
Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (Jan. 1 - June 19, 2013). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Damage and Management
Adults prefer to feed between soybean leaf veins, but can ultimately consume most of the leaf (Photo 2). The treatment threshold for Japanese beetles in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom. Most people tend to overestimate plant defoliation, but this reference can help with more accurate estimations. In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping silks during pollination (Photo 3). Consider a foliar insecticide during tasseling and silking if there are three or more beetles per ear, silks have been clipped to less than ½ inch, and pollination is less than 50 percent complete.

Photo 2. Japanese beetles skeletonize soybean leaves. Photo by Mark Licht, ISU.

Photo 3. Japanese beetles are strongly attracted to silking corn. Photo by Erin Hodgson, ISU.
source: iastate.edu