Track Pest Development with Growing Degree Days
Growing Degree Days (GDD)
This year's accumulated Growing Degree Days are 798 as of May 7, 2012 through May 14, 2012.
This information will be updated weekly from March through August to provide current data for use in making pest management decisions.
The concept of GDD presumes that both plant growth and pest development depend on the accumulated amount of heat received during the growing season. The generally accepted method for calculating GDD is to determine the difference between the daily average temperature (average of the high and low temperature for a given date) and the base temperature, usually 50°F. For example, if the average temperature on a given day were 65°F, the GDD accumulated for that day would be 15. Negative values are ignored, for on cooler days, plant growth and insect development do not regress, but simply slow, to speed up again when the weather warms.
When pest management is based solely on calendar dates, predicting the timing of control measures can be difficult. It has been determined that a given insect pest appears each year when the accumulated GDD reaches a critical value for that pest. Along with the current GDD data provided above, this information can be used to predict when a given pest will appear, thus allowing for improved timing of control measures. The same is true for plant development, e.g., bloom times for many ornamentals correlate well with GDD.
The following table utilizes the GDD for the years 2000-2009 for some of the common pests encountered in the Chesterfield County area. The “Minimum GDD” column shows the minimum GDD needed before the pest appears. As you can see, temperature differences from year to year can shift the appearance of these pests by more than three weeks. By monitoring the GDD on a daily or weekly basis, it is possible to predict a pest’s appearance and to respond accordingly with timely control.
Predicted Appearance Dates for Various Pests, 2000-2009
Chesterfield County, Virginia
| Pest | Minimum GDD | Average | Range |
| Eastern tent caterpillar | 92 | March 28 | March 13 - April 9 |
| Pine tip moth | 121 | April 3 | March 24 - April12 |
| Pine spittlebug | 148 | April 6 | March 27 - April18 |
| Azalea lace bug | 239 | April 23 | April 2 - April 25 |
| Two-spotted spider mite | 363 | April 29 | April 2 - May 10 |
| Euonymus scale | 406 | May 2 | April 25 - May 12 |
| Boxwood leaf miner | 448 | May 4 | April 28 - May 15 |
| Bagworm | 600 | May 14 | May 9 - May 26 |
| Fall webworm | 867 | May 31 | May 22 - June 8 |
| Japanese beetle | 970 | June 4 | May 25 - June 11 |
For more information on the use of Growing Degree Days (GDD) contact us.