How to nail your estimates and act at the right time When you use inaccurate data, the further you are into the growing season, the greater the estimate will differ from reality. For longer season crops, the difference could be quite significant, which is a problem because plant maturity, flowering, and pest/disease GDD targets often have tight windows. In this 20-minute webinar, Dr. Colin Campbell discusses what you need to know for more accurate models, so you can be confident in your management decisions.

2. WHYYOUR GROWING DEGREE
DAY (GDD) ESTIMATE
ISN’T GOOD ENOUGH
AND HOW TO FIX IT
Colin Campbell, PhD
METER Group, Inc. USA

3. UNITING MEASUREMENTS WITH
PEST MANAGEMENT

4. WHY USE
GROWING DEGREE DAYS (GDD)?
Plants and insects
develop on a clock that
includes BOTH time and
temperature
This combination is
called ‘thermal time’ and
has units of ‘degree
days’ or ‘day degrees’
The relationship
between specific stages
of development and
degree days has been
intensely studied for a
plethora of species
Knowing degree days for
specific stages of an
organism is a powerful
yet simple management
tool

5. FUNDAMENTALS OF
THERMAL TIME (GDD)
• All growth happens above a
minimum threshold called “base
temperature”
• There are two methods to
calculate degree days:
1) Average method – uses max and
min temperatures
2) Baskerville-Emin (BE) method – fits
a sine function to the data and
integrates under the curve
– The BE method is a little more
accurate, but their outputs are
similar

6. GDD CALCULATIONS
Growing degree days are simple to
calculate
• Requires at least maximum and minimum
temperature at the location of interest
• Essentially summing daily warming units (τn)
using the difference between an average
temperature and a base temperature (Tbase)
• Accumulating enough thermal time will
advance organisms from one stage to the
next
Example:Codling Moth (apple/pear)
Biofix:Trap moths two successive nights
Lower threshold (Tbase):10.0oC
Upper threshold (Tup): 29.4oC
τn=∑𝑖𝑖=1
𝑛𝑛 𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚
+𝑇𝑇𝑇𝑇𝑖𝑖𝑖𝑖
2
− 𝑇𝑇𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎 ∆𝑡𝑡
Stage GDD (oCd)
Eggs 87.8
Larvae 261.7
Pupae 239.4
Adult 588.9

7. DEGREE DAYS AS A
MANAGEMENT TOOL
• Important management steps need
to be taken at specific
phenological stages
(plants/insects)
• Degree day models prompt the
user when to act
• Timing of these actions is critical
to have the maximum effect
• Accurate data feeding into the
models provides a stable platform
from which to make decisions
Degree Day Illustration for wheat from ”Irrigated Wheat”
by Rawson and Macpherson, 2000, fao.org

8. ACCURACY OF
TEMPERATURE DATA
• Sources of weather data for GDD
modeling
– Virtual data (gridded from models)
– Regional weather (nearest weather
station)
– In field (aspirated or non-aspirated)
• Estimates of thermal time are only
as good as the data that are fed to
the model
• Virtual weather is all around us
• How well does it predict
temperature in natural systems?
AgWeatherNet Publication: https://extension.wsu.edu/wam/can-
virtual-weather-stations-replace-real-weather-stations/

9. VIRTUAL WEATHER DATA
A PANACEA?
• Many are starting to use virtual weather
data to feed disease pest models
• AgWeatherNet study by Dr. David Brown
and others show significant errors over
time
• Example:
– Target 375 GDD
– 33% of station-season combinations showed
more than 5 calendar day error
• Take home:virtual weather alone
cannot replace an actual in field
weather station
AgWeatherNet Publication: https://extension.wsu.edu/wam/can-
virtual-weather-stations-replace-real-weather-stations/

10. REGIONAL WEATHER STATION
GOOD ENOUGH?
• Network of weather stations in
agricultural areas
• Could one of these ‘regional’
stations provide accurate enough
data for management decisions?
• Comparison between in-field and
regional weather shows systematic
temperature bias
• Regional temperature data
significantly overestimates
thermal time compared to in-field Regional Weather
(Degree Days)
In-field Weather
(Degrees Days)
545 486

11. IN-FIELD DATA
ARE ALL TEMPERATURE
MEASUREMENTS EQUAL?
• In-field temperatures are far
closer than regional or virtual
temperatures
• Lower-cost, non-aspirated
temperature sensors tend to
overestimate maximum
temperatures due to solar
heating of radiation shield
• Aspirated (active airflow
across temperature sensor) or
radiation/wind-corrected
sensors are superior

12. SUMMARY
• Plants and insects move through
phenological stages according to
thermal time
• Critical, timely action can only be
taken on crops and pests using
accurate degree days
• Degree days from virtual and
regional weather info can be off by
multiple days
• Local temperature data can
provide good degree day estimates
• Best accuracy from aspirated or
corrected air temperature