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Questions About Japanese Beetles Abound

July 17, 2003

The unknowns about Japanese beetles and their potential effect on corn yields seemed to outweigh the knowns this past week. We have received numerous calls about infestations of Japanese beetles in cornfields, but few people have a good handle on whether this pest is having a significant effect on pollination. Japanese beetles are notorious silk clippers, but they also move around a lot. Consequently, after they have clipped silks from one ear, the silks continue to extend after the beetles leave for another food source. If the field has adequate moisture and silk extension is not impeded, silks that are 1/2 inch or longer can still intercept pollen.

And then there's the question about the uneven distribution of Japanese beetles in a field. Most reports have indicated that the beetles usually are prevalent along field edges. However, we also have received reports that Japanese beetles are spread throughout the field. In at least one instance, a grower had only the perimeter of a cornfield sprayed because of very high numbers of Japanese beetles. Within a couple of days, Japanese beetles were found clipping silks throughout the interior of the field. In the article "Japanese Beetles Are a Widespread Concern in Illinois" in last week's issue of the Bulletin (issue no. 16, July 11, 2003), I addressed the challenge of scouting and determining average densities of Japanese beetles when they appear in clumps in a field.

We've written numerous words about Japanese beetles in corn, but we've said very little thus far about Japanese beetles in soybeans. Suffice it to say that Japanese beetles also have been found in abundance in some fields of soybeans in Illinois. Rich Metzger, crop specialist with Madison Service Company, observed a section of a soybean field that was completely stripped of leaves. Others have observed significant defoliation of R1-R2--stage soybeans. Defoliation of soybeans during bloom and pod fill is more critical than defoliation of vegetative-stage soybeans.

Japanese beetles feeding on soybean leaves in Henry County.

Close-up of Japanese beetles feeding on soybeans leaves in Henry County.

It's important to scout fields of flowering soybeans to determine the presence of Japanese beetles and percentage of defoliation if you wish to determine whether control is justified. In general, for all defoliating insects, the threshold is 20% defoliation between bloom and pod fill. This number can be adjusted depending on the value of the soybeans and cost of control. Insecticides suggested for control of Japanese beetles in soybeans are listed in Table 1. If you decide to apply an insecticide to control Japanese beetles when soybeans are in bloom, keep in mind that bees visiting the soybean flowers can be killed. Please coordinate with local beekeepers before applying insecticide sprays. Insecticide applications early in the morning or late in the evening are preferential. Bees are less likely to be present in flowering soybeans at those times of day.

In issue no. 15 (July 3, 2002), Mike Gray wrote an article titled "Novel (Amusing) Insect Sampling Technique Reported in Britain." Apparently this article inspired our own Emerson Nafziger, crop specialist in the Department of Crop Sciences. He offered the following information after driving through southern Illinois: "As you know, Japanese beetles make a distinctive click when a speeding windshield hits them most satisfying in that they almost certainly died, but they don't leave much windshield residue to clean off. On a trip of 400 miles through southern Illinois yesterday, I would estimate the Interstate Japanese Beetle Contacts per Mile (IJBCM) to have been about 4 to 5, with high variability. They were on crops at Belleville, but I did not note presence or absence on a map. There is also a time-of-day correction that needs to be incorporated in this index (I didn't contact many before noon), which I did not do." A few more reports like these and we may have to consider a statewide effort.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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