Extension Communications 3614 Administrative Services Building Ames, Iowa 50011-3614 (515) 294-9915
7/22/03 Contacts: Marlin E. Rice, Department of Entomology, (515) 294-1101, merice@iastate.edu Rich Pope, Department of Entomology, (515) 294-5899, ropope@iastate.edu Laura Sternweis, Continuing Education and Communication Services, (515) 294-0775, lsternwe@iastate.edu Scout Now for Western Bean Cutworms AMES, Iowa -- Now is the time to start scouting for western bean cutworm eggs, according to Iowa State University Extension specialists. "Western bean cutworms have caused significant damage to some cornfields in Iowa since 2000," said Marlin E. Rice, ISU Extension entomologist. "To assist in scouting efforts for this pest, we've placed a network of pheromone traps in areas of historical western bean cutworm activity. These traps should reflect the emergence pattern within a county area. Some of the highest trap catches have been in Audubon, Crawford and Shelby counties in west central Iowa, plus Butler County in north central Iowa." Moths are now being collected and the results can be found at http://www.ipm.iastate.edu/westernbeancutworm, Rice said. Cornfields in the late-whorl stage are most attractive to the females for egg laying, added Rich Pope, ISU Extension program specialist. Eggs are laid in masses of five to 200, usually on the upper surface of the top leaves. The eggs are about the size of a pinhead. The eggs are white when first laid, then turn tan and finally purple just before hatch. Newly hatched larvae are approximately 1/4 inch in length and are dark brown, Pope continued. 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 1/2 inches. Larvae first feed on pollen and then move to the corn ears, feeding there for several weeks before they drop to the soil to 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. Rice recommends scouting for the western bean cutworm with the beginning of moth flight in mid-July. "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 if young larvae are found in the tassel, you should 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 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," Rice said. "Once the larvae reach the ear tip, control is nearly impossible," Rice added. "If an insecticide is needed, time the application so that 90-95 percent tassel emergence has occurred. If the tassels have already emerged, time the application for when 70-90 percent of the larvae have hatched. If an insecticide application is needed, check cornfields for the presence of spider mite colonies. If you find mites, select a product that does not stimulate mite reproduction." -30- Sidebar Insecticides labeled for western bean cutworm in field corn. Insecticide Rate per acre Comments Ambush* 3.2-6.4 oz. May cause mite flare up. Asana XL* 2.9-5.8 oz. May cause mite flare up. Baythroid 2* 2.1-2.8 oz.   Capture 2EC* 2.1-6.4 oz.   Lorsban 4E* 1-2 pt.   Mustang Max * 1.76-4.0 oz.   Penncap M* 2-4 pt.   Pounce 3.2EC* 2-4 oz. May cause mite flare up. Sevin XLR Plus 2 qt.   Warrior* 1.92-3.2 oz.   * restricted-use insecticide
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