Insect emergence during the growing season appears to be based on a variety of factors, particularly weather factors. As cold-blooded creatures, insects are sensitive to outdoor temperatures. It has been demonstrated that various species develop above certain threshold temperatures; at lower temperatures, development is sharply reduced or halted.
A variety of methods are used to predict when a pest insect is at a susceptible stage. Calendar date has been used for years but does not take into account warm springs, which accelerate insect development, or cold springs, which retard it. Using degree days is more accurate in predicting insect emergence, but this method requires calculation or at least entering daily high and low temperatures into a calculator or computer program.
Plant phenology is the process of associating a plant event, such as blooming, with the presence of a susceptible stage for pests. This is a very practical method because landscapers or nursery personnel can observe the plants in the course of their daily work as an indicator of when to expect susceptible insect stages. Phenology should be used to help determine when to scout for an insectand not as a stand-alone insect control guide. As with any system, the presence, number, and potential damage of an insect must be verified before insect pest management is undertaken.
The book Coincide by Donald A. Orton provides susceptible-stage information on a wide range of landscape insect pests based on the observation of blooming and other phenological events on common landscape plants. The book is available for about $30 through White Oaks Group, P.O. Box 1, Flossmoor, IL 60422 and by telephone at (708) 755-9700. We are particularly fortunate in Illinois because Don Orton, an Illinois Department of Agriculture nursery inspector, made most of his observations within our state.