To treat or not to treat: Japanese beetles abundant in corn and soybean

When walking along the edges of corn and soybean fields the last couple of days, I noticed many Japanese beetles feeding on both corn and soybean. A lot of Japanese beetles (Figures).

Japanese beetle ID. Japanese beetles are oval shaped and have an iridescent metallic sheen. They have a green head and thorax and copper colored outer wings with a green stripe down the center. There are also five white 'racing stripes' along each side of the beetle under the wings towards the posterior end, and two on the rear end. These are actually small white tufts of hair called setae (Figures). There are several Japanese beetle look-alikes, but the Japanese beetle can be distinguished by these white tufts of hair.

Japanese beetle behavior. Feeding damage on soybean and corn was also observed (Figures). This damage can be alarming, particularly if individual leaves are in tatters. Often-times the beetles tend congregate on field edges, and so damage occurs on the edge of the field (this is where I saw the damage in corn). Scouting more than just the outside edge of the field should help to inform your decision on whether to consider an insecticide spray. Just sampling the outer edges of the field may over-estimate beetle density throughout the field.

Management recommendations: Soybean. Ms. Kelly Estes and Dr. Mike Gray of the University of Illinois have summarized information about the Japanese beetle on field crops in a fact sheet on the U of I's IPM website. For soybean, treatment recommendations depend upon the growth stage of the plants. Thresholds for treatment are 30% defoliation before bloom and 20% defoliation between bloom and pod-fill.

It may be difficult to train the eye to estimate defoliation, but a photo from Dr. Marlin Rice of Iowa State University, may help with visualizing the differences among the different levels of defoliation. One way to help train the eye is to imagine taking all of the holes in the leaf/leaflet and condensing them into one area. You can then more easily estimate how much is gone by relating this 'image' to the total leaf area. This is something that takes practice.

Corn. Corn recommendations are a little bit more cut and dried. Consider using an insecticide if during silking if you have 1) three or more beetles per ear and 2) silks are clipped shorter than ½ inch and 3) pollination is less than 50% completed.