No. 17 Article 1/July 18, 2008

Japanese Beetles Are Causing Serious Concern in Some Areas

Over the past week and a half, reports of Japanese beetle activity in and around corn and soybean fields have increased substantially. The pests are particularly noticeable in some intensely infested areas throughout the southern two-thirds of Illinois. Many individuals have described--using lots of interesting words--the sights and sounds of Japanese beetles hitting windshields. One seasoned veteran whom I have known and trusted for many years indicated that "Japanese beetles in the Springfield area [in 2008] were the worst I have ever seen."

The captures of Japanese beetles in the five trap locations listed in "The Hines Report" (Franklin, Marion, Pulaski, Sangamon, and St. Clair counties) maintained by Ron Hines (FS seed agronomist, Southern Region, Growmark) have increased steadily, and in some locations dramatically, over the past 3 weeks. There were 126,703 Japanese beetles captured in the trap at Marion during the week ending July 15. The trap in Sangamon County comes in a distant, but still impressive, second at 70,934 during the same week. And the numbers still are increasing. We recently learned from Ron that the daily capture at the Marion County location approached 70,000 over the past couple of days (July 15 and 16). For comparison, one weekly capture of Japanese beetles in Massac County in 2007 exceeded 300,000 Japanese beetles, with the largest daily capture of more than 73,000 beetles. Captures of such large numbers of Japanese beetles require trap cooperators to replace the commercial "bullet trap" with a version that has a 3-gallon or larger container.

Commercial Japanese beetle "bullet trap" (photo courtesy of Ron Hines, University of Illinois, 2002).

Converted Japanese beetle trap with a larger capacity (photo courtesy of Ron Hines, University of Illinois, 2006).

Following is a quick review of some biological facts about Japanese beetles:

In last week's issue of the Bulletin (issue No. 16, July 11, 2008), we addressed the threshold for Japanese beetles clipping silks in cornfields. However, many soybean fields are infested with Japanese beetles, too, so percentage of defoliation of soybean leaves is of primary concern at the moment. We have questioned, and will continue to question, the value of the currently published thresholds for percentage of defoliation, but at least we have some foundation for discussion. The published economic thresholds for all soybean defoliators (bean leaf beetles, grasshoppers, green cloverworms, etc.) is 30% defoliation before bloom and 20% defoliation between bloom and pod fill. Even without today's high prices for soybeans ($15 to $16 per bushel), one can question these thresholds, which were established during the 1970s and 1980s. Much has changed with soybean production since then, including production practices and yield potential. However, simple economics suggest that a soybean producer needs to protect only about 1 bushel of soybeans to offset the cost of control of insects this year. In addition, yield potential of soybeans has been reduced by late planting and slow development this year, so many growers are seeking to protect what's there.

Is a threshold less than 20% defoliation this year more realistic? Probably, but we can't know for certain without conducting the research with modern expectations. Dr. Doug Jones, University of Illinois Extension specialist in IPM at Mt. Vernon, has initiated an experiment (funding provided by the Illinois Soybean Association) to address the relationship between insect defoliation and soybean yield components with a modern soybean variety. However, we obviously won't have preliminary information until later this year, so quantitative answers are not immediately available. We hope to expand research efforts in 2009, based on the results in 2008.

Japanese beetle defoliation of both soybean and smartweed, Champaign County, 2007 (University of Illinois).

Examples of a range of percentages of insect defoliation to a soybean leaflet (from M. Kogan and D. E. Kuhlman. 1982, Soybean Insects: Identification and Management in Illinois, Bulletin 773, Agricultural Experiment Station, College of Agriculture, University of Illinois at Urbana-Champaign).

As we have stated so many times already this year, use your best judgment regarding the need to control Japanese beetles in either soybeans or corn. Signs of "substantial" defoliation in soybeans and the presence of lots of Japanese beetles likely warrant an insecticide application. However, application of an insecticide to prevent an infestation of Japanese beetles is not warranted. Because of the beetles' feeding behavior, some fields will not be infested. Also, as noted by many individuals, much of the feeding in soybean fields is along field edges, in which case a field-edge application might be the wise economic choice. However, one needs to make certain that the rest of a seemingly edge-infested field is not infested before deciding on a field-edge application. As indicated in the photograph, Japanese beetles may appear in pockets within a field, possibly attracted initially by a flowering weed (smartweed in the photograph) but moving onto soybeans after they have completely defoliated the initial host.

Insecticides suggested for control of Japanese beetles in Illinois soybeans are listed in Table 1 (page 12) of the 2008 Illinois Agricultural Pest Management Handbook (Adobe PDF). Follow all label directions and precautions.--Kevin Steffey and Mike Gray

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