The overall mild winter that we had in 2001-2002 has caused some to wonder if that will result in more insect pests during the 2002 growing season. Mild winters or cold winters have little effect on the number of insects that people typically notice. Many, if not most, of the insect species that occur in Illinois range from Georgia in the south to Hudson Bay in the north. With the Illinois climate being moderate between those two extremes, it is very rare to have a winter that has any major effect on insect survival. Perhaps the el Nino winter of 4 or 5 years ago was warm enough to have an effect. Similarly, the cold spell in the early 1980s when the high temperature over a 3-day span barely reached the high teens below zero with little snow cover may be another example. Even the 35-below-zero temperature during the winter in the late 1990s had little effect on insect survival because a deep snow cover insulated the wintering insects.
A few species, however, are affected by mild or severe winters. Bagworm, honeylocust plant bug, mimosa webworm, and elm leaf beetle have a more southern distribution and are uncommon north of Interstate 80 in Illinois. Typically, after a couple of mild winters, these insects become pests in Rockford, Chicago, and other more northern areas of the state. Conversely, many sawflies prefer colder winters and become less common after mild winters. European pine sawfly is likely to be less common in southern Illinois after a mild winter, while redheaded pine sawfly and white pine sawfly are likely to be less common in the northern third of the state. It is true that in a mild winter, more pest insects survive, but so do more predatory insects, parasitic insects, and other natural enemies. This increased survival of natural enemies typically brings insect populations into line rather quickly. Typically, the most important impact on insect numbers during the growing season is spring weather. Cool, damp springs encourage the development of fungi that attack insects and slow the development of insects. The result is fewer caterpillars and other insects surviving through the spring.