Have You Seen This Pest? Japanese Beetles In Field Crops

Above: Japanese beetle adult. Beetles are large, between 1/3 and 1/2 inch long.

The Japanese beetle (Popillia japonica) is becoming an increasingly prevalent pest in the North Central region, and it can occasionally be an economic problem in soybeans or corn. The Japanese beetle should not be confused with the Asian lady beetle (which is often called a Japanese beetle by mistake). Asian lady beetles are familiar to many as the yellow or orange lady beetles that come into houses in the fall, and they are beneficial predators of crop pests. Japanese beetles are large (up to 1/2 inch long) and metallic green and copper colored. Adults feed on the leaves and flowers of over 300 plant species. They are an introduced pest first found in the U.S. in 1916 in New Jersey. Only in recent years have they become common in the Midwest. The South Dakota Department of Agriculture monitors for this pest with traps, and it has been detected in several counties, particularly in the Southeastern part of the state.

Japanese beetle immatures are soil-dwelling, white grubs that feed on roots and organic material, and they are often pests of turfgrass. The adults typically feed between the veins of leaves causing a characteristic lacy or “skeletonized” damage. They feed on a wide range of plants including various ornamentals, fruits, and vegetables. Though they are more common in horticultural settings, they will also feed in field crops, including corn and soybeans. In soybeans they cause defoliation of leaves, which reduces photosynthesis, and in corn they feed on silks, reducing kernel set. Though still a minor field crop pest, Japanese beetle outbreaks are becoming more common in Illinois and Iowa soybeans and corn. So far in South Dakota most reported problems with Japanese beetles have been in gardens near urban centers, but as it becomes more common in South Dakota, producers should also be on the lookout for this insect in crops.

Above: Japanese beetle feeding damage illustrating "skeletonization" on a leaf.

Japanese beetles have one generation per year and overwinter as grubs in the soil. Adults emerge from the soil in late May or early June and can be found through early September. Feeding damage is most noticeable in July and August. Japanese beetle feeding damage in soybean may be confused with bean leaf beetle feeding because both make holes in the leaves, but bean leaf beetle feeding produces more smooth-edged “shot-holes” in the leaves, whereas Japanese beetles create a lacy patchwork of holes between the veins. Also, unlike bean leaf beetles, Japanese beetles are not shy or skittish and are usually found easily at the scene of their crimes. Damage often appears first at field edges.

Soybeans can bear a fair amount of defoliation before yield is lost, so modest numbers of Japanese beetles and other defoliators can be tolerated.