This is a repeat from an article from last year at about this same time. Japanese beetles are beginning to show up in numbers. Fortunately, they seldom cause economic injury to field crops. In soybean, treatment is recommended when 35% or more defoliation occurs prior to bloom or when 20-25% defoliation occurs from R1 to R6. My experience suggests that 1 beetle per sweep may cause this level of injury, but this depends on the size of the beans and how long the beetles persist.
In corn, the only real concern is silk feeding. We’ve done research on this. The suggested threshold is 3 or more beetles per ear during the first week of silking while ears are pollinating. It is unusual to see this level of infestation, except perhaps on field edges. Our data suggest there is more chance of yield loss when plants are under significant drought stress.
Pyrethroid insecticides are typically used to control Japanese beetles. Suggested insecticides and rates are listed in UT’s Insect Control Recommendations for Field Crops.