European Corn Borer
Ostrinia nubilalis (Hubner), Pyralidae, LEPIDOPTERA)

DESCRIPTION

Adult - The female moth has a robust body and a wingspread of about 25.5 mm. It is pale yellow to light brown. The outer third of the wings is usually crossed by two dark, zigzag lines. The male moth is smaller, more slender, and darker than the female. The outer third of its wings is usually crossed by two zigzag streaks of pale yellow, and often there are pale yellow areas on forewings.

Egg - Each white egg is about half the size of a pinhead. It changes to pale yellow and darkens just before hatching as the brown head of the borer inside becomes visible. The eggs overlay each other like fish scales. Color plate.

Larva - The newly hatched larva, about 1.5 mm long, has a black head, five pairs of prolegs, and a pale yellow body bearing several rows of small black or brown spots. It develops through five or six instars to become a full-grown larva about 25.5 mm long. Color plate 1, Color plate 2.

Pupa - The brown pupa is 13 to 15 mm long with a smooth, capsule-like body.

BIOLOGY

Distribution - Within the United States, this pest is found in all the major corn-producing states of the Plains, Midwest, and South. In North Carolina, the highest numbers occur in the Coastal Plain where 75 percent of the stalks in some fields have been attacked. However, 40 percent lodging due to borers has been observed where harvesting was delayed in the Piedmont.

Host Plants - The European corn borer has been found on more than 200 host plants, but corn is a preferred host. In addition to corn, crops likely to suffer primary economic damage include barley, beans, millet, oats, Irish potatoes, and sorghum.

Damage - Although a prevalent insect pest of sorghum, the European corn borer is less damaging than pests which attack the grain portion of the plant. In corn, however, this borer is a serious pest. Both corn and sorghum are attacked in approximately the same manner.

Initial feeding occurs on the leaf surface, generally in the whorl. Later the larvae bore down midribs of leaves into stalks. Frass and silk, near entrance holes, are evidence of their presence. Primary damage to conducting tissue may result in a reduction of starch and sugar reaching the grain. Serious tunneling damage by subsequent generations may weaken plants so extensively that tassel and stalk breakage, ear dropping, and small ears may occur.

Life History - Mature larvae overwinter inside tunnels in stubble, stalks, ears, or other protective plant material. They pupate in the spring. During late spring, the adult moths emerge and mate. Each female lays 500 to 600 eggs in small masses of 15 to 20 on the underside of leaves. Eggs hatch in 3 to 12 days, depending upon the temperature. The young larvae usually begin feeding on the leaf surface and, as they mature, begin boring in the midribs of the leaves. During the 4th instar, stalk- or ear- boring commences and continues until pupation. In Florence, South Carolina, the European corn borer completes four generations per year and may do so in parts of North Carolina too. If this is the case, eggs of the second generation are laid in mid- to late June, those of the third generation in late July, and those of the fourth generation in September. This last generation is not a problem on corn.

CONTROL

Many natural parasites of this corn borer, mainly flies and wasps which have been introduced from Europe, exist in corn-growing areas. Other biological control agents such as ladybird beetles, predaceous mites, and downy woodpeckers have also been responsible for some borer reduction. The bacterium Bacillus thuringiensis, however, shows most promise for borer control.

The most effective cultural method of control has been stalk destruction. This is accomplished by cutting corn for silage; disking; using stalk cutters, shredders, and flailers; and clean fall plowing. If all stalks and other debris in corn fields is completely plowed under in the fall, few borers will survive the winter. However, a neighborhood effort is necessary since the adults are strong fliers.

In North Carolina, crop destruction, resistant varieties, and early harvest provide the most logical control. In former years, insecticides as a first line defense proved ineffective in this state, but recent research indicates that chemical control of first-generation larvae may now be practical. However, thresholds have not yet been established in North Carolina. For further control information, consult the current North Carolina Agricultural Chemicals Manual.