Description:
Adults are ½ inch in length, are metallic green with bronze-colored elytra (wing covers). Just below the elytra, on each side of the abdomen are six tufts of white hair. Eggs vary in color from translucent to creamy white. The larvae, or grubs, are about 1 inch in length. The C-shaped grubs are creamy white with a brown head capsule. Japanese beetle grubs can be identified by the V-shaped pattern of bristles on the raster. Pupae of Japanese beetles resemble the adult, except the legs, antennae, and wings are closely folded into the body. As the pupal stage nears its end, the color of the pupa changes from a pale tan color to metallic green.
Damage Symptoms:
In soybeans, grubs also feed on the root hairs of seedling plants, though the injury is generally not of economic importance. The main threat of Japanese beetle adults in soybeans is the defoliation of soybean plants. Much of the defoliation of soybeans occurs during the reproductive stages of plant development. Individually, Japanese beetles may not cause sufficient injury to warrant treatment, but when combined with the feeding of other soybean defoliators, treatment may be needed occasionally.
Scouting: It’s important to scout flowering soybean fields for the presence of the Japanese beetle and the extent of defoliation. Estimate the number of beetles per foot of row and examine the plants to determine if any blooms are being destroyed. Estimate the percent defoliation in at least five different areas of the field. Insecticidal treatments should be considered when defoliation reaches 30 per cent before bloom and 20 per cent before bloom and pod fill. Japanese beetle traps are most useful for detecting Japanese beetle emergence and monitoring population fluctuations.
Management:
Management of adult beetles focuses on the prevention of defoliation in soybeans and silk clipping in corn from exceeding economic levels. Scouting entire fields is essential in determining the extent of injury of a crop. Japanese beetle injury will add to other stresses that the crop is experiencing. There are many insecticides labeled to control Japanese beetles. When making a decision regarding the use of an insecticide, take into account several factors: the density of Japanese beetles, amount of injury (silk clipping in corn, defoliation in soybeans), level of crop stress, yield potential, and cost of control.
Sources:
University of Illinois – http://ipm.illinois.edu/fieldcrops/insects/japanese_beetles/
Iowa State University Extension, Soybean Disease and Pest Management Field Guide. 2008: 44.
