Integrated Crop Management

Japanese beetles defoliating soybean near Cedar Rapids

Jim Fawcett, Extension field specialist-crops, and Brad Buchanan, independent crop consultant, both report large numbers of Japanese beetles in soybean in the Cedar Rapids area. On July 14, Brad was collecting as many at 60 to 100 beetles per 20 sweeps with a net (hot spots in the field) and small areas of defoliation were probably showing 50-60 percent defoliation. I collected adults in a soybean field near the Iowa State University Horticultural Farm in Story County on July 19.

Adult Japanese beetles have a metallic grean head and bronze-colored wing covers.

Japanese beetles feeding on soybean leaves.

The Japanese beetle is easy to recognize. 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. When they are fully warmed up by the sun, they can fly rapidly. There is a single generation in the Midwest.

This pest can feed on soybean from late June to early September, with the heaviest defoliation in July and August. Japanese beetle 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. Before spraying an entire field, determine the location of the population and spray only those areas that have large numbers of beetles and defoliation that has reached the threshold.

Insecticides labeled for Japanese beetles in soybean

Product Product/Acre
Asana XL* 5.8-9.6 oz
Baythroid 2* 1.6-2.8 oz
Mustang Max* 2.8-4 oz
Penncap-M* 3-4 pints
Pounce 3.2EC* 2-4 oz
Sevin XLR Plus 0.5-1 qt
Warrior* 3.2-3.84 oz

*Restricted use insecticide.

This article originally appeared on pages 131-132 of the IC-490(18) -- July 28, 2003 issue.

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