Ask an Agronomist
What Can We Do To Control Japanese Beetles?
Aug 15, 2011
Question: This is the first year we have had trouble with Japanese beetles. We keep checking the silks on corn and they are OK. We pulled back the shucks, and they are eating the kernels on the stalks that are still green. Anyone else having this problem? Nothing we can do, but it sure is depressing. Guess they might move to soybeans when all of the corn dries, and then we need to spray. How long will they continue to eat and lay eggs? They are doing more damage than an earworm.
Answer: We are seeing quite a few Japanese beetles this year across the eastern and western Corn Belts. Erin Hodgson, Department of Entomology, Iowa State University, says, "In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping silks during pollination. Consider a foliar insecticide during tasselling 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% complete."
At this point in the season, a majority of fields are probably beyond 50% pollination, so Japanese beetles may not be economically important anymore. As you’ve indicated, however, soybeans are at risk as well and require careful scouting. Traditional defoliation levels suggest economic thresholds are 30% before bloom and 20% between bloom and podfill. Because the soybean crop is so valuable this year, growers may want consider lower thresholds.
Notes Hodgson, "There are many insecticides labeled for Japanese beetle control; however, do not expect season-long control from a foliar application. Adults are highly mobile and move frequently in the summer. Japanese beetles release a strong aggregation pheromone and are commonly seen feeding and mating in clusters. Beetles present during the application will be killed, but beetles migrating into sprayed fields may not be controlled. If soybean defoliation continues, additional applications may be necessary to protect the seed-filling stage."