Corn injury from Japanese beetle

Corn injury from Japanese beetle feeding in a Saunders County, Neb., field.

Editor’s note: The following was written by Robert Wright, University of Nebraska Extension entomologist, and Justin McMechan, crop protection and cropping systems specialist, for the Extension Crop Watch website June 28.

Japanese beetle adults are beginning to emerge in eastern Nebraska. Their distribution has been increasing in Nebraska the last few years, and they are being seen in corn and soybeans more frequently.

They will continue to emerge for the next few weeks.

Japanese beetles have one generation per year. They often feed in clusters due to an attraction to the female sex pheromone and an attraction to volatile chemicals produced by damaged plants.

Japanese beetles — along with a complex of other insects, such as bean leaf beetles, grasshoppers and several caterpillar species — can contribute to defoliation in soybeans. They feed by skeletonizing the leaves, leaving only the leaf veins. They feed primarily in the upper canopy, making the damage very visible.

In soybeans, insecticide treatment is recommended when insects are present and damage is expected to exceed 30% defoliation in vegetative stage and 20% in reproductive stage soybeans.

Similar to corn rootworm beetles, Japanese beetles will scrape off the green surface tissue on corn leaves before silks emerge, but prefer silks once they are available. They may interfere with pollination if abundant enough to severely clip silks before pollination. University of Illinois Extension recommends an insecticidal treatment should be considered during the silking period if:

  • there are three or more Japanese beetles per ear,
  • silks have been clipped to less than ½ inch, and
  • pollination is less than 50% complete.

Be aware that Japanese beetle numbers are often highest on field margins, so scout across the whole field before making a treatment decision. Japanese beetle adults are about 1/2 inch long and have a metallic green head and thorax. A key characteristic is a series of white tufts of hair on each side of the abdomen.

A variety of insecticides labelled on corn and soybeans would be expected to provide control of Japanese beetles. See product labels for rates and restrictions.

In some cases people have mistaken the Japanese beetle for its look-alike, the false Japanese beetle, or sand chafer.

False Japanese beetle adults are about the same size as Japanese beetles, but do not have a metallic green head. They may vary in color from coppery brown to black. They may have some white hairs on the side of the abdomen but they are not organized into tufts of hair.

Sand chafers are often noticed because they have a habit of landing on people and seem to be attracted to people wearing light-colored clothing. They have not been reported to cause economic damage to crops as adults, although the immature white grub has been reported to cause damage to potato tubers.