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Will Another Mild Winter Promote Insect Problems?

March 16, 2001
Remember the cold temperatures of December? At that time, many of us were braced for a rather severe winter, which would have been a break in a string of mild winters dating back to at least 1997. Well, severe winter weather never really developed. Sure, it was cold in December, and we had a lot of snow on the ground that month. But freezing cold temperatures never set in for very long this winter. So, as is always the case, we (and a lot of others) wonder what effect the winter has had on a host of insects that often invade our fields. In this article, we'll offer a "snapshot" of some key insect pests of alfalfa, corn, and soybean and try to guess the impact of the winter environment on their survival.

The two key insect pests of alfalfa in Illinois are the alfalfa weevil and potato leafhopper. Only the alfalfa weevil overwinters in the Midwest. Potato leafhoppers arrive in the Midwest later in the spring, usually borne by prevailing winds and storm fronts. Therefore, winter conditions in the Midwest have no bearing on their survival.

On the other hand, alfalfa weevils spend most of the winter as eggs and adults in southern Illinois and as adults in northern counties. On warm (temperatures generally above 50°F) days during the winter, alfalfa weevil adults may break their winter dormancy and become somewhat active in alfalfa fields; females often lay eggs during extended periods of warm weather. In addition, mild winter conditions favor survival of overwintering eggs. Although ambient temperatures were low in December, the snow cover undoubtedly offered protective insulation. John Shaw, coordinator of the Insect Management and Insecticide Evaluation Program with the Illinois Natural History Survey, has already found a lot of healthy alfalfa weevil eggs in some projected plot areas in Champaign County. Although we have not sampled alfalfa fields in southern counties, we anticipate very good survival of alfalfa eggs in that region, too.

In southern Illinois, overwintering eggs hatch relatively early in the season, and larvae feed on the tender terminal leaves of growing alfalfa plants. Therefore, alfalfa growers in central counties should anticipate alfalfa weevil activity soon. Refer to the article "Alfalfa Weevil Will Be the First Insect We'll Watch For" in this issue of the Bulletin for more information.

In the past, our questions about the effect of winter weather conditions on insect pests of soybean have focused solely on bean leaf beetles in the Midwest. However, the discovery of the soybean aphid last year adds a new twist to our conjecture. Refer to the article "What Can We Expect From Soybean Aphids This Year?" in this issue of the Bulletin for more information about this invasive species.

We have known for a long time that mild winter weather conditions favor the survival of bean leaf beetles. The adults spend their winters beneath leaf litter and debris usually in wooded areas. So, the milder the winter, the better they survive. As mentioned previously for alfalfa weevils, snow cover insulates, reducing the potential negative effects of low ambient temperatures. Consequently, we anticipate significant numbers of bean leaf beetles this spring, especially in areas of Illinois where densities of bean leaf beetles were high late last summer. Entomologists in Iowa also reported very high numbers of bean leaf beetles toward the end of the summer last year. So, we should be ready early for these pests this year. When spring temperatures rise to daytime highs of 50°F or greater, bean leaf beetles will leave their overwintering quarters and fly to nearby fields of alfalfa or clover. Although bean leaf beetles feed on alfalfa foliage, the extent of their feeding is not sufficient to cause economic damage. As soon as soybean plants begin to emerge, bean leaf beetles leave alfalfa and clover fields to colonize soybean fields. The earliest planted fields usually have the heaviest infestations of bean leaf beetles.

The primary insect pests of corn are the European corn borer and western and northern corn rootworms. These pests usually are first and foremost on most corn growers' list of concerns every year. However, some of the secondary insect pests that attack corn early in the season have garnered considerable attention during the past few years. Consequently, inquiries about the potential effects of winter weather on many pests of corn have been numerous.

Corn rootworms overwinter as eggs in the soil. The depth at which adult corn rootworms deposited eggs last summer depends on species and levels of soil moisture. Western corn rootworm adults tend to lay their eggs deeper than northern corn rootworms do. However, eggs of both species can be found from just below the soil surface to depths of 12 inches or greater. During dry summers, adults of both species will crawl into drought cracks and lay eggs well below depths of 12 inches. In general, eggs of both corn rootworm species are well adapted to winter conditions in the Midwest. But survival of overwintering eggs that were deposited near the soil surface last summer may not be as good as survival of eggs laid deeper in the soil. This occurs most frequently when winter temperatures are unusually cold for prolonged periods of time and snow cover is lacking, so we doubt that mortality of corn rootworm eggs this past winter has been significant.

European corn borers overwinter in cornfield residue (stalks and cobs) as fully developed (fifth-instar) larvae. Like corn rootworms, European corn borers have adapted quite well to Midwestern winters, so their survival usually is not affected greatly by winter weather conditions.

Densities of corn rootworm and European corn borer are influenced more significantly by environmental conditions in the spring. In general, survival of these pests is benefited by spring weather that encourages early planting. We'll discuss other factors in forthcoming issues of the Bulletin.

Black cutworms do not overwinter in Illinois. Rather, the adults migrate into Illinois from southern states each spring. Therefore, winter weather in the Midwest has little impact on the potential for infestations of black cutworms during any given year. Anticipating black cutworm problems depends on when the moths arrive to begin laying eggs and the condition of fields at that time. We would appreciate hearing from any of you who use pheromone traps to monitor flights of black cutworms in the spring.

The most prominent so-called secondary insect pests of corn during the past few years have been the corn flea beetle, grape colaspis, southern corn leaf beetle, white grubs, and wireworms. It's unlikely that winter weather conditions have much impact on grape colaspis larvae, white grubs, and wireworms, all of which overwinter relatively deep (at least 8 to 10 inches or more) in the soil. Again, weather conditions in the spring are more likely to influence the impact of these pests. We believe that favorable spring weather that encourages early planting will increase the potential for these pests to cause significant problems.

On the other hand, corn flea beetles and southern corn leaf beetles overwinter as adults in debris on the ground surface. We know very little about the ecology of the southern corn leaf beetle, but we imagine their populations benefit from mild winters. We know quite a bit more about corn flea beetles. Although the corn flea beetle is not a major pest of hybrid corn, seed-corn inbreds are highly susceptible to Stewart's wilt, caused by a bacterium transmitted by the corn flea beetle. For years we have estimated the survival of the flea beetle by adding the average monthly temperatures for December, January, and February. If the sum is less than 90°F, survival of corn flea beetles is not low to moderate. If the sum is more than 90°F, survival of corn flea beetles is good. However, the snow cover in December may have insulated overwintering flea beetles, so use this rule of thumb cautiously. In the next issue of the Bulletin, we'll report the sums of the average temperatures during December, January, and February for all areas of Illinois.

So, there you have it; one size does not fit all. Although mild weather conditions during the winter enable some overwintering insects to survive, winter weather has little or no impact on other species. Therefore, predicting the potential for infestations of any insect pest is risky business. As the spring season unfolds, we should get a better handle on occurrences of all of our little insect foes and friends.--Kevin Steffey and Mike Gray

Author: Kevin Steffey Mike Gray


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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