Cooperative Extension

Growing Degree Days (GDD)

The Growing Degree Days is 418 as of 5/1/08

This concept of pest management relies on actual temperature data to plan monitoring and control strategies for ornamental plant pests. Here is a comparison of GDD for the years 1996 - 2002 for some
of the common pests encountered in the Chesterfield County area.

As you can see from this chart, temperature differences changed the activity time for many of these pests from two to four weeks. The most extreme being the Two spotted mite from 4/21 in 1995 to 5/13 in 1997 (a total of 22 days).

The concept of using GDD in pest management is rapidly increasing in use. When pest management is based on the calendar, daily temperature is not taken into consideration. However, the weather has a major impact on the development of insects. As a result, predicting the timing of control measures can be difficult.

GDD takes into account the average daily temperature by calculating the number of heat units received. Thus, this system can be more accurate than the calendar method for estimating insect development and timing management strategies.

The easiest method of calculating GDD is to average the daily minimum and maximum temperatures and subtract the result from the base temperature as follows:

For each day that the average daily temperature is one degree above the base temperature, one degree-day accumulates. For most situations, a base temperature of 50 degrees F is used.

For more information of GDD contact the Extension Office at (804) 751-4401 or e-mail likins@vt.edu.