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Wintering Insects

November 20, 2000
Every winter, when we have a particularly cold spell, warm up, or just when it's been normal and nothing else is happening in the world, I get calls from news reporters wanting predictions on the number of insects next year. I respond that if their weather forecasters can accurately predict the spring weather conditions, then I can predict the number of insects that are likely to be present. Of course, their response is always the same, that predicting the weather several months away is essentially impossible.

Many of the insect species that occur in Illinois range from Arkansas to Hudson Bay. They are adapted to a very wide range of weather conditions, so the winter has little effect on their numbers. Species such as bagworm, mimosa webworm, and elm leaf beetle that are more common farther south may be more numerous in northern Illinois after a couple of particularly warm winters. Other northern species such as cottony maple scale, honeysuckle aphid, birch leafminer, and imported willow leaf beetle may be more numerous in central Illinois after a series of cold winters and cooler summers.

Wet, cool springs tend to favor fungus development on plants and insects. Just as there are fungi that are specific to certain tree and shrub species, there are fungi that specifically attack insects. In wet weather, many insects succumb to fungus infections, particularly caterpillars and other larvae. In dry springs, more survive. Thus, if the spring weather pattern is known, one can usually predict the frequency of spring-occurring larvae such as eastern tent caterpillar, cankerworm, and European pine sawfly.

In general, winter and spring conditions that favor the survival of insects not only help pests survive but also their insect predators and parasites. So although there may be more caterpillars in a dry spring following a mild winter, there are also more parasitic wasps to attack them. When weather conditions have resulted in fewer pests, there are also fewer natural enemies. The result is that although some insects may be more numerous, most are similar in abundance from year to year. It's alerting you to those insects that are more numerous in one year than normal, as well as reminding you when insect pests are susceptible to control efforts, that helps make this newsletter worthwhile. (Phil Nixon)

Author: Phil Nixon


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