No. 13 Article 3/June 23, 2006

Get Ready for . . . Japanese Beetles

The advice in this article title may be a little late for people in southern Illinois, who have witnessed enormous numbers of Japanese beetles already in the past week. Since the last issue of the Bulletin (issue no. 12, June 16, 2006), when I relayed some noticeable numbers of Japanese beetles captured in traps in southern Illinois, capture numbers have skyrocketed. Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, reported that from June 16 to June 20, the Japanese beetles captured in a trap in Pope County increased from 1,000 to 3,000 per day! Read his full report for the week ending June 20.

Ron is not alone in such observations. We have received several additional reports of large numbers of Japanese beetles in southern Illinois, and people in central counties are also starting to observe Japanese beetles in cornfields and elsewhere. Emerson Nafziger, Extension crop production specialist in the Department of Crop Sciences, saw his first Japanese beetle of the year on the South Farms on June 15. On June 20, I saw many Japanese beetles in a couple of cornfields on the Ag Engineering Farm in southeast Urbana. Other reports of Japanese beetles in central Illinois have come in from Christian, McLean, and Sangamon counties.

With such potentially large numbers of Japanese beetles flying into cornfields and soybean fields, the potential for economic damage is real. At this time, because most cornfields are not tasseling, silking, and pollinating, the only threat to corn is defoliation, which, if severe, could warrant control. The same concern about defoliation applies to soybean. Remember that Japanese beetles are attracted to flowering plants, so they also will be found on flowering weeds.


Japanese beetles defoliating a corn leaf in 2002 (photo courtesy of Shawn Jones, Pioneer Hi-Bred International, Inc.).


Japanese beetles defoliating smartweed plant.

However, the real concern with Japanese beetles and corn production is the threat of silk-clipping injury, which interferes with pollination and can result in poor kernel set on affected ears. Because so many people have had bad experiences with Japanese beetles when corn is pollinating, some producers are considering "pulling the trigger" on this pest sooner rather than later. However, there is no way to assess the amount of silk clipping that might occur. We encourage people to fight the urge to spray insecticides to control Japanese beetles before pollination begins, unless defoliation is excessive. An insecticide applied too early may not have enough residual to control Japanese beetle adults that may fly into cornfields at a later date. One well-timed insecticide application makes more economic sense than two applications.


Poor kernel set as a result of Japanese beetles clipping cornsilks in 2002 (photo courtesy of Matt Montgomery, University of Illinois Extension).

The operating threshold for Japanese beetles in corn is three or more beetles per ear during tasseling and silking when pollination is not complete. We are among the first to admit that this threshold probably needs to be updated, but it is what we have to work with at the moment. People who have had considerable experience with Japanese beetles may use a different threshold, and we won't argue with experience. However, even under the best circumstances, determining the impact of silk clipping by Japanese beetles on corn yield is difficult.

Table 1 lists the insecticides suggested for control of Japanese beetles in corn and soybean. Please read the labels and follow all directions and precautions. Also, remember that the efficacy of pyrethroid insecticides is compromised by high temperatures. I have listed the class of each insecticide for additional information.--Kevin Steffey

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