University of Illinois

No. 1/March 19, 1998

Winter and Insect Survival:
What Can We Expect?

Because winter was exceptionally mild this year, the question of insectsurvival has been a topic for conversation and much speculation. As with mostquestions related to insect survival as affected by environmental conditions,there is no clear-cut answer. The potential for increased pest pressure variesaccording to the specific insect and crop. For some insects, such as blackcutworms and potato leafhoppers, our winter will have essentially no directeffect on survival. Black cutworms migrate into Illinois each spring,beginning as early as mid-March. Egg-laying moths seek out the fields with theheaviest densities of winter-annual weeds. Potato leafhoppers, anothermigratory insect pest, most often are sighted soon after the first cutting ofalfalfa has occurred. Other migratory insects that have the potential tobecome pests each year in Illinois include familiar examples such as cornearworms (pupae overwinter in the soil south of 40 degrees North latitude),corn leaf aphids, fall armyworms, and green cloverworms (moths overwintersouth of 41 degrees North latitude). As a point of reference, 40 degrees N isabout the latitude just north of Decatur.

Many familiar insect pests do indeed overwinter in Illinois--such as alfalfaweevils, bean leaf beetles, corn rootworms, European corn borers, fleabeetles, grasshoppers, stalk borers, white grubs, and wireworms. These "hardy"insect pests are most often superbly adapted to survive winters throughoutmuch of the Midwest.

Due to the mild winter, we anticipate an early egg hatch of alfalfa weevils;and growers, even those in northern Illinois, should be alert for an early"flush" of weevil activity this spring. An early cutting of alfalfa, althoughgenerally a great approach for weevil management in northern areas of thestate, may not work as well this season, particularly if a warmer than averagelate March and early April take place.

Flea beetle survival should be outstanding, based upon the balmy temperaturesof December, January, and February. We anticipate impressive densities of fleabeetles this spring; and seed-company personnel should be alert for this tinyinsect because it has the potential to vector Stewart's disease, a bacterialwilt. Many inbreds are especially susceptible to this disease.

European corn borer larvae and corn rootworm eggs will most likely not beaffected by the mild winter in any significant fashion. Corn rootworm eggsoverwinter in the soil at depths that can be deeper than 1 foot. A highconcentration of western corn rootworm eggs, however, generally can be foundin the 4- to 8-inch soil profile. Compared with environmental conditionsduring the winter, the degree of soil moisture at the time of corn rootwormegg hatch is a much more critical factor in the survival of corn rootworms. Ifeggs hatch (late May to early June) into saturated, waterlogged soils,producers should expect reduced numbers of surviving larvae. With respect toEuropean corn borers, spring weather has far more impact on survival than dowinter temperatures. If egg-laying moths encounter stormy, windy, and rainyevening weather from early to mid-June, expect pressure from thefirst generation of this pest to be curtailed.

Bean leaf beetle survival throughout the winter should have been enhanced thisyear, so early planted soybean fields could be at risk to damaging levels ofthis insect pest. Bean leaf beetles overwinter away from soybean fields innearby wooded areas beneath leaf litter. Early planted soybean fields nearwoodlots are prime candidates for bean leaf beetle injury this spring.

It is to be hoped that these forecasts will give you an early alert for whatmay prove to be another very interesting season from the insect world!
Mike Gray (, Extension Entomology, (217)333-6652