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Entomopathogenic nematodes and protozoans— mode of action.ppt

  1. Entomopathogenic nematodes and protozoans— mode of action and field efficacy Bioresources 2nd Sem.
  2. PROTOZOA  Protozoan pathogens naturally infect a wide range of insect hosts.  One important and common consequence of protozoan infection is a reduction in the number of offspring produced by infected insects.  Although protozoan pathogens play a significant role in the natural limitation of insect populations, few appear to be suited for development as insecticides.  Species in the genera Nosema and Vairimorpha seem to offer the greatest potential for use as insecticides.  Pathogens in these genera attack Lepidopteran larvae and insects in the order Orthoptera (the grasshoppers and related insects).
  3.  The one protozoan currently available in a registered insecticidal formulation is the microsporidian Nosema locustae, a pathogen of grasshoppers.  It is sold under such trade names as NOLO Bait® and Grasshopper Attack®.  It is most effective when ingested by immature grasshoppers (the early nymphal stages).  Infections progress slowly; where the pathogen kills the grasshopper, death occurs three to six weeks after initial infection. Not all infected hoppers are killed.  Nosema locustae has been used to reduce grasshopper populations in rangeland areas, and adequate control has been achieved when treatments were applied to large areas while hoppers were still young.
  4.  Although not all grasshoppers in the treated area are killed by Nosema locustae, infected hoppers consume less forage and infected females produce fewer viable eggs than do uninfected females.  Nosema locustae persists on egg pods to provide varying degrees of infection the following season. The effectiveness and utilization of Nosema locustae for rangeland grasshopper control are likely to increase as research continues.  Unfortunately, small, one-pound packages of Nosema locustae preparations developed for sale to gardeners and homeowners offer much less utility (or none).  The mobility of grasshoppers, coupled with the fact that infected hoppers are not killed until a few weeks after they ingest the pathogen, means that application of baits containing Nosema locustae to individual lawns or gardens is unlikely to reduce grasshopper densities or damage substantially.
  5. Nematodes  Traditionally, soil-inhabiting insect pests have been managed by the application of pesticides to the soil. Interest in biological control to manage crop pests has increased because of concerns about the economic, environmental, and health costs of chemical crop protection and because of the need to develop production systems that are environmentally and economically sustainable.  Insect-parasitic nematodes show promise as biological control agents for soil pests.  Nematodes are microscopic, whitish to transparent, unsegmented round worms. Nematodes in the families Steinernematidae and Heterorhabditdae have been studied extensively as biological control agents for soil-dwelling insects.  These nematodes occur naturally in soil and possess a durable and motile infective stage that can actively seek out and infect a broad range of insects.  They do not infect birds or mammals. Because of these attributes, as well as their ease of mass production and exemption from EPA registration, a number of commercial enterprises produce insect-parasitic nematodes as biological “insecticides.”
  6.  Nematodes in the Steinernematidae and Heterorhabditidae carry bacteria of the genus Xenorhabdus in their oesophagi.  The nematode larvae are free-swimming and infect a very wide range of insects. These larvae require free water in order to swim to and penetrate into the orifices of their insect hosts.  In the haemocoel they release the bacterium which kills the insect and the nematodes then feed on the decomposing tissues until the substrate is exhausted, whereupon they are liberated to seek new prey.  Their host-seeking ability is a great advantage in pest control, but the requirement for free water is a serious limitation.  The infectious larvae can be produced by fermentation and several products are currently on the market for control of pests in high value crops such as glasshouse ornamentals.
  7. Life Cycle. o The infective third-stage (IJ stage) is the only life stage wherein the nematode exists outside of the host insect. o This is the stage that is available as a commercial product. o Upon locating a suitable insect host, the infective juvenile enters the insect through natural openings (the anus, spiracles, and mouth) and penetrates the insect body cavity. o The nematodes develop and reproduce in the infected insect. Two to three generations occur in the insect cadaver.
  8. o Application. Some nematodes that are commercially available are Steinernema carpocapsae, S. feltiae, S. riobrave, Heterorhabditis bacteriophora, and H. megidis. o Treatment with these nematodes can be expensive, and they are most commonly used for managing soil insect pests in high-value crops, such as turf, nurseries, citrus, cranberries, and mushrooms, as well as in home lawns and gardens. o As production technologies improve, however, the cost of using nematodes is falling, and it may be economical to use them in lower value crops. o The efficacy of nematodes in the field can be variable. Their successful use probably depends on favorable environmental conditions — adequate moisture, temperatures within the tolerance levels for the nematode, and protection from ultraviolet radiation during application.
  9. o Nematodes are very sensitive to destruction by bright sunlight and to desiccation if they are applied to plant foliage. This limits their use mainly to soil, although there are some instances where they have been successfully applied above-ground to insect tunnels or mines in plant tissue. o Nematodes are formulated as suspensions in liquid, on sponge, in gels, or as semi-dry granules. o The main application approach is as suspension in water at a typical rate of 1 billion per acre, but this rate varies depending on the crop. o They can be applied with conventional chemical application equipment, but screens in nozzles should be removed when applying nematodes with a back-pack sprayer or spray rig.
  10. o Nematodes tend to settle in the tank, so agitation must be provided for uniform application. Nematodes can also be applied with irrigation. o However, some irrigation systems, especially low-volume trickle systems, may not move water fast enough to keep nematodes suspended. o When in doubt, check periodically by taking a sample at the emitters to determine if live nematodes are being moved through the system. • Some appropriate nematode host targets are known: • S. scapterisci against mole crickets; • S. carpocapsae against cutworms and other mobile caterpillar pests; • S. riobrave against cutworms and other noctuid larvae and pupae and citrus weevils; • S. feltiae against sawfly larvae and fungus gnat maggots; and • H. bacteriophora against white grubs and other sedentary soil-dwelling beetle larvae.