No. 16 Article 2/July 11, 2008

Preparing for Silk-Clipping Insects

We know already this year that Japanese beetles will pose a serious threat to corn pollination in several areas of Illinois. (Refer to "The Hines Report" for July 8.) In addition, some samples of root injury caused by rootworm larvae suggest that numbers of corn rootworm adults could be large in some fields. I reported on some injury to corn roots in Joe Spencer's plots (Illinois Natural History Survey) in Champaign County in last week's issue of the Bulletin (No. 15, July 3, 2008, "Still Waiting on Corn Rootworm Adults"), which prompted a couple of additional reports of similar levels of rootworm larval injury in some fields in east-central Illinois. And a few folks have begun to find western corn rootworm adults in cornfields, although the numbers found thus far have not been particularly high.

Japanese beetles clipping silks on a corn ear, 2001 (photo courtesy of Robert Bellm, University of Illinois Extension).

Western corn rootworm female (University of Illinois Extension).

Given the high level of concern about protecting corn yields that have already been compromised by late planting or slow development, silk-clipping insects will be high on the list of targets when pollination commences. To protect pollination from silk clipping by corn rootworm adults, an insecticide application may be warranted when there are 5 or more beetles per plant, pollination is not complete, and silk clipping is observed. To protect pollination from silk clipping by Japanese beetle adults, an insecticide application may be warranted when there are 3 or more beetles per ear and pollination is not complete. Even if numbers of either or both species seem overwhelming, please place emphasis on the amount of silk clipping. As many experienced agronomists have observed over the years, some corn hybrids seem to escape the ravages of silk-clipping insects, either by rapid silk growth or possibly nonpreference (i.e., insects present but not feeding on silks). In such situations, an insecticide application is unnecessary.

If you determine that an insecticide application is needed to protect yield, several insecticides are labeled for control of corn rootworm adults (page 5) and Japanese beetles (page 7, Chapter 1, of the 2008 Illinois Agricultural Pest Management Handbook). Please follow all insecticide label directions and precautions.--Kevin Steffey

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