No. 16 Article 4/July 11, 2008

First Captures of Western Bean Cutworm Adults

In 2005 we joined with other university extension personnel and people in the ag industry in the north-central states to monitor for western bean cutworm adults with pheromone traps. From 2000 through 2007, the distribution of this pest, a native species, has expanded eastward from the western Iowa border to as far east as Michigan and Ohio. We recently learned that western bean cutworm adults were found in Ontario, Canada, during the week of June 30. Why the rapid spread? Speculation, with supporting data in the scientific literature, currently focuses on replacement by western bean cutworms of other ear-infesting caterpillars killed by the widely planted corn hybrids that express the Cry 1Ab endotoxin (YieldGard and Agrisure corn hybrids). Western bean cutworm larvae are not killed by the Cry 1Ab endotoxin. Consequently, western bean cutworms seem to be replacing other insects (e.g., European corn borer) in the vacated niche.

Although they are not currently a persistent or widespread threat to corn production in Illinois, western bean cutworms are of interest in our state because of experiences with the pest in Iowa between 2000 and 2004. If the western bean cutworm becomes firmly established in our state, we will have to add it to our list of insects to manage regularly, either by planting corn hybrids with Herculex Insect Protection 1 (which express the Cry 1F endotoxin that kills western bean cutworms) or by scouting for eggs and larvae and applying an insecticide if 8% or more of the corn plants are infested before larvae move downward to invade corn ears.

You can watch the progress of captures of western bean cutworm adults in pheromone traps at Iowa State University's "Western Bean Cutworm Monitoring Network" Web site. Click on "Trap sites" to view numbers of western bean cutworm adults captured in individual traps throughout the Midwest. Thus far, the numbers of moths captured have been very small. The earliest capture reported in Illinois was on June 24 in Mason County.

Western bean cutworm adult (photo courtesy of Marlin Rice, Iowa State University).

Western bean cutworm moths floating in antifreeze in a pheromone trap (photo courtesy of Marlin Rice, Iowa State University).

I remind everyone again that if you are operating a pheromone trap to monitor for western bean cutworms, make certain you have honed your insect-identification skills. Although the western bean cutworm pheromone used in the traps theoretically should capture only western bean cutworm males, other insects often are found in the rather crudely constructed milk-jug traps. Some of the insects captured are other species of Noctuidae (the family of moths that includes black cutworms, fall armyworms, and western bean cutworms) that might be easily confused with western bean cutworms, especially after the moths have floated in the antifreeze for a few days. A one-page flyer developed by John Obermeyer, IPM specialist at Purdue University, provides helpful information and photographs of the imperfectly preserved moths often found in western bean cutworm pheromone traps (Adobe PDF, 84 kb).

For an excellent overview of western bean cutworms, including identification of look-alike species, go to the North Central IPM Center Web site to access a short course that was presented and recorded on February 29, 2007. Time spent refreshing your knowledge now could save time, aggravation, and money in the near future.--Kevin Steffey

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