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Issue No. 18, Article 1/July 25, 2008

Apparently a Good Year for Japanese Beetles

The reports of significant infestations of Japanese beetles in corn and soybean fields throughout Illinois continue to mount. We have received word of heavy infestations from border to border in every direction. This is not to say that Japanese beetles have blanketed the entire state (although some people in some areas may feel otherwise), but this pest certainly is more problematic over a larger area this year than it was in 2007. A lot of people have made the decision to control the beetles by having corn or soybean fields, or both, sprayed with an insecticide, often tank-mixed with a fungicide. We suspect that some of the applications are unnecessary (e.g., where Japanese beetles are only along field edges or are not causing much injury), but as we have indicated ad nauseam, higher commodity prices have triggered a "protect the investment" response. It is clear that we need some research to address this insect's behavior in and effects on soybeans and corn.

In correspondence we had with Kevin Black (insect/plant disease technical manager, Growmark, Bloomington, Illinois) and John Obermeyer (IPM specialist, Department of Entomology, Purdue University), the consensus was that populations of Japanese beetles in Illinois and Indiana in 2008 are highly variable from one location to another. Evidence for one extreme is the 417,102 Japanese beetles captured during the week ending July 22 in a trap in Marion County (Alan Mosler, Effingham-Clay Service Company), part of a five-trap network in southern Illinois (refer to "The Hines Report"). This is a record catch, eclipsing (to use Ron Hines's word) the 2007 capture of 300,000+ Japanese beetles in the trap in Massac County. On the other hand, all of us have heard from people in other areas that their problem with Japanese beetles is much reduced from last year. Regardless, this insect is capturing a lot of attention.

In previous articles in the Bulletin this year, we have written quite a bit about Japanese beetle biology and making management decisions in both corn and soybeans. Not much has changed over the years, although we might expect some developments that will aid our management of the pest in the future (refer to the related article in this issue of the Bulletin). In the meantime, I'll highlight once more some important information about Japanese beetles and their control:

  • Pyrethroids (Ambush, Asana, Baythroid, Capture, Mustang Max, Pounce, Proaxis, Warrior) kill Japanese beetles on contact, but they also are repellent to beetles.
  • High temperatures may reduce the efficacy of some pyrethroids.
  • Tank-mixing different insecticides should not be necessary in most situations. However, we are aware that some people want to add malathion with their "primary" insecticide for quick knockdown. Remember that the residual activity of malathion is about 24 hours.
  • Assess the situation for the entire field. Most people report the heaviest infestations along field edges, with only pockets of heavy infestations in the rest of the field. Depending on field size, an insecticide may not be necessary for the entire field.
  • Japanese beetles essentially become a non-issue in cornfields after pollination is complete.

If anyone takes the opportunity to leave an area of a field untreated to assess the effectiveness of an insecticide application, please let us know. We have very little data associated with Japanese beetle injury and yield loss. Obtaining some preliminary information might help us refine some of the management guidelines, or at the very least provide a foundation for future research efforts.--Kevin Steffey and Mike Gray

Kevin Steffey
Mike Gray

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