Insects and plants depend upon the temperature to develop. They begin developing when the temperature exceeds a base temperature. Each development stage of an insect has its own accumulated heat requirement.
Growing Degree Days (GDD) are a measurement of the growth and development of plants and insects and takes into account the average daily temperature accumulations. GDDs can be used to time control measures for the pests of plants. Insect pests appear when the GDD reaches a critical value for that particular pest.
The formula for GDD looks something like this:
| Max Temp. + Min. Temp. 2 |
50 | = | Daily GDD |
50 F is used as the base for these GDDs because this is the temperature at which woody plants in the northeast begin to grow. For each day that the average temperature is one degree above the base temperature, one degree day accumulates.
GDDs can also be used to predict other events related to the accumulation of warmth such as the blooming of plants.
Predicted Appearance Dates for Various Pests, 2000-2009
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 |
Additional Information
Growing Degree Days - Cornell University
Using Growing Degree Days for Insect Pest Management
Track Pest Development with Growing Degree Days
What is phenology? - Ohio State University
