During the past two weeks Japanese beetle adults began their annual emergence in many Missouri counties. In most areas their numbers will steadily increase through late June when peak numbers will result in damage to many trees, ornamental plants, and fruit and field crops. Adult Japanese beetles typically feed on green silks and tassels in corn, foliage feed on soybean, and damage the foliage and fruit of over 400 flower, shrub, and tree species.
This beetle was first found in the United States in 1916, following its accidental introduction from its native country of Japan. It is thought that grubs of this pest were introduced in pots of iris plants imported into the US prior to the initiation of federal plant and animal inspections in 1918. In Missouri, infestations of Japanese beetles were first found in the urban area of St. Louis in the early 1960’s followed by infestations being reported in Kansas City, Columbia and Springfield. These urban infestations were initially associated with golf courses and plant nurseries where grubs of this pest were again introduced in soil and plants imported from states with earlier infestations. Populations of this pest remained mainly in these urban area until about 10 years ago, when this pest spread into more rural areas of the state. The Japanese beetle in Missouri is still in a colonization stage of population growth with continued dispersal in most counties of the state. At present, most rural areas of Missouri will experience increasing populations of this pest for the next 7 to 10 years and maybe beyond. Beneficial biological pathogens and agents will eventually slow these expanding populations, resulting in annual population fluctuations at levels below peak populations experienced in earlier years.
Japanese beetle adults are approximately ½-inch in length, metallic green in color with bronze or copper colored wing covers. A diagnostic characteristic is the presence of twelve white tufts of hair or bristles located around the edge of the shell (five running down each side and two located at the very back end). Without magnification, these structures are seen as white dots. Japanese beetles can be confused with adult green June beetle, but are smaller in size. Adult beetles typically begin emerging from the soil in late May or early June, reach peak numbers in June into early July and then diminish during late July into August. Adults emerge, mate and feed for approximately 60 days. During this time each beetle female typically lays 40 to 60 eggs in groups of 1 to 8 into the soil with larvae emerging in about 2 weeks. Larvae will feed on plant roots and decaying material before overwintering in the soil as 3rd instars (worm or grub stage). The following spring larvae quickly finish development, pupate, and emerge as adult beetles. To this point in 2011, the adult beetles have been slow to emerge due to very cool soil temperatures being experienced earlier this spring. Emergence of Japanese beetle adults is about 1-2 weeks behind schedule, depending on their location in the state.
Feeding damage of adult Japanese beetle is often observed as a lace-like pattern of defoliation of host plant foliage as beetles avoid leaf veins when feeding. Beetles gather high (often in full sunlight) on host plants that exude strong odors. This attracts high numbers of beetles. Tassels and developing silks of corn can be severely damaged by adult feeding, whereas leaf feeding is common on soybean and many other plants. Feeding on corn silks can disrupt pollination and result in substantial yield losses. Foliage feeding on soybean is less damaging, although late planted or double-crop soybean may sustain economic damage if beetle numbers are high. The grub stage of this pest will feed on plant roots of both corn and soybean with most feeding occurring after egg hatch in late June, July and possibly early 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.
Economic thresholds for corn and soybean can quickly be reached as these beetles often aggregate on host plants and feed in high numbers. In field corn, an insecticidal treatment is justified if during the silking period an average of 3 or more beetles are present per ear tip, silks have been clipped to ½ inch or less in length, and pollination is less than 50% complete. For soybean insecticide treatment is justified if foliage feeding exceeds 30% prior to bloom and 20% from bloom through pod fill. The following insecticide tables 1 & 2 (pg. 106) are recommended for control of Japanese beetles in field corn and soybean in Missouri.