Japanese Beetle in Soybean

Crop Damage

Japanese beetle (Popillia japonica) is an invasive insect pest established throughout much of the United States and Canada. Adults feed on more than 300 species of ornamental and agricultural plants. In soybean, larvae feed on roots but rarely cause economic injury. Most crop damage occurs when large numbers of adult beetles aggregate on plants and feed on leaves. Adults consume leaf tissue between the veins, creating a characteristic skeletonized appearance. Severe infestations can result in substantial defoliation and yield loss.

Pest Description

Adult Japanese beetles are approximately 1/2 inch long with metallic green bodies, bronze-colored forewings, and clubbed antennae. The forewings do not completely cover the abdomen, exposing the tip of the body. A distinguishing characteristic is the presence of six white tufts of hair along each side of the abdomen.

Larvae are cream-colored, C-shaped grubs that reach approximately 1 inch long when fully grown. They have an orange-brown head capsule and a body covered with fine brown hairs. The end of the abdomen is typically darkened and bears distinctive V-shaped rows of spine-like hairs.

Scouting

Japanese beetles often become established and persistent once introduced into an area. Fields with grassy areas, weeds such as smartweed and evening primrose, high organic matter, or cover crops may be at greater risk for infestation. After cover crops are terminated, adult beetles may move to nearby soybean fields in search of suitable hosts.

Begin scouting in high-risk areas, including field edges and locations where weeds or cover crops are present. If beetles are detected, scout the entire field. Adult beetles often aggregate in large groups and feed on the upper canopy, causing localized areas of severe defoliation. Because feeding injury is often unevenly distributed, evaluate plants across the field and throughout the entire canopy when estimating defoliation levels.

Management

If Japanese beetles are confined to field edges or isolated areas, spot treatments may be sufficient to manage infestations. When injury is widespread, consider treatment when defoliation reaches 30% during vegetative growth stages or 20% during reproductive growth stages. Use the Crop Protection Network’s Insect Defoliation Tool to improve the accuracy of defoliation estimates and treatment decisions. Lower treatment thresholds may be warranted when other defoliating insects are present.

Several insecticides are labeled and effective against adult Japanese beetles. However, a single foliar insecticide application may not provide season-long control because adults are highly mobile and can readily reinfest soybean fields from surrounding hosts. Research on soybean resistance to Japanese beetles is ongoing. Cultural practices such as weed management, cover crop selection, and tillage may help reduce future populations. Although several natural enemies of Japanese beetles have been identified, their impact on soybean infestations is not well understood.

Developed and reviewed by Carol Pilcher, Iowa State University and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.