Estimating Development of Insects with Accumulation of Degree–Days

April 2, 1999
Insects are cold–blooded animals; their growth and development are influenced by temperature. Within certain limits, the growth of insects is related directly to temperature, so insect development usually can be predicted by keeping track of temperature accumulations, a more reliable method than predicting insect development based on the calendar.

Optimum development of insects usually occurs within a range of temperatures. Most insects have a minimum developmental temperature and a maximum developmental temperature, below and above which, respectively, insect development ceases. Degree–days (or heat units) are a measure of the amount of temperature available for insect growth. Degree–days may be calculated simply as the average daily temperature minus the minimum developmental temperature, or the base temperature ([maximum temperature + minimum temperature] ÷ 2 — base temperature). For example, the base temperature for development of alfalfa weevils is 48 deg F. If the daily maximum temperature was 70 deg F and the daily minimum temperature was 40 deg F, a total of 7 degree–days occurred on that day ([70 + 40 ÷ 2] — 48 = 7).

This simple equation works well when both the minimum and maximum temperatures are above the base temperature. However, temperatures may fluctuate during a 24–hour period, so more complex equations and computer programs may be needed to predict insect growth and development accurately.--KS

Author: Kevin Steffey