This week I heard the first reports of Japanese beetles (JB) becoming active throughout Iowa. I also saw a few around the Insectary in Ames. Although they are beautiful beetles, most homeowners and growers do not like to see these insects around. They have a wide host range and defoliate over 300 plants, including corn, soybean, fruit trees and many ornamentals. JB aggregate when they feed and can cause severe defoliation. Sometimes they will clip corn silks and interfere with pollination. Foliar treatments may be warranted in corn if: there are 3 or more beetles per ear, silks have been clipped to less than ½ inch, AND pollination is less than 50% complete. Treatment thresholds in soybean are based on percent defoliation (30% before bloom and 20% after bloom). JB may be clustered around field edges, so be sure to sample throughout the field to get an accurate estimation of density. A border treatment may be possible to save money and protect beneficial insects.
Japanese beetle adults are metallic green and bronze with white tufts along the side. Photo credit to Marlin E. Rice.