University of Illinois

No. 14/June 25, 1998

Japanese Beetles Observed in Several Areas of the State

Robert Bellm, crop systems educator, Edwardsville Extension Center, observed Japanese beetle adults in Madison County in margins surrounding some soybean fields. In east-central Illinois, Michael Stanyard, a technical service representative with American Cyanamid Company also reported that Japanese beetles were showing up in some fields. These observations suggest that this versatile insect pest has made its appearance sooner than in many previous years. Adult Japanese beetles typically begin to appear in July and are common inhabitants of corn and soybean fields well into August. Japanese beetles are about 3/8-inch long; metallic green, with coppery brown wing covers; with five small tufts of white hair along the sides of the body (edges of wings) and two small tufts of white hair near the tip of the abdomen.

In corn, Japanese beetles can clip silks, similar to adult corn rootworms, and interfere with pollination. A treatment in a commercial cornfield may be justified if you find three or more beetles per ear, silk clipping is evident, and pollination is not complete. In soybeans, these colorful beetles can cause extensive defoliation. Generally, before the bloom and pod-fill stages of development, soybeans can withstand at least 30 percent defoliation before an insecticide is warranted. Before a treatment decision is made, make certain the entire field is scouted. Japanese beetles frequently congregate near field edges, and their fondness for each other in these margins may not be representative of their numbers further within the field. We will provide more discussions of this insect pest in future issues of this Bulletin.

Mike Gray ( and Kevin Steffey (, Extension Entomology, (217)333-6652