Humans and other warm-blooded animals can generate their own heat to regulate their body temperature. In contrast, insects are ectotherms, which means they cannot generate their own heat and their development is driven by the temperatures they experience in their environment. This is the same for plants, fish, amphibians, and reptiles. Most people in agriculture are familiar with using growing degree days (GDD; also called “heat units”) to monitor and predict crop development. We can apply the same concept of predictable development to insects to monitor development and estimate important benchmarks (e.g., egg hatch, peak adult emergence, etc.).
Knowing when pests become active or when certain growth stages are reached is important for integrated pest management of crops because it can help us refine scouting windows and make timely management decisions, if needed. Each year brings different challenges for crop production and different risks for pest infestations. Additionally, many factors influence when migratory pests arrive in the Midwest and, combined with monitoring efforts such as trapping, GDD can help determine when scouting should occur. This article goes over important terminology, how to calculate GDD for key insect pests, and resources for obtaining relevant information.
What is a degree day? A degree day is a measurement of heat unit accumulation over time.
Because insect development depends largely on the temperatures they experience, their development is predictable using GDD. There is an ideal temperature range, which differs by species, for optimal reproduction and longevity. There are three key things we need to know about insect development to use the formula:
Lower developmental threshold: The lowest temperature at which the insect can complete development. This varies by species but usually tells us when the pest becomes active after overwintering in Iowa. This is not the same as the lowest temperature that will kill an insect. See Table 1 for examples of lower developmental thresholds.
Upper developmental threshold: The highest temperature at which the insect can develop. Some insects do not have an upper threshold, but we use 90°F as the upper threshold for many species. Some insects cannot withstand warm temperatures and have a lower maximum. See Table 1 for examples of upper developmental thresholds.
Biofix: The starting point for accumulating degree days. This varies by species, but most of the time we use January 1 as a starting point even though insects are not generally active during the winter months. See Table 1 for examples of various biofix indicators.
The formula for calculating growing degree days uses the daily minimum and maximum temperatures to find an average daily temperature. Then, the organism’s lower developmental threshold is subtracted to give a measurement of GDD.
[(daily maximum temperature + daily minimum temperature) ÷ 2] – lower threshold
For almost all GDD calculations in agriculture, we use the Modified Average Method, which accounts for the organism’s developmental thresholds. Using the Modified Average Method, the organism’s lower developmental threshold is used in the formula if the daily minimum temperature is below the threshold, and the upper developmental threshold is used if the daily maximum temperature exceeds the threshold.
Examples 1-3 will walk through a 3-day period in Ames, Iowa to demonstrate how to use the Modified Average Method for calculating GDD for seedcorn maggot.
Example 1: April 11, 2022
- Daily minimum temperature: 41°F
- Daily maximum temperature: 69°F
- Lower developmental threshold: 39°F
- Upper developmental threshold: 84°F
Since neither the daily minimum nor maximum temperatures were reached, the formula is straightforward:
[(69 + 41) ÷ 2] – 39 = 16 GDD
Example 2: April 12, 2022
- Daily minimum temperature: 38°F
- Daily maximum temperature: 77°F
Here, the daily minimum temperature is below the lower developmental threshold for seedcorn maggot. Therefore, we substitute 39°F in place of the daily minimum temperature in the formula.
[(77 + 39) ÷ 2] – 39 = 19 GDD
Example 3: April 13, 2022
- Daily minimum temperature: 28°F
- Daily maximum temperature: 47°F
Again, the daily minimum temperature is below the lower developmental threshold, so we substitute the lower developmental threshold in the formula.
[(47 + 39) ÷ 2] – 39 = 4 GDD
Over this 3-day period, seedcorn maggot in Ames, IA accumulated 39 GDD.
Variables for Key Pests
For most of the important agricultural insect pests, scientists have estimated the lowest temperature for development and the GDD requirements to reach key developmental stages. Table 1 lists several key pests, their developmental thresholds, and the biofix for GDD accumulation. Click on the pest name to see an encyclopedia article about that pest with more scouting information.
Table 1. Variables for key pests.
|bean leaf beetle
|Egg-laying (soybean emergence)
|Significant moth flight
|common stalk borer
|European corn borer
|First spring moth capture
|western corn rootworm
Growing degree days for insect pests can be calculated based on temperature readings from a home thermometer or from data on the Internet from local weather stations. The Iowa Environmental Mesonet has archives of data and various tools for retrieving weather data or calculating GDD.
A new tool is the Pest Forecasting Maps page on the Iowa Environmental Mesonet. Use this tool to get a current estimation of GDD for key pests at sites across Iowa, and a 7- or 14-day prediction of GDD accumulation.