The growth rate of many biological organisms is controlled primarily by temperature. As a result of this relationship between temperature and growth, it is possible to predict when specific developmental events should occur by calculating the average heat accumulation. Growing Degree Days (GDD) is defined as the number of degrees above a certain threshold base temperature at which growth begins for the particular organism. Below this base temperature, growth is zero. GDD are calculated by determining the mean daily temperature and subtracting it from the base temperature needed for growth of the organism.
GDD = (Tmax + Tmin)/2 - TBase
One degree day is one day when the average daily temperature is at least one degree above the lower developmental threshold; the base temperature. For example, if the low for the day was 35oF and the high was 65oF, then the day had an average temperature of 50oF. If an organism has a temperature base of 32oF (for example, wheat), then that day counts as 18 degree days. Accumulated GDD are calculated by summing GDD for each day during a period and are useful for tracking the development of crops and insect pests.
Growing Degree Days have many applications in crop management. An example is the characterization of crop development in order to estimate growth stages, including maturity and harvest dates. Growing Degree Days are useful in Integrated Pest Management programs where they are used to predict insect life stages, which in turn help to improve the efficiency of insect scouting efforts. Growing Degree Days can also be used to assess the suitability of a region for production of a particular crop, predict the best timing of irrigation, fertilizing or pesticide application. Growing Degree Days can also estimate heat stress on crops, and plan spacing of planting dates to produce separate harvest dates.
The following table contains accumulated GDD and the corresponding developmental stages for the Alfalfa Weevil.
| GDD °F | Event | Comments |
|---|---|---|
| 310 | Eggs hatch, first instars | Monitor for early detection |
| 425 | Peak of 2nd instar | Increasing population |
| 550 | Peak of 4th instars | Most damaging instars present |
| 850 | Adult emergence |
