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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Japanese beetles feeding and mating on corn, June 19, 2012.
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Japanese beetles are abundant

Posted by Angie Peltier - Insects

When walking along a tree line on the NWIARDC last week, I noticed large dark spots on some of these weeds along this line. Upon closer inspection (getting out of the car), these 'spots' were Japanese beetles. A lot of Japanese beetles.

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 (Figure). There are several Japanese beetle look-alikes, but the Japanese beetle can be distinguished by these white tufts of hair. Matt Montgomery, University of Illinois Small Farms Local Foods Educator, recently wrote a detailed article describing the differences between Japanese beetles and look-alikes.

The Warren County Extension Office has had homeowners stop by with jars of Japanese beetles and weed and vegetable leaves stripped clean between the veins. Control recommendations may be different in a home garden setting than for a cash grain operation. Matt Montgomery and Rhonda Ferree of the U of I have summarized information on the Japanese Beetle on a blog devoted to this pest, Japanese Beetle News.

The beetles at the NWIARDC did not just stick to feeding on weeds. 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.

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 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.

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