What Can We Expect from Insect Pests Following Another Mild Winter?

March 17, 2000
The decade of the 1990s will long be remembered as producing some of the hottest summers and warmest winters. For several consecutive winters, we have had very mild temperatures. The winter of 1999­2000 will likely go down in the record books as one of the meekest since we began to amass meteorological data. How many of us can recall so many record-breaking warm and balmy days in February? Because of the mild temperatures, many folks logically begin to ask questions regarding the likelihood of insect pest outbreaks for the upcoming growing season. I'll try to address this topic by offering a quick snapshot of the life cycles and overwintering habits of some key insect pests in alfalfa, corn, and soybeans.

Alfalfa weevil adult.

The two key insect pests of alfalfa in Illinois include alfalfa weevils and potato leafhoppers. Alfalfa weevils spend the greater part of winter in Illinois as either eggs or adults. On warm (temperatures generally above 50°F) sunny winter days, many adult alfalfa weevils break their winter dormancy and begin to move about stands of alfalfa. In fact, winter egg laying can take place during extended periods of warm weather. Most of the early-season injury to alfalfa occurs in southern Illinois when larvae begin to feed on tender terminal leaves of alfalfa plants. It stands to reason that more winter egg laying can take place in the warmer latitudes of southern Illinois. In winters, such as this most recent one, some winter egg laying (even in northern Illinois) by adult weevils most likely took place during some of those record-breaking 70°F days in February. This being the case, alfalfa growers in central and northern Illinois counties should anticipate earlier-than-normal larval feeding this spring. Harvesting alfalfa early in northern Illinois typically can prevent economic losses. However, because of the mild winter and subsequent early egg laying, producers should be extra vigilant this spring and be prepared to apply rescue treatments to prevent excessive larval feeding.

Potato leafhopper adult.

The other major insect threat to alfalfa is the potato leafhopper. Many alfalfa growers throughout Illinois will long remember 1999 as one of the worst "leafhopper years" in recent memory. Because potato leafhoppers are migratory insects and travel to midwestern stands of alfalfa from more southern states, the mild winter we have experienced has little bearing on the likelihood of infestations for this growing season. So we promise to monitor the migration of leafhoppers into Illinois later this spring and let you know when they've arrived.

Bean leaf beetles.

The insect pest of soybeans most likely to be a problem following a mild winter is the bean leaf beetle. Bean leaf beetle adults spend their winters tucked away beneath leaf litter and debris within woodlot areas surrounding fields. When spring temperatures return to consistent daytime highs of 50°F or greater, bean leaf beetles stretch and shake off the winter blues and travel to nearby stands of alfalfa. Although initial feeding occurs in alfalfa, bean leaf beetles are not considered pests of this crop. Producers' soybean fields most at risk to bean leaf beetle defoliation include those that are planted earliest. In fact, many very early-planted soybean fields may serve as a trap crop for bean leaf beetles. Many other insect pests of soybeans, such as green cloverworms and thistle caterpillars (painted lady butterfly), migrate annually each spring and summer into soybean fields, and winter conditions have no effect on their abundance in the Midwest.

Black cutworm adult.

European corn borer larva.

Some of the major insect pests of corn include black cutworms, European corn borers, and western and northern corn rootworms. Corn rootworms spend the winter in Illinois as eggs. The depth at which corn rootworm eggs were laid last summer varies according to species and soil moisture levels. Western corn rootworm adults tend to lay their eggs a bit more deeply than northern corn rootworms. Both western and northern corn rootworm eggs can be found just below the soil surface and also at depths of 12 inches or greater. During dry summers, both species of corn rootworms have been observed crawling into drought cracks and laying eggs well below depths of 12 inches. In general, corn rootworm eggs of both species are adapted superbly to midwestern winter conditions. Egg survival throughout a winter can be compromised most for those eggs laid nearest the soil surface. This most often occurs when winter temperatures are unusually cold for prolonged periods of time and snow cover is lacking. We haven't had one of those winters for some time. European corn borers are tucked away safe and secure in cornfield residue (stalks and cobs) as fully developed (fifth-instar) larvae. Like corn rootworms, European corn borers are seldom affected by our winter conditions. Densities of both these major insect pests of corn are more influenced by spring environmental conditions. We'll discuss these factors in upcoming issues of the Bulletin later this year. Of the three major insect pests of corn, only black cutworms migrate into Illinois each spring. As you monitor your pheromone traps later this spring, let us know when you begin to capture moths.

Although we don't consider corn flea beetles as major pests in commercial corn, seed corn producers know that inbreds are often at economic risk from this insect. Of most concern to the seed industry is the potential for transmission of Stewart's disease or wilt to susceptible inbreds. It is commonly suggested that if the average monthly temperatures for December, January, and February sum to more than 90, flea beetle survival throughout the winter may be good. In our next issue of the Bulletin, we'll report what these winter averages look like for all areas of the state. Our guess is that the news will not be good concerning flea beetles this spring.

So, in predicting the effect of a mild winter on an insect population, it seems clear that one explanation does not fit all insects. Many of our most significant pest-management challenges come from those insects that migrate into our state. For several species that overwinter in Illinois, spring weather is a more critical environmental factor in setting the stage for a potential outbreak.--Mike Gray

Author: Mike Gray