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Western Bean Cutworm Guide

western bean cutworm larva

Western Bean Cutworm Management

Biology, Scouting and Control of the Western Bean Cutworm

Bob Wright, South Central Research & Extension Center
Ron Seymour, Extension Educator, Adams County, Nebraska

Updated July 24, 1996

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Western bean cutworm (WBC) moths are beginning to emerge. These large (3/4" long) moths are dark brown and have a white stripe on the leading edge of the upper wing.

Moths lay eggs on corn or dry edible field bean leaves. Corn fields in the late whorl stage are most attractive to the females for egg laying. Eggs are laid in masses of 5 to 200, usually on the upper surface of the top leaves. The eggs are about the size of a pinhead. When first laid, the eggs are white. As the eggs develop they turn tan, then purple just before hatching.

Newly hatched larvae are about 1/4 inch long and are dark brown. Young larvae are tan with a darker, faint diamond shaped pattern on their backs. As the larvae grow toward maturity, they become a plain pinkish-tan or pale brown and reach a body length of 1-1/2 inches. When the eggs hatch, the larvae first feed on pollen and then move to the corn ears or bean pods. The larvae will feed there for several weeks before they drop to the soil to form a subterranean overwintering chamber. By the end of the five larval instars, considerable feeding damage can occur. In corn, one larva per plant usually will not cause severe damage but the ears may contain up to 10 larvae, which can significantly reduce yield. This is because western bean cutworms are not cannibalistic, as compared to corn earworms.

Start scouting for the western bean cutworm with the beginning of moth flight in mid-July. In corn check 20 consecutive plants at five locations. If eight percent of the plants have an egg mass or young larvae are found in the tassel, consider applying an insecticide.

Timing of the application is critical. If the tassel has not emerged when the larvae hatch they will move into the whorl and feed on the developing pollen granules in the tassel. As the tassel emerges, the larvae will migrate down the plant to the green silks. The larvae will move down the silk channel and feed on the developing ear.

Once the larvae reach the ear tip, control is difficult. If an insecticide is needed, time the application so that 90-95% tassel emergence has occurred. If the tassels have already emerged, theapplication should be timed for when 70-90% of the eggs have hatched.

If an insecticide application is needed, corn fields should be checked for the presence of spider mite colonies. If mites are found, select a product that will not stimulate mite reproduction. Products that contain permethrin (Pounce, Ambush) or esfenvalerate (Asana) have been associated with increased mite reproduction. Other products labelled for western bean cutworm control on corn include Sevin 80S and XLR Plus, Lorsban 4E, Penncap-M, Capture 2EC and Warrior 1EC.

Suggested Treatments for Controlling Western Bean Cutworm in Corn