Scout Corn and Soybean for Japanese Beetles

Figure 1. Japanese beetles will first feed on the leaves of soybean (left) or corn. In corn they'll move to the silks (right) when they become available. (Photos by Justin McMechan)
Figure 1. Japanese beetles will first feed on the leaves of soybean (left) and corn. In corn they'll move to the silks (right) when they become available. (Photos by Justin McMechan)

Scout Corn and Soybean for Japanese Beetles June 25, 2018

Nebraska map of Japanese beetle distribution
Figure 2. Shaded counties have established populations of Japanese beetles in one or more locations.

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 more frequently in corn and soybeans (Figure 1), in addition to feeding on landscape trees and shrubs. They will continue to emerge for the next few weeks. First identified in counties along the state's eastern border several years ago, the beetles were found as far west as Lincoln County in 2016 (Figure 2).

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 beetle numbers are often highest on field margins, so scout across the whole field before making a treatment decision.

Japanese beetle (Popillia japonica Newman)
Figure 3. Japanese beetle (Popillia japonicaNewman) (Photos by Jim Kalisch)
Sand chafer
Figure 4. Sand chafer, also known as the false Japanese beetle (Strigoderma arboricola)

Japanese beetles (Popillia japonica Newman, Figure 1) can contribute to defoliation in soybeans, along with a complex of other insects, such as bean leaf beetles, grasshoppers, and several caterpillar species. 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. For more information see Managing Soybean Defoliators, NebGuide G2259.

The following infographic describes a process to accurately assess defoliation levels by sampling from all portions of the canopy.

 
Infographic showing the steps to estimating insect defoliation in 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. Japanese beetles feed on corn silks, and 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:

  1. there are three or more Japanese beetles per ear,
  2. silks have been clipped to less than ½ inch, AND
  3. 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 ½ 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 or the Insecticides for Field Crops section of the 2018 Guide for Weed, Disease, and Insect Management in Nebraska (EC130) Nebraska Extension EC130 for rates and restrictions.

False Japanese Beetles

In some cases people have mistaken the Japanese beetle for its look-alike, the false Japanese beetle or sand chafer, Strigoderma arboricola, which is a native Nebraska insect found across most of the state. Sand chafers (Figure 4) are commonly found along the Platte River valley and other river valleys in Nebraska. 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.

They 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.