University of Illinois

No. 9/May 23, 1997

Webworms Damage Some Seedling Corn

As people have scouted cornfields this spring, they have encountered an array of insects that cause injury to corn seedlings early in the spring. Although we do not hear about webworms causing problems in corn very often, these infrequent pests occasionally cause concern, especially in corn planted after grass pasture or alfalfa. I am aware of webworms' having caused some injury in fields in Champaign, Randolph, and at least one other (unidentified) county in western Illinois. I verified the identification of the webworm found in Champaign County.

Webworm larvae usually are gray-brown, with black spots and coarse hairs (Figure 3). Full-grown larvae are about 1 inch long and very active. Initial observation of webworms sometimes leads people to believe the larvae are corn borers. However, webworms are more strongly spotted and have longer hairs. Besides, corn borer larvae at this time of year are fully grown, possibly still in diapause in most areas of the state, and they do not feed in the spring.

Figure 3. Sod webworm larva.

Webworms that attack corn overwinter as larvae inside cocoons spun the previous summer. Larvae become active in the spring and feed on corn until they are full-grown in June. Pupation occurs next to corn plants and a short distance underground. Injury caused by webworm larvae may resemble cutworm injury. Larvae defoliate plants, feed on leaf margins, cut plants near ground level, or feed just below the soil surface. Larval tunneling in furled leaves results in straight rows of shot holes across the leaves. The most severe injury occurs when plants are cut below ground or when larvae chew holes into underground portions of the stem and injure the meristem, which results in wilting or tillering of the plant.

Although no thresholds have been developed for webworms in corn, thresholds established for cutworms have been suggested because the injury is similar. In the 1997 Illinois Agricultural Pest Management Handbook, we suggest Lorsban 4E applied at 1 to 2 pt per acre for control of sod webworms. Because webworms tend to remain below ground, shallow incorporation with a rotary hoe or other suitable equipment immediately before or soon after treatment is recommended.

Kevin Steffey, Extension Entomology, (217)333-6652