Get Ready for Rootworm Adults

June 30, 2000
More reports regarding emergence of adult western and northern corn rootworms came in during the week of June 26, so we can assume that emergence will be in full swing all over the state within the week. The presence of emerging adults also means that larvae are completing their development, and the amount of root damage in any given field will be at its zenith soon. Anticipating the end of rootworm larval feeding, we will begin evaluations of rootworm research trials during the week of July 10.

Silvia Rondon, one of Mike Gray's graduate students, captured two western corn rootworm adults in emergence cages in her plot in Champaign County on June 26. Corey Gerber, an entomologist from Purdue University, found the first western corn rootworm adult in our areawide rootworm management study site in Iroquois County on June 27. But the reward for the most surprising report goes to Rich Archer with Pike County Service Company. He observed as many as six to seven western and northern corn rootworms per plant in a field of continuous corn in western Illinois. The field was pollinating, and the adults were clipping the silks. In addition, he indicated that rootworm larvae had damaged the root systems significantly; the plants were goosenecked as a result. Unfortunately, the field had been treated with a granular soil insecticide applied at planting time.

Rich's latter observation emphasizes the message I offered in an article in last week's Bulletin (issue no. 13, June 23, 2000): You can begin assessing corn roots for rootworm larval damage right now. Rich also found both rootworm larvae and pupae in the same field in which he found adults, so larval feeding will continue for a little while yet. Corey Gerber and Larry Bledsoe, also an entomologist at Purdue University, stated in a brief report that "Empirical observations of western corn rootworm larvae in the region [areawide rootworm management site] suggest that beetles are abundant and that 2000 may be a year with significant plant injury to unprotected, at-risk fields." Get set for diagnosing cornfields for rootworm larval damage.

Rootworm adults clipping silks on a corn ear.

As pollination begins in various regions of the state, you should be alert for large numbers of rootworm adults. If they clip enough silks, they can interfere with pollination. For commercial field corn, a spray to protect pollination may be warranted if you find five or more beetles per plant, pollination is not complete, and silk clipping is observed. For seed corn, control may be warranted if silks on 20% of the plants have been clipped to a length of 3/4 inch or less, pollination is still taking place, and rootworm beetles are still present. Suggested insecticides for control of corn rootworm adults are provided in Table 1.--Kevin Steffey

Author: Kevin Steffey

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

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