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It's Time to Consider Monitoring Western Corn Rootworm Adults in Soybean Fields

June 15, 2001
The monitoring program is in its sixth year, and data from this program have shown that the new strain of western corn rootworm is now present in 40 counties in Illinois. Extension entomologists continue to recommend the use of the monitoring program to predict root injury in rotated cornfields and track the spread of the new western corn rootworm strain. In 2000, we revised the thresholds used to predict larval root injury in first-year corn. The old threshold of 2 to 7 beetles per trap per day has been raised to 5 to 10 beetles per trap per day. Data indicate 5 beetles per trap per day may result in a 3.0 root injury rating, and 10 beetles per trap per day may result in a 4.0 root injury rating. The level of root injury that may result in economic loss varies according to growing season and hybrid selection. In general, economic injury occurs with root injury ratings between 3.0 and 4.0. However, for certain corn hybrids during poor growing conditions, recent research has shown that yield losses can occur with root ratings between 2.0 and 3.0. The scouting protocol and revised predictive threshold are described in our insect information sheet that is available from your local Extension office and also on the Web at infosheets/1-wcornr/wcornr.html.

Based on monitoring data from 2000, Bureau, Christian, Henderson, Henry, Lee, Menard, Ogle, Peoria, Pike, Sangamon, Shelby, Stephenson, Warren, and Whiteside are considered low-risk counties. Counties in the moderate-risk area are Clark, Coles, DeKalb, Kane, Kendall, LaSalle, Logan, Macon, Marshall, Moultrie, Putnam, and Tazwell. The 14 counties considered in the high-risk area for western corn rootworm larval damage in first-year corn are Champaign, DeWitt, Douglas, Edgar, