Western bean cutworms are active in sweet corn

Adult flight and egglaying is underway in many areas of Michigan.

Western bean cutworm is a relatively new pest of corn in Michigan, being first detected in 2006 in field corn. This insect spread to the Midwest from western states through expanding its range. We now have resident populations since it can overwinter in Michigan inside earthen chambers in the soil. Adults start emerging at 1320 degree days (base 50ºF); therefore these are late season pests, and they complete only one generation in Michigan. Check the MSU Enviro-weather website for degree day accumulations in your area.

In Michigan, adults usually start flying around mid-July. Eggs are laid in groups of 5 to 200 on the upper side of corn leaves. Newly hatched larvae initially feed on the developing tassel, pollen, leaf tissue and eventually move to the kernels. Larval development lasts for about a month. This insect is not cannibalistic, unlike the corn earworm, thus there can be multiple larvae per ear that feed anywhere on the ear. Superficially, the larvae resemble corn earworm larvae, but two distinctive dark brown stripes immediately behind the head are characteristic for western bean cutworm larvae.

Monitoring and scouting

Western bean cutworm flight monitoring in sweet corn is underway by MSU Extension educators across the state. The first report of adult moths caught in pheromone traps was received the week of July 11, 2011 from the Saginaw area. At this time, there were only two moths caught in one trap. A week later (last week, the week of July 18) two adult moths were captured in the Hart area. Since most of the state has surpassed the 1320 degree day accumulation already, this means that attention to monitoring is increasingly important at this time. You can follow the numbers of insects caught in traps placed in sweet corn fields at the MSU Vegetable Entomology website, or for field corn at the MSU Field Crops Entomology website.

If you are interested in making your own monitoring trap from a milk jug, read the Western Bean Cutworm milk jug trapping factsheet for instructions.

Scout for the presence of eggs or young larvae by inspecting the upper four leaves of 100 plants per field.

Control recommendations

The timing of insecticide applications is critical (more so than the type of insecticide used) since once the larvae enter the ear, control is nearly impossible. If the eggs have not hatched yet, time the insecticide to when most of the eggs are expected to hatch and when 90 percent of the tassels have emerged. Eggs turn from creamy-white to dark-purple one to two days before hatching. In sweet corn, the treatment threshold is 4 percent (four of 100 plants with eggs or larvae) for processing sweet corn and 1 percent for fresh-market sweet corn.

For more information on the biology of the insect and to see pictures, visit the following websites.

Dr. Szendrei's work is funded in part by MSU's AgBioResearch.

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