Growing Degree Days and Insect Development

Insect development is directly tied to temperature not calendar dates. Each stage of the insect requires a certain amount of growing degree day (GDD) accumulation to advance from one stage to the next (eg. egg to 1st instar lava). There is also a minimum temperature (base temperature) that must be reached for their development to begin, as well as a maximum temperature that, once reached, hinders or stops their development. Calculating daily degree days based on these development requirements allows us to predict when to expect a certain stage of the insect that is best suited for scouting or management.

All GDD models use the same equation to calculate daily degree days but the base temperature and biofix date (the start date for which we should begin calculating the daily degree days each year) are specific for each insect. We cannot just take the daily degree day calculations for European corn borer and use it to predict western bean cutworm development, for example.

The Daily GDD equation is: ((T max + T min) ÷ 2) - T base

• T max = the daily maximum air temperature
• T min = the daily minimum air temperature
• T base = the GDD base temperature for the organism being monitored

To determine the GDDs of a given insect, it starts with basic research. The bulk of the work is done by rearing the insect in a number of growth chambers, each set at a different constant temperature to monitor their development. This helps determine what the appropriate base temperature and maximum temperatures are for that insect. Since constant temperatures do not occur in the real world, additional research is done in the field, monitoring both the insect development and daily maximum and minimum temperatures. These studies often also determine what the biofix date is. When the research has been done elsewhere it requires validation to determine if it is also accurate at predicting the pest in Ontario's climate. There is often more than one GDD model to compare with (Table 1). Always pay attention when GDD model are coming from the US, as they are often calculating their degree days in Fahrenheit instead of Celsius degrees.

Table 1. Examples of GDD Requirements for Specific Insect Pests

Insect	T Base (°C)	Annual Start Date (Biofix)	Reference
Alfalfa weevil	9	April 1st	Harcourt 1981 Peterson and Meyer 1995 Beauzay et al. 2013 Soroka 2015
Black cutworm	10.4	Begin when cumulative trap catch of 9 or more moths over 2-day period	Troester et al. 1983 Story et al. 1984 Showers et al. 1985 Kullik et al. 2005
Cereal leaf beetle	8	January 1st	Guppy and Harcourt 1978 Blodgett et al. 2004 Philips et al. 2012 Evans et al. 2014
European corn borer	10	April 1st	McLeod 1976 Boivin et al. 1986

The original research on growing degree days for alfalfa weevil was done here in Ontario (Harcourt 1981). There are now several GDD models used in North America, many using at least part of the original Ontario research (Table 2). Research will need to continue as our climate changes and impacts the start dates of these models.

Table 2. Accumulated Degree Day Requirements for Alfalfa Weevil Development

Alfalfa Weevil Stage	Accumulated Degree Days (Base 9°C) Start Date April 1st
	Soroka 2015	Harcourt 1981
Egg Hatch	155-167	Peak hatch at 155
1st Instar	176-206	+ 42 (= 197)
2nd instar	218-243	+ 42 (= 239)
3rd Instar - Major Feeding	260-280	+ 46 (= 285)
4th Instar - Major Feeding	306-331	+ 58 (= 343)
Pupation	N/A	+ 39 + 80 (= 462)
Adult emergence	N/A	N/A

Growing degree day calculations come in handy especially when the growing season is far from "normal". In some years (like this spring) we can accumulate daily GDDs very quickly, especially when the nights are also warm. Planting delays result in the insect getting ahead of crop development. Once the crop finally emerges, the insect might already be in its most damaging stage. If there is a significant delay in planting, the crop may escape any injury if it does not emerge until after the damaging stage of the insect has passed (Table 3). Based on this year's GDDs up to May 27th, feeding should begin in fields from Harrow to Vineland, while from Guelph to Eastern Ontario egg hatch about to start.

Table 3. Accumulated Degree Days (Base 9°C) for Various Ontario Locations in 2018

Location	Accumulated Growing Degree Days as of May 27th, 2018 - Start Date April 1st	Predicted Alfalfa Weevil Stage
Harrow	199	1st instar
London	191	1st instar
Guelph	122	Nearing egg hatch
Vineland	167	Egg Hatch ending, 1st instars begin
Peterborough	109*	Nearing egg hatch
Kemptville	140	Nearing egg hatch

Weather data obtained from Environment Canada
*missing data for one or more days