ISU Field Agronomists around the state have noted Japanese beetle feeding injury in corn and soybean over the last week. Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. Adults prefer to feed between soybean leaf veins but can ultimately consume most of the leaf. They are easy to spot because of their size and metallic color, and are usually found along field borders. They are also very mobile and like to hang out in big groups. So when you see one, you usually see 10 more.
The treatment threshold for Japanese beetle 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 injury comes from clipping silks during pollination. Consider a foliar insecticide during tasseling and silking if: there are 3 or more beetles per ear, silks have been clipped to less than 1/2 inch, AND pollination is less than 50% complete.