Several Iowa State University Extension and Outreach Field Agronomists have reported fields with high numbers of grubs this spring. There are a number of grub species in Iowa, including Japanese beetle. With warm temperatures accelerating insect development, expect adult Japanese beetles to begin emergence in southern Iowa counties this weekend (Figure 1). The emergence is about 7-10 days ahead of the last few years. Literature shows Japanese beetle adults need about 1,030 growing degree days (base 50°F) to complete development and will continue emergence until around 2,150 degree days.
Plant Injury and Management
Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. On corn, silk clipping can interfere with 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. On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf. The treatment threshold for Japanese beetle in soybean is 30% defoliation before bloom and 20% defoliation after bloom. It is important to note most people overestimate plant defoliation. Migrating adults could reinfest the field in after knocking down an initial population. I recently published a review article for Japanese beetle if you want to learn more about this corn and soybean pest.
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