Japanese Beetles are beginning to emerge

Wednesday, June 24, 2015

The annual emergence of Japanese beetles has started. As of June 15, we have received reports of significant counts from Columbia and Forsyth. These counts do not mean that immediate control measures are necessary, but that scouting for this insect should begin. With the weather that we have experienced, the delayed planting and resulting plant stress, in general the plants are smaller than usual for this time of year. A large population of Japanese beetles could inflict relatively more damage than usual.

Japanese beetle adults are approximately -inch in length, metallic green in color with bronze-or-copper colored wing covers. A diagnostic characteristic is the presence of 12 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. 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 AugustEach beetle female typically lays 40 to 60 eggs in groups of one to eight into the soil. Larvae emerge in about two weeks and feed on plant roots and decaying material before overwintering in the soil as third instars (worm or grub stage). The following spring they finish development, pupate and emerge as adults, and the cycle begins again.

Adult Japanese beetles typically feed on green silks and tassels in corn, foliage feed on soybean, and damage the foliage and fruit of more than 400 flower, shrub and tree species. Feeding damage is often observed as a lace-like pattern of host plant foliage. Beetles often gather in high numbers on host plants. Tassels and developing silks of corn can be severely damaged, disrupt pollination and result in substantial yield loss. In field corn, an insecticidal treatment is justified if during the silking period an average of three or more beetles are present per ear tip, silks have been clipped to inch or less, and pollination is less than 50 percent complete. In soybeans, foliage feeding is less damaging but can be significant. For soybean, insecticide treatment is justified if foliage feeding exceeds 20-30 percent prior to bloom and 10-20 percent from bloom through pod fill. Use the lower threshold numbers if soybean plants are under drought stress.

There are several sites across Missouri where Extension is monitoring the emergence of the Japanese beetle, including Marshall and Brunswick. We will provide updates on Beetle emergence.

The Japanese beetle 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 seven to ten years and maybe beyond, according to Wayne Bailey, Extension entomologist.

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.