Japanese Beetles (Popillia japonica) Numerous in 2010

Published: June 13, 2010

During the past week reports of Japanese beetle adults have been received from across the state of Missouri. Populations of this pest vary by location with heavy numbers occurring in many locations in eastern, central, and southwestern Missouri counties. This is the time of year when most annual grub, including the Japanese beetle, complete their grub stages of growth, pupate, and emerge as adult beetles. Although the peak emergence for Japanese beetles usually occurs in late June into early July, high numbers of beetles are actively feeding on many ornamental plants and trees at this time. This pest will readily feed on most field crops with corn and soybean being favorite hosts for Japanese Beetles. This insect continues to disperse across Missouri with beetles being reported in areas where they have not been found in past years.

Infestations of this pest was first found in the United States near Riverton, New Jersey during 1916, following its accidental introduction in shipments of iris from its native country of Japan. During the mid 1900s infestations of this beetle in were found in the urban areas of St. Louis, Columbia, Kansas City, and Springfield, Missouri where they were probably introduced in the soil of container plants coming from infested areas of the US. About ten years ago these urban populations began to expand and disperse to the more rural areas of Missouri. This colonization of rural areas of Missouri continues today with ornamental plants, shrubs, and trees often the first to be attacked. Once populations are established in an area, damage to field crops is common. Japanese beetles are approximately 1/2–inch in length, metallic green in color with bronze or copper colored wing covers. A diagnostic characteristic is the presence of five white tufts of hair or bristles running down each side of the shell and two tufts of hair located on the tail end of the insect. Without magnification, these structures are seen as white dots. Japanese beetles can be confused with adult green June beetle, but are smaller in size than the June beetle. Adult Japanese beetles emerge from the soil in late May, June, and early July to feed for approximately 60 days. During this time the beetles mate and females deposit eggs in the soil. Each female may lay 40 to 60 eggs with larvae emerging in about 2 weeks. Females typically feed then move to the soil to deposit a few eggs. These actions will be repeated until all eggs are laid. Larvae will typically feed on grass roots and decaying material before overwintering in the soil as 3rd instars. The following spring larvae quickly finish development, pupate, and emerge as adult beetles beginning in early June in most years.

Japanese beetle adults often congregate in large numbers to feed on foliage and fruit of over 400 different host plants worldwide (approximately 220 in US). Hosts include a wide variety of ornamental, tree, small fruit, and field crop plants. Typical feeding damage by the beetles is often seen as a lace-like pattern on host plant foliage as beetles avoid leaf veins when feeding. Beetles often begin feeding on the top of plants and move downward. They tend to select plants which emit strong odors and often feed in large groups on host plants. Tassels and silks of corn can be severely damaged by adult feeding, whereas foliage feeding is common on soybean. Feeding on corn silks can disrupt pollination and result in substantial yield losses. Foliage feeding on soybean is less damaging, although small double-crop soybean may sustain economic damage. The grub stage of this pest will feed on plant roots of both corn and soybean with most feeding occurring in late June, July and August. Damage to plant root hairs may result in poor uptake of water and nutrients or be more severe and cause reduced stands through plant mortality.

In field corn, an insecticidal treatment is justified if during the silking period there are an average of 3 or more beetles present per ear, silks have been clipped to ½ inch or less in length, and pollination is less than 50% complete. For soybean, treatment is justified if foliage feeding exceeds 30% prior to bloom and 20% from bloom through pod fill. The following insecticides are recommended for control of Japanese Beetle in field corn and soybean in Missouri.