Many areas of Missouri are supporting high populations of Japanese beetle adults. Damage is occurring to many species of plants including ornamental, fruit, vegetable and field crops. Japanese beetle adults typically begin emerging in early June and continue to emerge in high numbers through the third week of July. Adult beetles feed for about a month during which time female beetles lay 40-60 eggs in the soil. These eggs hatch into white grubs within a couple of weeks and remain in the soil to feed and grow until next summer when they emerge as adult beetles. Japanese beetle adults often cause excessive damage to the 220 host plants on which they feed in the US. Although ornamental and fruit crops are most at risk, these beetles do attack corn and soybean crops in Missouri. 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 were 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 many host plants being 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 twelve white dots.
Japanese beetle adults often congregate in large numbers to feed on the foliage and fruit of host plants. 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 just foliage feeding is common on soybean. Damage to soybean foliage takes on a lacelike pattern as beetles avoid leaf veins when feeding. Feeding on corn silks can disrupt pollination and result in substantial yield losses. 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 pollination is less than 50% complete, 3 or more beetles are present per ear, and green silks have been clipped to ½ inch or less from the husk. 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.
For more information, consult Integrated Pest & Crop Management: volume 20, number 12 (https://ipm.missouri.edu/newsletters/IPCM/archives/fullissue/v20n12.pdf)
REVISED: September 30, 2013