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Growing Degree Days
Effective woody ornamental pest management is dependent on our ability not only to accurately diagnose the causal organism, but also to predict when the vulnerable life stage of a pest will be active. Insect and mite development sometimes varies from one year to the next by several days or even weeks. It is important for pest managers to consider additional methods to better time their management strategies. The use of the calendar method and experience is helpful, but when applying management materials with a short residual life, you need to more precisely determine when a pest will be in a vulnerable stage.
The development of arthropods (insects and mites), like many other organisms on earth, is affected by temperature. Insects and mites begin development when the temperature of their environment exceeds a lower developmental threshold or base temperature. The rate of their development increases as the environmental temperature exceeds the threshold temperature and decreases as the temperature declines. For example, arthropod development will occur more quickly during warmer growing seasons and is slower during cooler growing seasons.
The concept of growing degree days (GDD) is an arithmetical conversion of daily temperature records to heat units. While the concept has been recognized for many years, the base or threshold temperature is known for just a few of our key woody ornamental pests. The threshold or base temperature varies from one insect or mite species to another. Organisms that are active when temperatures are cool usually have a lower threshold temperature than those that become active during warmer months of the growing season. For the sake of consistency, a threshold or base temperature of 50°F has been adopted by most woody ornamental pest managers. Several equations such as the average, triangulation, and sine can be used to calculate GDD based on minimum and maximum temperature. A simple method that is often used involves averaging the daily maximum and minimum temperatures and subtracting the base or threshold temperature from the daily average as described below:
If this system is to work, you will need to collect the maximum and minimum temperatures each day from March 1 through September 30. You also will need to keep a record of the total accumulated GDD for your landscape. If the average temperature is below the threshold or base temperature for the day, enter a zero for that day. The total accumulated GDD in the early season will occur slowly. As daily temperatures increase during the growing season, GDD will accumulate more quickly from one day to the next. This information will help you to determine when pest management actions should be applied. Remember to continue monitoring your landscapes to determine if treatment is indicated against a key pest population. Consult Table 1 below for assistance in tracking growing degree days in your landscape or nursery.
Table 1. Growing degree day (GDD) at a threshold temperature of 50°F.
Use this table as you would use a department of transportation highway map mileage chart. For example, when the maximum air temperature was 70°F and the minimum air temperature was 50°F for a 24-hour period, the GDDs indicated in bold text (10) is the number of GDDs accumulated on that day.
