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Issue No. 1, Article 4/March 22, 2012

Mild Winter, Record-Breaking March Temperatures: How Will Field Crop Insects Respond?

The mild winter and string of record-breaking highs in March have generated considerable interest in the potential effects on insect survival and infestations this growing season. To address the issue most effectively, I think it is useful to separate insect pests of field crops into two broad categories--those that spend the winter in much of the Corn Belt and those that migrate into the Midwest from more southerly latitudes. Obviously, the survival of insects that migrate into Illinois during the spring and summer will not be affected by the mild winter we’ve experienced. Provided here is commentary on (incomplete lists of) insect pests of field crops most familiar to many of us.

Selected Migratory Insects: Pests of Illinois Field Crops

Armyworms--overwinter in soil as pupae in southern states; northward migration of moths begins in April and May.

Black cutworms--overwinter in Gulf Coast states. Moths migrate northward beginning in February, with flights intensifying during April and May in Corn Belt states. Late-planted fields infested with winter annual weeds are most susceptible to economic infestations.

Corn leaf aphids--overwinter on grasses in southern states; winged adults (alates) fly northward in early spring. Colonies are most often noticed in whorl- to late-whorl-stage corn plants.

Corn earworms--overwinter as pupae in the soil. Moths are robust and strong flyers; migration occurs from northern Mexico, Texas, Florida, and North Carolina to cornfields in the Midwest and as far northward as Canada. Fields with fresh silk are prime targets for egg-laying moths.

Fall armyworms--overwinter as partially grown larvae in Gulf Coast states where the ground does not freeze; migratory moths fly into Midwest during summer and early fall.

Green cloverworms--overwintering occurs south of 41°N latitude (Champaign, Illinois, is 40.11°N; DeKalb, Illinois, is 41.9°N), more northern latitudes are colonized by migrating moths.

Potato leafhoppers--overwintering occurs in southern Gulf Coast states; migrating adults typically begin to occur in alfalfa fields in Illinois by late May through early June.

Selected Non-Migratory Insects: Pests of Illinois Field Crops

Bean leaf beetles--overwinter as adults in plant debris, often in wooded areas; adults become active in early to late April, initially flying to stands of alfalfa. Early-planted soybean fields are at most risk.

Corn flea beetles--overwinter as adults within clumps of grasses near cornfields. Beetles become active in the spring, serving as vectors of Stewart’s wilt (Pantoea stewartii), a bacterial pathogen.

European corn borers--overwinter as full-grown larvae within corn residue (stalks, corn ears). There are two generations per year across much of the Corn Belt; early planting favors establishment of the first generation, while late planting tends to favor the second.

Grape colaspis--overwinters as partially grown grubs in the soil. Larvae begin feeding on roots in early spring, and adults (beetles) commonly emerge in July in Illinois cornfields.

Southwestern corn borers--overwinter as mature larvae in overwintering cells in corn stalks slightly below the surface of the soil. Pupation occurs in the spring; emerging moths lay eggs on whorl-stage corn.

Soybean aphids--overwinter as eggs on their primary host, common buckthorn. Early-planted soybean fields are most at risk to potential injury.

Stalk borers--overwinter as eggs; hatch occurs late April through early May. Larvae feed on many hosts, including broadleaf weeds. Moths emerge in late August; females lay eggs on several grass species as well as some broadleaf hosts, such as giant ragweed.

Stink bugs--overwinter as adults in plant residue near fields; the most suitable overwintering sites are alfalfa, wheat, and rye cover crops.

Western bean cutworms--overwinter as prepupae in the soil and pupate in late spring through early summer; moths emerge in midsummer.

Western corn rootworms--overwinter as eggs in the soil, with most eggs laid in the top 4 inches. Low soil temperatures and lack of snow cover for extended periods can reduce survival. Hatch usually occurs in late May through early June.

White grubs and wireworms--overwinter as grubs (larvae) in the soil below the frost line.

The mild winter will very likely improve the survival of some species that overwinter in Illinois (e.g., corn flea beetles, bean leaf beetles, soybean aphids, white grubs). For others the mild winter may be a neutral factor, since many insect species, including European corn borers and western corn rootworms, are superbly adapted to survive even the most severe winters, especially if snow cover is present.

The record-breaking warm temperatures this March will likely hasten the emergence and development of some overwintering insect pests of field crops as well as those that are new arrivals, such as black cutworm larvae that will hatch from eggs laid on winter annual weeds. Ultimately, insect pest densities in Illinois will likely be more affected by spring and summer weather conditions.

Spring precipitation patterns and how they affect planting dates of corn and soybeans are critical influences on the densities of many insect pests. A summer drought may lead to a twospotted spider mite outbreak, whereas a mild to moderate summer can help intensify soybean aphid problems. The bottom line? Each growing season is different, and accurately predicting insect outbreaks is not easy this early in the year.--Mike Gray

Mike Gray

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