Insects do not maintain a constant temperature. Insect body temperature varies with ambient temperature. Insect rate of growth and development is proportional to temperature. Below a minimum temperature, there is no development. Above the minimum temperature (Developmental Threshold), the growth rate increases to a maximum, then declines as insects suffer heat stress near the maximum tolerated temperature.
Each insect species has its own developmental threshold and development vs temperature curve. These parameters must be measured empirically and are known for most economically important insects. Knowing these parameters enables insect biologists to create models that predict when insects will be present based on temperature data alone. Good models can focus scouting and pest management measures to only those times when it would be possible for an insect stage of interest to be present. Combined with weather forecasts, the emergence of some pest species can be predicted in advance with the same certainty as the weather forecast.