Integrated Crop Management

Japanese beetles in corn and soybean

The Japanese beetle is a minor pest in Iowa. Kevin Steffey, extension entomologist at University of Illinois, reported significant problems during mid-July in both corn and soybean, and 10,000 acres was sprayed for this pest in Illinois. In Iowa, Jim Fawcett, Iowa State University extension field specialist-crops, has found them again in soybean near Cedar Rapids in Linn County. Some fields have been sprayed because of defoliation. This year large populations of Japanese beetles have been found in Story County in central Iowa. John Kennicker, Iowa State University extension field specialist-crops, and I found large populations in both corn and soybean southwest of Nevada in Story County. This is the first report of Japanese beetle injury from central Iowa in either corn or soybean.

The Japanese beetle is an imported pest and is not native to Iowa. The adults have a metallic green head and pronotum (neck region) and reddish bronze wing covers with a row of white hair tufts along the abdomen. There is a single generation in the Midwest. Beetle feeding on soybean occurs from late June to early September, with the heaviest defoliation in July and August. This insect has been a significant pest in soybean in Illinois, Indiana, and Ohio for many years. There are no thresholds based on the number of beetles per plant or unit area. An economic threshold in reproductive-stage soybean is 20 percent defoliation. A threshold in corn would be the silks clipped to within 1/2 inch of the husk.

Japanese beetles feeding on soybean leaves.

Japanese beetles feeding on corn silks.

Japanese beetle adults feed on corn leaves, tassels, silks, and pollen and in soybean on leaves and flowers. They are capable of interfering with pollination in both crops, which may threaten yield potential. The pollination problem is more of a concern in corn where silk clipping by beetles can cause ears to be only partially pollinated. Most of the damage to either corn or soybean should be finished by the end of August.

This article originally appeared on pages 161-162 of the IC-488(20) -- August 19, 2002 issue.

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