A network of pheromone traps has been placed throughout the state to assist in scouting efforts for western bean cutworm. Iowa State University Extension is cooperating with Pioneer Hi-Bred agronomists to survey for western bean cutworm emergence. Most traps are now in place and results are being posted on a western bean cutworm website. Traps cannot be used to predict which fields should be sprayed; rather, they can indicate those areas that have significant moth flights and where fields should be scouted.

Western bean cutworm pheromone trap.

Western bean cutworm pheromone trap.

Agronomists in western Iowa have found occasional egg masses, and one report in Butler County had up to 15 percent of plants with egg masses on July 7.

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 placed on the upper surface of the top leaves. The eggs are about the size of a pinhead. The eggs are white when first laid, and then they turn tan, and finally purple just before the eggs hatch. New larvae are approximately 0.25 inch in length and are dark brown. Young larvae are tan with a darker, faint diamond-shaped pattern on their backs. As the larvae mature, they become a pinkish tan or pale brown and reach a body length of 1.5 inches. Larvae first feed on pollen and then move to the corn ears, feeding there for several weeks before they drop to the soil where they overwinter. One larva per plant usually does not cause severe damage, but the ears may contain up to 10 larvae, which can substantially reduce yield because western bean cutworms are not cannibalistic, compared with corn earworms.

Western bean cutworm eggs are white when first laid, then turn tan, and finally become purple just before the eggs hatch. (Marlin E. Rice)

Start scouting for the western bean cutworm now. In corn, check 20 consecutive plants at five locations. The University of Nebraska recommends that if 8 percent of the plants have an egg mass or young larvae are found in the tassel, consider applying an insecticide. The western bean cutworm directions on the Sevin® XLR Plus label state that treatment should be made when "the infestation averages 15% and at 90 to 100% tassel emergence." Timing of the application is critical. If the tassel has not emerged when the eggs hatch, larvae will move into the whorl and feed on the developing pollen grains in the tassel. As the tassel emerges, the larvae will move down the plant to the green silks and then into the silk channel to feed on the developing ear.

If an insecticide is needed, apply it when 90 to 95 percent tassel has emerged. If the tassels have already emerged, the application should be timed for when 70 to 90 percent of the eggs have hatched. Once the larvae reach the ear tip, control is nearly impossible. 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 does not stimulate mite flare ups (increased population growth). Mite flares are especially a concern in moisture-stressed fields, which are common in many parts of Iowa.

Insecticides labeled for western bean cutworm in field corn.

*Restricted-use insecticide