Insect I.D.: March 2001, Planning Ahead: Predicting Insect Activity

DEGREE-DAYS. Since insect and mite pests are "cold blooded" animals, their development is slow at cool temperatures and faster at higher temperatures. The number of "heat units" above a baseline developmental threshold temperature an insect is exposed to each day (=Degree-Days), has a direct influence on how fast it will develop. The base line developmental temperature for most insects is considered to be 50 degrees Fahrenheit and air temperatures are often used.

The "daily average" method is most commonly used to calculate the number of heat units above 50 degrees F to which the pest is exposed. For example, if the low temperature for a day was 40 degrees F and the high temperature was 70 degrees F, the average temperature would be 55 degrees F [(40 + 70)/2]. For this day 55 degrees - 50 degrees = 5 degree-day units. When these units are added to the previous days’ units, one gets the cumulative degree-day units. (When calculation yields a negative number or zero, no degree-day units are accumulated - and zero is added for that day). Once a certain number of degree-day units have been accumulated, pest activity can be expected. The following chart contains examples of degree-day accumulations for several turf pests.

PHENOLOGICAL INDICATORS. Apparently, certain plants and insect pests develop at similar rates, and visible or measurable events in nature (= phonological occurrences) occur at the same time. The best phonological indicators are plants that have easily observed events (e.g., bud break, flowering - beginning, full bloom, or petal drop).

We recommend development of a phonological record book. Each year, record in chronological order the observable events seen and record pest activity at this time. After several years of keeping such records, it will be seen that certain plant events consistently occur in the same order and at the same time as certain pest events. Now, integrate these records into your Pest Spectrum and Target Calendar and you really have a useful tool.