foot rest (a) Do not eat, drink or smoke while working. I.Nozzle (b) Wash your hands and face with soap and water after Nozzle body spraying and before eating, smoking or drinking. Nozzle tip (c) Shower or bathe at the end of every day’s work and change Nozzle cap into clean clothes. • 912 •
  • 13. A. Sprayer tank (c) Operate the pump using both hands and with foot on B. Shoulder strap the footrest. Pump to the working pressure of 55 psi C. Lid (Fig. - 7). D. Pump (handle) E. Pressure gauge Fig. - 7 F. Lance G. Strainer H. Hose I. Nozzle - check correct type of nozzle is fitted and is not damaged or worn (flat fan nozzle with 55º to 60º swath and 0.75 l/min flow rate at 700 g/ sq cm). J. Trigger on/off valve. Is the strainer inside valve handle clean? K. Footrest Before using an insecticide, use clean water to ensure that the equipment operates properly and does not leak. Wear protective clothing. To check, follow the steps below : (a) Pour clean water into the tank (never fill tank more than 3/4 full) (Fig. - 5). Fig. - 5 (d) Check tank is holding pressure. Listen for hissing sound of escaping air (Fig. - 8). Fig. - 8 (b) Fit the lid. Turn the handle to lock the lid in position (Fig. - 6). (e) Check to make sure there are no leaks along lance and Fig. - 6 hose, especially where hose joins tank and trigger on/off valve (Fig. - 9). Fig. - 9 • 913 •
  • 14. (f) Operate trigger on/off valve to make sure that spray is container with water for several hours before the blockage is emitted from the nozzle (Fig. - 10). removed by a very soft toothbrush. NEVER clean nozzle with a hard pin or piece of wire and NEVER put a nozzle to your Fig. - 10 mouth to blow through it. Mixing, Handling and Spray Techniques Prepare the insecticide spray according to the manufacturer’s instructions. The insecticide may be mixed separately in a bucket and poured into the sprayer. Water soluble sachets, tablets and insecticides granules are added directly to the water filled tank. These formulations mix readily with water and reduce the hazards associated with handling and mixing in a separate container. When the sprayer has been filled with water to the maximum level indicated on the tank, the lid of the tank is fitted and the sprayer pumped to a pressure of 55 psi by pumping 55 times (700 g/sq cm). The contents of the tank should be thoroughly mixed by shaking the tank before starting to spray (Fig. - 12). Fig. - 12 (g) Check the spray pattern from the nozzle by spraying a dry wall surface. Look to see that the pattern is even and without streaks. Ensure nozzle does not drip when trigger on-off valve is released (Fig. - 11). Fig. - 11 Spraying in a room should commence from the backside of a door clockwise completing the plain surfaces of walls. Then the crevices on the walls and inside portion of windows etc. should be sprayed. Thereafter the pillars, under surfaces of furniture and lastly the ceilings should be taken for spray. (h) Calibrate the nozzle with water in the tank. Pump to 55 Spray is done from roof to floor, using downward motion, to psi (700 g/ sq cm). Open the trigger on-off valve for one complete one swath; then stepping sideways and spraying minute, collect the discharge and measure the amount in a upwards from floor to roof. Spray is applied in vertical swaths measuring jug. Empty the jug. Discharge for a further one 52-56 cm wide. Swaths should overlap by 5 cm and spraying minute and measure the amount. Repeat again and calculate should be undertaken as shown in Fig. - 13. Normal swath the average of the three, one-minute measurements. coverage will take 2.7 sec if height of wall is assumed to be With the above procedure, the average discharge of an 8002 3 meters and hence in one minute 22-23 swaths will cover a nozzle is about 750 ml per minute. If the discharge is incorrect, wall of 10-11 metres length and 3 metres height i.e. 30 - 33 sq check the nozzle and the screen filters to ensure they are not m. It takes about 5 minutes to spray a house with an average clogged. If necessary replace nozzle. Repeat the calibration. surface area of 150 sq m. If the nozzle is clogged : The opening in a nozzle is very small and must not be damaged. Clogged nozzles should be put in a • 914 •
