For the last few weeks, Iowa State University Extension crop specialists have been seeing Japanese beetle activity throughout Iowa. Ornamentals, trees and shrubs have large numbers of adults feeding and mating, and defoliation is becoming obvious in our small plot research. In addition, ISU field agronomists have been reporting defoliation in soybeans.
"Although Japanese beetles have been reported here since 1994, plant damage has been erratic," says Erin Hodgson, ISU Extension entomologist. "I strongly encourage growers and crop consultants to scout corn and soybean fields for this pest this year." Hodgson provides the following information and suggestions.
You need to scout Japanese beetles and understand their life-cycle
Japanese beetles have one generation per year. Adults emerge from grass in late June and begin to feed on low-lying plants, and eventually move up on trees and field crops to feed and mate. Mated females move back to grass in August and September to lay small egg masses in soil cavities.
The eggs hatch into small white grubs that feed on roots underground until late September when the temperature cools. The almost fully-grown grubs burrow down in the soil and remain inactive all winter. In the early spring, grubs become active again and feed until turning into resting pupae. The pupae hatch into adults that emerge from the soil.
How much damage do these beetles cause? Can you control them?
Adults prefer to feed between soybean leaf veins, but can ultimately consume most of the leaf. The treatment threshold for Japanese beetles in soybeans is 30% defoliation before bloom and 20% defoliation after bloom. Most people tend to overestimate plant defoliation, but this reference can help with estimations.
In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping corn silks during pollination Consider applying a foliar insecticide during tasseling 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.
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. If corn pollination is complete, Japanese beetles may not be economically important anymore. Also consider a border treatment if these beetles are aggregated in the edge rows.