How will insects respond to this strange weather?
How will the mild winter and record-breaking high temperatures in March affect insect survival and infestations in Illinois in the coming growing season?
According to professor of entomology and crop sciences Extension coordinator Mike Gray, it depends on whether the insect pests spend the winter in the Corn Belt or whether they 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," said Gray.
Gray lists the patterns that are typical of the most common migratory and non-migratory insect pests of Illinois field crops.
Selected migratory insects:
• Armyworm pupae overwinter in the soil in the southern states. The moths begin to migrate northward in April and May.
• Black cutworms overwinter in the Gulf Coast states. The moths begin to migrate northward beginning in February. Flights intensify during April and May in Corn Belt states. Late-planted fields infested with winter annual weeds are most susceptible to economically-important infestations.
• Corn leaf aphids overwinter on grasses in the southern states. The winged adults (alates) fly northward in early spring and form colonies in whorl to late-whorl stage corn plants.
• Corn earworms overwinter as pupae in the soil. The moths, which are robust and strong flyers, migrate from northern Mexico, Texas, Florida, and North Carolina to corn fields in the Midwest and as far northwards as Canada. Fields with fresh silk are prime targets for egg-laying moths.
• Fall armyworms overwinter as partially grown larvae in the Gulf Coast states where the ground does not freeze. Moths migrate to the Midwest during summer and early fall.
• Green cloverworms overwinter south of latitude 41ºN (Champaign, IL is 40.11ºN, DeKalb, IL is 41.9ºN); migrating moths colonize more northern latitudes.
• Potato leafhoppers overwinter in southern Gulf Coast states. Migrating adults typically appear in Illinois alfalfa fields from late May through early June.
Selected non-migratory insects:
• 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 greatest risk for damage from these pests.
• Corn flea beetles overwinter as adults in grasses near corn fields. 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). Two generations occur per year across much of the Corn Belt. Early planting favors the establishment of the first generation; late-planting favors the second generation.
• Grape colaspis overwinter as partially grown grubs in the soil. Larvae begin feeding on roots in early spring. Adults (beetles) commonly emerge in July in Illinois cornfields.
• Southwestern corn borers overwinter as mature larvae in overwintering cells of corn stalks slightly below the soil surface. 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 for damage from these pests.
• Stalk borers overwinter as eggs on their weed hosts. Hatch occurs in late April through early May. Larvae feed on many hosts, including broadleaf weeds. Moths emerge in late August. The females lay eggs on several grass species as well as on some broadleaf hosts such as giant ragweed.
• Stink bugs overwinter as adults in plant residue, particularly near fields of alfalfa, wheat, or rye cover crops.
• Western bean cutworms overwinter as pre-pupae in the soil, pupating in late spring through early summer. The moths emerge in midsummer.
• Western corn rootworms overwinter as eggs in the top four inches of the soil. Low soil temperatures and lack of snow cover for extended periods during the winter can lead to reduced survival. Hatching is usually in late May through early June.
• White grubs and wireworms overwinter as grubs (larvae) in the soil below the frost line.
According to Gray, "The mild winter will very likely improve the survival of some insect species, such as corn flea beetles, bean leaf beetles, soybean aphids, and white grubs that overwinter in Illinois." For other overwintering species, the mild winter may be a neutral factor. "Many insect species, such as European corn borers and western corn rootworms are superbly adapted to survive even the most severe winters, especially if snow cover is present," he said.
Moreover, the record-breaking warm temperatures this March will likely hasten the emergence and development of some overwintering pests as well as those that are new arrivals. For example, black cutworm larvae will hatch and develop faster on winter annual weeds. "Ultimately, insect pest densities in Illinois will likely be more affected by the spring and summer weather conditions that develop," said Gray.
Spring precipitation patterns and their influence on planting dates of corn and soybeans is a critical factor influencing the densities of many insect pests. A summer drought may lead to a twospotted spider mite outbreak; more mild to moderate summers can help trigger more intense soybean aphid problems. "Bottom line," he said, "each growing season is different, and accurately predicting insect outbreaks is not easy this early in the year."
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