## The role of temperature in scouting and management of insect pests

Growing degree days is an accumulation of heating days that can help determine the stage of pest development and the proper timing of control measures for landscapes and nurseries.

Last summer I was called out to a nursery that had a large number of pine trees with dead terminal leaders (the newest growth at the top of the trees). The dieback was due to feeding from white pine weevils. This type of damage can be difficult to control because the adult weevils deposit their eggs into the terminal leaders very early in spring.

### What Growing Degree Days can do for you

When to apply a pesticide treatment is critical for this insect since there is a short time period between when the insect emerges from its overwintering site in leaf litter to depositing eggs into the terminal leaders and egg hatch. If treatments are timed solely on a calendar date alone, it may not be effective since insect development is related to temperature, and weather varies from year to year. Insect development begins once a certain temperature threshold is reached. A system that uses temperature thresholds to predict stages in an insect’s life cycle is called “Growing Degree Days.”

Growing Degree Days (GDD) uses a base temperature, which for many insects is 50°F. Above this temperature insect development begins to occur. As warming occurs in early spring, the average temperatures begin to climb above 50 degrees. The number of degrees above the 50 degree base is used to calculate the number of degree days for that day.

### How GDD are calculated

For example, on a day where the temperature starts out at 39 and climbs to 65, the average temperature is determined by adding 39 + 65 then dividing by 2 which equals 52. Fifty-two minus the base temperature of 50 = 2. The 2 represents the number of accumulated degree days. GDD is just the sum of all the accumulated degree days greater than the base of 50 degrees that occur at various points during the season. This method for calculating GDD is called the Averaging Method.

### Let MSU do the math for you

There are other ways to calculate GDD that are more precise. Recording temperature changes throughout the day on a graph creates a curve of rising and falling temperatures. The degree days are determined by measuring the area under the curve that is greater than the base temperature. If math is not your subject, don’t worry. MSU’s Enviro-weather website calculates the data and provides degree day information for growers. Enviro-weather also provides GDD information from numerous weather stations around the state.

Localized weather stations provide Growing Degree Day information that is specific to that area of the state. So what does this mean for managing pests like the white pine weevil? Records indicate that the weevil begins to emerge to lay eggs starting at 25 GDD. Last year (2011) in Oakland County, Mich., 25 GDD occurred on April 11 while 200 miles north near Rodgers City, Mich., the degree days did not reach this level until two weeks later. Comparing 2010 to 2011 data for GDD at the Oakland County site, you would find that 25 degree days occurred two weeks earlier in 2010. Treatments based on a calendar date can easily miss the intended target. This means that money and pesticides can be wasted with no resolution to the problem.

MSU’s Enviro-weather website is an excellent tool for accurately timing applications for maximum effectiveness. It can also help reduce unnecessary pesticide use by helping you to determine while scouting if a specific pest is present.

Visit MSU’s AgBioResearch website for more in-depth information on calculating degree days.