Using Growing Degree-Days for Insect Pest Management
When pest management is based on calendar timings, daily temperature is not taken into account. This can result in misreading information regarding current insect activity. Depending on weather conditions, insect development may vary from year to year by a few weeks, consequently predicting the proper time for control measures is difficult.
Insects, like plants and many other organisms, are dependent on temperature to develop. These organisms begin developing when the temperature exceeds the lower developmental threshold or base temperature. The rate of development increases as the temperature exceeds the base temperature and decreases as the temperature drops. Thus, insect development is accelerated during warm years and delayed during cooler years. Upper developmental thresholds, temperatures above which growth slows or ceases, are seldom used for insects since these thresholds are either not known, or they live in habitats where the upper threshold is seldom exceeded. Growing Degree-Days (GDD) takes into account the average daily temperature by calculating the number of heat units received. Thus, this system can he more accurate than the calendar method for estimating insect development and timing management strategies.
Several mathematical equations, such as the Average, Triangulation, and Sine, can be used for calculating GDD based on minimum and maximum temperature. The easiest method is to average the daily maximum and minimum temperatures and subtract from-it the base temperature as follows:
For each day that the average temperature is one degree above the base temperature, one degree-day accumulates. Depending on the species, the base temperature can vary. Cool weather organisms will have lower base temperatures while other types of organisms will have higher ones. For most situations though, a base temperature of 50oF: is satisfactory. If a development prediction for a particular insect varies more than expected, using a lower base temperature could be necessary for that organism.
Some of the other methods used for calculating GDD are complicated. Forecaster, a computer program from the Department of Entomology at the University of Minnesota, calculates GDD using the sine method and can calculate the most appropriate base temperature for that species using daily minimum and maximum temperatures along with emergence information. Also, Forecaster can predict insect activity based on previous observed data along with normal and observed temperatures for your region. Biophenometers are instruments that record the temperature every few minutes and accumulate the GDD as that portion of a 24-hour period. This is the most accurate way of calculating GDD but it does not retain minimum and maximum temperature data which would he important to calculate the most appropriate base temperature to use. When using GDD from other sources it is important to determine the method and 'base temperature used for calculations.
Minimum/maximum temperatures can be acquired from newspapers, TV, weather reporting services, etc. Ideally, taking temperature readings from your property or area would be most accurate. If taking temperature readings on your property, several things should be considered. Minimum/maximum thermometers or any other devices used to detect temperature should be placed in a well ventilated, white shelter. Shelters should be placed in the full sun, ideally in areas free from excessive radiant heat from driveways, sidewalks, buildings, etc. Call your local Cooperative Extension for details on constructing a weather shelter.
Each day between March 1 and September 30 the daily GDD are calculated and totaled to determine the accumulated GDD (Table 1). If the average temperature is below the base temperature, which would return a negative daily GDD number, just enter zero (0) for the day. For the system to work you must collect the maximum and minimum temperature every day. Early in the season the numbers will accumulate slowly but as the average daily temperature increases the GDD will accumulate faster.
The GDD for insects are listed in Table 2. The time for pest control is expressed in a range of numbers beginning with first perceptible feeding injury and continuing until approximately the end of the insects' plant injury cycle. In other cases, ranges indicate optimum control periods. If more than one range of numbers appears, this is indicative of multiple generations and/or control- periods in an insect's life cycle. For example, Cooley spruce gap adelgid GDD are 22 - 92 and 1500 - 1775. This means the insect is active starting around 22 GDD and control measures can be implemented until approximately 92 GDD. Cooley spruce gall adelgid also has another period during the growing season when controls may be effective and necessary. This period is between 1500 and 1775 GDD.
GDD should be used as a guide as to determine when pest control actions should be utilized. Monitoring should be employed at some point before the GDD number is reached to determine if a pest problem exists and if some type of control is warranted. Decisions as to whether or not to use control measures will be dependent upon such things as the level of damage or potential damage and the life stage of the insect. Treatment, if decided upon, would be timed to correspond with some point within the GDD range.
Temperature data to calculate GDD are being collected at 11 different locations in Western Pennsylvania from Uniontown to Erie. GDD accumulations for the corresponding locations are currently available on Penn State Cooperative Extension Woody Ornamental Web Site. Accumulated GDD are available from March 1 to September 30.
Tom Kowalsick and Scott Clark, Cornell Cooperative Extension