Bryan Jensen, UW Extension and IPM Program
A number of questions and several sightings of Japanese beetles have occurred this past week. Emergence is well under way and damage has been evident in susceptible ornamentals as well as soybean. This summer’s flight seems to heavier than normal. Likely a result of the mild winter.
Japanese beetles complete one generation/year. Adults began emergence around the July 4th weekend and egg laying is probably underway. Immatures (grubs) are not a pest on field crops but will feed on the roots of turf and other ornamental plants. Larval survival is highest when soil moisture is adequate and winters are mild. Adults will feed and thrive through August before populations start to decline in September. Adult Japanese beetles are approximately ½ inch in length, have a metallic green thorax, bronze elytra (wing covers) and 6 white tufts of hair on each side of the abdomen below the elytra.
Adults tend to feed in groups making spot treatment possible. On soybean, they feed on the upper leaves and this damage is best described as “net-like” or “lacy” in appearance. The economic threshold for Japanese beetles and other defoliators on soybean is dependent on crop stage. Vegetative soybean can withstand a moderate amount of feeding (30%). The economic threshold on reproductive soybean is scaled back to 15%. Defoliation estimates should be made on a whole plant basis and is why many people tend to overestimate injury.
In corn, Japanese beetles will feed on green corn silk that can result in poor ear fill. They rarely feed on leaves. Feeding damage is similar in appearance to corn rootworm adults and populations may be mixed. The established threshold for Japanese beetles is 3/plant (5-6/plant for rootworm adults) and silk clipping is within ½ inch of the ear tip. Presence of these numbers does not indicate an immediate need for treatment unless green silks are being clipped. If populations are mixed you will have to use your best judgement. However, once silks turn brown pollination is over and that field is no long susceptible to injury.