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Issue No. 15, Article 3/July 7, 2006

Japanese Beetles Will Not Go Away Any Time Soon

With reports of ridiculous numbers of Japanese beetles continuing to come into our offices, I'm not certain we can provide much more information than we already have. The insects are far more than a nuisance in many Illinois corn and soybean fields, and homeowners are battling them on flowers, fruits, and ornamentals. For an update on numbers of Japanese beetles in southern Illinois, refer to "The Hines Report".

We have been asked about "combination silk clipping injury" by both the Japanese beetle and western corn rootworm. Each species has its own threshold (three or more Japanese beetles per ear, five or more western corn rootworm adults per plant), but there is no threshold for both insects clipping silks in the same field. Under such circumstances, people making decisions about whether insecticide application is necessary will have to use their best judgment, hopefully based on some experience. About the best we can do is to suggest that the focus should be on the extent and longevity (how many days) of silk clipping by both insects, rather than on the numbers of either or both insects. Also, remember that silk clipping should be observed before a decision is made to apply an insecticide. Once again, we have received some reports that Japanese beetles, in particular, seem to feed preferentially on the silks of some hybrids and not others. The presence of the insects without observed silk clipping may suggest that an insecticide application is not warranted. Nonetheless, continued monitoring through the pollination period is strongly encouraged.

Insecticides for control of Japanese beetles in corn were published in "Get Ready for . . . Japanese Beetles," issue no. 13, June 23, 2006, of the Bulletin. Insecticides suggested for control of western corn rootworm beetles in corn are included in "Tassel High by the Fourth of July: Prepare for Silk Clipping by Western Corn Rootworms" in this issue. Please note that not all insecticides are registered for control of both insects. If both insects must be controlled to protect pollination, select an insecticide labeled for control of both insects. And remember, the efficacy of pyrethroid insecticides declines as temperatures increase above 90°F.--Kevin Steffey and Mike Gray

Kevin Steffey
Mike Gray

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