  • 15. Time your spray speed to cover one meter every 2.2 seconds, Fig. - 13 i.e. 4.5 seconds for a 2 m high wall. Timing may be aided by mentally counting “one thousand and one - one thousand and two - one thousand and three -…”. Adjust the mental counting procedure according to the local language (Fig. - 15). If spray stops due to a blockage in nozzle, unscrew the nozzle cap, remove blocked nozzle and replace with a new one. The blocked nozzle should be cleaned as explained above. Do not let spray drip on the floor. Re-pressurize the tank when the pressure falls. Fig. - 15 To ensure the correct swath width, keep the spray tip about 45 cm from the wall. Lean forwards as you spray from top of the wall and move back as you bring the nozzle downwards (Fig. - 14). Fig. - 14 Procedures after Spraying (a) Advise the occupants to stay outside until the spray is dry. (b) Instruct the householder to sweep or mop the floor before children or pets are allowed to re-enter. (c) Instruct the householder not to clean the sprayed surfaces. Disposal of Remains of Insecticides and Empty Packaging: At the end of the day’s work, put the washings from the sprayer into pit latrines, if available, or into pits dug especially for this purpose and away from sources of drinking water. Dilute any insecticide with more water before putting into pits. It is advisable to prepare only sufficient insecticidal solution to avoid disposal of remaining insecticidal solution. Never pour the remaining insecticide into rivers, pools or drinking water sources. All empty packaging should be returned to the supervisor for SAFE disposal. Never re-use empty insecticide containers. Empty insecticide containers should NOT be burned or buried. Maintenance of Equipment : After completing the day’s work, de-pressurize the tank and empty any remaining insecticide, following the instructions given in the previous section. Clean the tank as explained below : The flow of liquid from the nozzle tip at 700 g/sq cm pressure ●● De-pressurize the tank. is 750 ml/minute. Hence 30 sq m surface will be covered with ●● Fill the tank half-full with clean water. 750 ml of the insecticide solution. ●● Replace the lid. • 915 •
  • 16. ●● Shake the tank so all inside surfaces are washed Fig. - 19 (Fig. - 16). Fig. - 16 ●● With lid open, turn tank upside down, open the on/off valve and let all the water drain out of the hose and lance. ●● Ensure the lance is parked to protect nozzle when not in use. ●● When storing the sprayer for a long period, hang it upside down with lid open to allow air circulation. Allow lance to hang from D-ring on the tank with the trigger valve kept open (Fig. - 20). Fig. - 20 ●● Pump up to 700 g/ sq cm pressure. Spray water through nozzle (Fig. - 17). Fig. - 17 ●● De-pressurize the tank and pour out any remaining water into pit latrines or into a pit away from sources of water. ●● Unscrew trigger on/off valve handle and check and clean the strainer. ●● Reassemble the trigger on/off valve (Fig. - 18). Space Spraying Fig. - 18 It is an ideal method for bringing about rapid control of vectors in emergency or epidemic situations and may also be used for seasonal control of flying insect pests or vectors. An additional objective may be to reduce or interrupt the transmission cycle of insect-borne diseases. However, it may not be ideal for all vectors or situations and as such may not be an economical method of control. Among the disease vectors affecting public health, the most important and widespread are mosqui­ oes, t ●● Remove the nozzle tip and wash houseflies, sandflies and other biting flies; some of these may ●● Refit the nozzle be tar­ eted for space treatment. g ●● Clean the outside of tank. Immediate killing of actively flying insects requires a cloud of insecticide droplets that they will encounter in flight. To • 916 •
  • 17. be cost-effective and obtain good biological efficacy, space Cold Fog spraying requires : The cold fog is produced by a special device (cold fogger) that (a) Knowledge of the behaviour and biology of the target breaks up the chemical into microscopic droplets by mechanical species - to understand where and when space treatments means, basically with a high-pressure pump and an extremely will be effective; fine nozzle. The spray droplets are generated without any (b) Knowledge of insecticides and formulations most suitable external heat. With cold fogs, the volume of spray is kept to for space spraying; a minimum. Ultra-low-volume insecticide formulations are (c) Knowledge of pesticide application technology - to know commonly used for such applications. The cold fogger may which equip­ ent is needed and how to use it; and m dispense formulations in a very concentrated form and generate (d) Monitoring and surveillance of the target species and the droplets (usually in the 5-30 micron diameter range) in a vector-borne disease problem to evaluate the efficacy of precise manner. However, its ability to penetrate dense foliage the programme. or obstacles is not as good as that of the thermal fogging. Cold A space spray - technically a fog (sometimes referred to as an fogging is sometimes called Ultra Low Volume (ULV) treatment aerosol) is a liquid insecticide dispersed into the air in the form as it allows the utilization of only a very small amount of of hundreds of millions of tiny droplets less than 50 μm in chemical for coverage of a large area. diameter with a view to cause by contact, immediate knock Like thermal fogging, cold fogging also does not have lasting down of the flying or resting insects in confined spaces. Space residual effects. It is, therefore, essential to carry out fogging at sprays, even when they settle on surfaces do not have much the time when the vectors are most active to hit them directly. residual action. It is only effective while the droplets remain Advantages airborne. Therefore, they have to be repeated at frequent intervals. Space sprays are applied mainly as thermal fogs or ●● The amount of diluents is kept to a minimum, resulting in cold fogs. lower appli­ ation cost and increased acceptability. Some c formulations are ready to use, thereby reducing operator Thermal Fog exposure Thermal fog is produced by special devices known as thermal ●● Mostly water-based and water-diluted formulations foggers that use heat to break up the chemical into very small are used which pose a low fire hazard and are more droplets (usually in 5-30 µm diameter range) which then environmental friendly disperse in the air. When the chemical (usually diluted with ●● Application is more efficient because a lower volume of oil-based carrier) is heated, it is vaporized in a combustion liquid is applied chamber and then expelled via an outlet tube to form a dense ●● No traffic hazard as the spray cloud is nearly invisible fog cloud when it condenses on contact with cool ambient air. Disadvantages The insecticide used in thermal fogs is diluted in a carrier ●● Dispersal of the spray cloud is difficult to observe liquid, which is usually oil-based. Hot gas is used to heat the ●● Higher technical skills and regular calibration are required pesticide spray, decreasing the viscosity of the oil carrier and for efficient operation of equipment. vaporizing it. When it leaves the nozzle, the vapour hits colder Space Spray Equipment air and condenses to form a dense white cloud of fog. Most of Selection of appropriate equipment for space spraying depends the droplets are smaller than 20 µm. The droplet size is affected on the size and accessibility of the target area as well as the by the interaction between the formulation, the flow rate and human resources and operational capacity of the programme. the temperature at the nozzle (usually > 500°C). The volume Sometimes smaller machines may be needed in conjunction of spray mix­ ure applied in vector control is usually 5-10 litres t with vehicle-mounted equipment to treat narrow pathways and per hectare, with an absolute maximum of 50 litres per hectare. other areas inaccessible to vehicles or sheltered from prevailing The hot emission gas is obtained from engine exhaust, friction air movements. Cold fog equipment is recommended where plate/engine exhaust or from a pulse jet engine. thermal fogs may cause a traffic hazard. Aerial application Advantages of space sprays may be justified where access with ground ●● Easily visible fog, so dispersal and penetration can be equipment is difficult and/or extensive areas need to be treated readily observed and monitored; very quickly. ●● Good public relations in some circumstances as people can Equipment for Thermal Fogging see some­ hing being done about the problem; and t ●● Low concentration of active ingredient in the spray mixture Hand-carried Thermal Foggers : These are used for treating and reduced operator exposure. houses and certain outdoor areas of limited size or accessibility, Disadvantages e.g. markets, hotel grounds and parks. There are two types of hand-carried thermal foggers; pulse jet and fric­ ion plate. t ●● Large volumes of organic solvents are used as diluents, which may have bad odour and result in staining; Vehicle-mounted Thermal Foggers : Large thermal fog ●● High cost of diluents and spray application; generators use an air-cooled motor to run an air blower, fuel ●● Householders may object and obstruct penetration of fog pump and insecticide pump. Air from the “roots type air blower” into houses by closing windows and doors; is delivered into the combustion chamber. There it is mixed with ●● Fire risk from machinery operating at very high gasoline vapour and ignited so that temperatures reach 426- temperatures with flam­ able solvents; and m 648°C. The diluted insecticide liquid is pumped via a simple flow ●● Can cause traffic hazards in urban areas. delivery valve and injected into a cup in the fog head or directly • 917 •