By Wayne Bailey
During the past two weeks Japanese beetle adults began their annual emergence in many Missouri counties.
The emergence of this beetle is about 3 weeks earlier than normal with numbers of beetles emerging being substantially greater than in past years. In most areas their numbers will steadily increase through June then slowly decrease through when peak numbers will result in damage to many different tree, ornamental, 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 Riverton, New Jersey 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 southern portion of the City of St. Louis in 1934. A Missouri Department of Agriculture bulletin list 51 different tree, shrub, vegetable, and field crop species being damaged by Japanese beetles in St. Louis by the summer of 1936.
For many years the Japanese beetle infestation stayed in the St. Louis area although by the early 1960's infestations were reported in the urban centers of 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 Japanese beetle infestations.
Populations of this pest remained mainly in these urban area until about 10-12 years ago, when this pest began spreading mainly west and south 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 -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 1/2–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 45 - 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 beginning in very late May or early June in most years.
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 often gather high (often in full sunlight) on host plants which exude strong odors to feed in high numbers. Several tree species, roses, and mature fruit are favored hosts of this pest.
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 20% - 30% prior to bloom and10% - 20% from bloom through pod fill. Use the lower threshold numbers if soybean plants are under drought stress.
Source: Wayne Bailey, Integrated Pest Management