Developmental Rates and ThresholdsInsects are cold-blooded, mammals are warm-blooded. We mammals generate heat and control our own temparature. Insects do not generate body heat and control their own temparature, they remain at the same temperature as their environment. At a certain temperature an insect's biochemical reactions cannot proceed and development stops. This temperature is known as the insect's developmental threshold or developmental base and it varies among species.
Charting the ambient temperature makes it possible to track insect development, which is directly proportional to the amount temparature accumulated above the developmental threshold temperature. This accumulation of temperature over a day's time is a heat unit known as degree-days (DD).
Degree-Day Calculation MethodsThere are different ways to determine the quantity of degree day heat units. We calculate them by calculating the area under a temperature versus time graph on a given day. The methods are listed below in order of precision in measuring small changes during the day or departures from idealized heating and cooling trends (see figure).
Average or Max/Min Method - This method is the simplest and least precise. It assumes that the daily temperature graph is linear and that the area beneath it is triangular.
DD = [Daily max temp + Daily min temp*]/2 - Devel. Threshold (* If Daily min temp < Devel. Threshold, substitute Devel. Threshold)
Sine Wave (Baskerville-Emin) Method - This method is more precise and assumes that the daily temperature cycle takes the form of a sine wave. This method makes the same use of daily maximum and minimum temperatures and developmental threshold as does the Average Method. Using the Sine Wave Method tends to accumulate more DDs than the Average Method, particularly during the early part of the season.
Continuous Integration Method - This method is the most precise and requires multiple temperature readings hourly or more frequently throughout the day to obtain a temperature versus time graph that is truly representative of a field situation.
Relating Degree-Days to Life Cycle and Development These methods are attempts to correlate a pest event or activity with another event that can be measured more precisely. Events in an insect's life cycle often occur after the same heat units have accumulated each generation, but many generations' observations must be collected to measure this precisely. Degree-days can be used to predict events wherever weather data are available.
