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Issue No. 16, Article 3/July 13, 2007

Watch for Japanese Beetles in Soybeans

Not that Japanese beetles have finished feeding in corn, mind you, but as pollination is completed, the threat from Japanese beetles feeding in corn goes away. Consequently, the focus on Japanese beetles will be in soybeans (among field crops) for the remainder of the season. Many people already have observed defoliation of soybeans caused by Japanese beetles, but the accumulated effect of previous and continuing defoliation could eventually exceed the published economic threshold of 20% defoliation during reproductive growth. With the price of soybeans what they currently are, this relatively conservative threshold probably is appropriate.

We have referred you to "The Hines Report", compiled by Ron Hines (FS Seed Agronomist, Southern Region), many times already this year to "oooh" and "ahhhh" over the numbers of Japanese beetles captured in his network of traps. Well, you should keep watching the numbers, which continue to rise in several locations in southern Illinois. For example, only 2 Japanese beetles were captured in the St. Clair County trap during the week ending June 12, compared with 37,388 beetles captured during the week ending July 10. Many of the other sites also show an continual weekly increase in numbers of beetles captured from early June through mid-July.

Word on the street in central Illinois is that numbers of Japanese beetles generally are smaller this year than in 2006. Although this may be true, thus far, the continual increase in numbers of Japanese beetles in southern Illinois suggests that we shouldn't jump to conclusions just yet.


Japanese beetles defoliating smartweed, Champaign County, July 3, 2007 (Mike Gray, University of Illinois Extension).

On July 2, 2007, Mike Gray and I visited a soybean field in Champaign County, just a little ways south of Champaign. We were impressed with the numbers of Japanese beetles we observed in the field, although they were concentrating primarily on the flowering smartweed present there. Defoliation to soybeans on that date was very low. However, we all know what the Japanese beetles will begin feeding on after the they devour the smartweeds. Vigilance for defoliation in soybeans is pretty important right now.--Kevin Steffey

Author:
Kevin Steffey

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