Issue 12, July 17, 2009

Japanese Beetle

Japanese beetle adults are out throughout the state. They are very numerous in areas of southern Illinois. However, there appears to be only about one-quarter to one-third as many beetles as is usual in central and northern Illinois. In central Illinois, this reduction is occurring not only in the Rantoul to Charleston area that had smaller numbers last year but also in the Springfield area, which had high beetle numbers.

We are assuming that this drop in numbers in the northern two-thirds of the state is due to deep soil freezing over the winter. Most Japanese beetle larvae overwinter about eleven inches below the soil surface. Although they can tolerate being frozen during this time, they start to die after being frozen for three weeks. With the seasonably cold temperatures that we experienced last winter, colder than it had been for several winters, and the reduced snow cover during much of this time, the soil froze deeply and stayed frozen for several weeks. In northern Illinois, the soil froze in many areas 18-24 inches deep and stayed frozen for 6-8 weeks. In central Illinois, the soil froze to about 14 inches deep and stayed frozen for 5-6 weeks. This was deep enough and long enough to kill many of the overwintering Japanese beetle grubs. The soil did not freeze deeply in southern Illinois, so the overwintering grubs were not severely affected.

Even with a reduced number of beetles in much of the state, individual plants within these areas are being heavily damaged and control efforts are warranted. Certainly in southern Illinois, damage is very heavy in many areas. Japanese beetle adults prefer to feed on smartweed, rose, linden, crabapple, birch, willow, ornamental cherry, and many other trees and shrubs. They prefer to feed on the upper side of leaves at the top of the plant in full sun, chewing holes into the leaves or eating away all but the lower leaf surfaces, causing them to turn brown.

Adult Japanese beetles fly to new hosts every three days, but are strongly attracted to hosts that have already been damaged by other Japanese beetles. For this reason, it is important to get control early in the six-week heavy flight period. The insecticide spray applications are effective for about two weeks. If you are limited to only one application, applying it early is most effective.

Insecticides recommended for control are carbaryl (Sevin), clothianidin (Arena), cyfluthrin (Tempo, Bayer Advanced Multi-Insect Killer), permethrin (Astro, Eight Insect Spray), and dinotefuran (Safari). Removal of the beetles by hand is most effective during later afternoon and evening. Disturbed beetles will drop from the plant into a jar held under them containing rubbing alcohol or soapy water and be killed.--Phil Nixon

Phil Nixon

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