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Japanese Beetles Are Coming On Strong in Some Areas

July 5, 2002
During the past couple of weeks, Japanese beetles have made their presence known in several areas of Illinois, especially in southwestern and east-central counties. Reports of extremely large numbers of Japanese beetles have been common. Several callers have expressed concern about defoliation in both corn and soybeans and the potential for silk-clipping injury in corn. Steve McCoy, with PC LTD., and Shawn Jones, with Pioneer Hi-Bred International, reported extremely high numbers of Japanese beetles in Macon County. In Champaign County, Joe Spencer, entomologist with the Illinois Natural History Survey, reported that populations of Japanese beetles are "ramping up at an amazing pace." Keith Evans, with FMC Corporation, has observed large numbers in Piatt County. During a drive from Darien (DuPage County) to Champaign on June 30, I passed through Will, Grundy, Livingston, and Ford counties and witnessed large numbers of Japanese beetles throughout the countryside. In fact, the beetles were hitting my windshield producing a sound reminiscent of a popcorn popper. In southwestern Illinois, Kelly Roberston, with McNeil Consulting, has observed large numbers of Japanese beetles in Monroe County, especially in field edges.

As I indicated in a previous issue of the Bulletin (issue no. 13, June 21, 2002), the large numbers of reports of Japanese beetle grubs in corn this spring prepared us for the current onslaught. The beetles that have emerged are dispersing throughout the landscape, showing up in yards, gardens, and field crops. Homeowners in some areas have already seen Japanese beetles attack roses and other ornamentals, as well as fruit trees. Obviously our attention will be focused primarily on corn and soybeans.

As corn matures, people will have to keep tabs on Japanese beetle activity during the pollination process. In most cornfields right now, Japanese beetles are feeding on leaves. In some fields, they are skeletonizing corn leaves. No yield loss usually occurs as a result of this type of injury.

Injury to corn leaves caused by Japanese beetles. (Photo courtesy of Shawn Jones, Pioneer Hi-Bred International.)

A "gang" of Japanese beetles on a corn leaf. (Photo courtesy of Shawn Jones, Pioneer Hi-Bred International.)

Over the past few years, several people have reported that Japanese beetle adults were observed feeding almost exclusively on tassels and, presumably, pollen. Although we have little information about the effect of feeding by Japanese beetles on corn pollen, some people within the seed industry and with companies that market high-oil corn believe that Japanese beetles feed preferentially on tassels bearing pollen. As Larry Bledsoe, entomologist at Purdue University, noted in a conversation we had earlier this year, Japanese beetles feed on many flowers, so they naturally would consume pollen as they consumed the flowers on a corn tassel. If Japanese beetles feed preferentially on tassels, as some people speculate, the insects might threaten pollination in high-oil corn where only 9% of the plants shed pollen. This situation probably is more threatening if pollination in a given field is not synchronized with pollination in other fields (i.e., corn planted earlier or later than nearby fields). Unfortunately, no threshold or treatment guideline has been developed for such a situation. I recommend that people develop their own "nominal" (based on experience) thresholds or rely on the threshold for Japanese beetles in seed corn, suggested by extension entomologists at Purdue University: To protect against silk clipping and interference with pollination, control of Japanese beetles in seed corn may be necessary if the silks on 20% of the plants have been clipped to a length of 3/4 inch or less, pollination is still taking place, and beetles are still present.

Speaking of preferential feeding, Shawn Jones shared an observation made by Pioneer's research group in southwestern Indiana over the past two to three years. They have witnessed preferential feeding by Japanese beetles on different hybrids. I have received similar comments from several other people during the past few years, so I suspect there is truth to this observation. However, no one knows why this occurs. Consequently, we'd like to hear from you if you observe this in your area. The more information we gather, the more likely we will be able to answer questions in the future.

Japanese beetles on corn silks. (Photo courtesy of Omar Koester, Monroe County Extension.)

Thresholds for Japanese beetles feeding in commercial hybrid corn, as well as in soybeans, were provided in last week's Bulletin (issue no. 14, June 28, 2002). Insecticides registered for control of Japanese beetles in both crops were listed in the same article. Please note that Mustang (restricted-use insecticide) also is labeled for control of Japanese beetles in corn (2.9 to 4.3 ounces per acre) and soybeans (3 to 4.3 ounces per acre).--Kevin Steffey

Author: Kevin Steffey

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

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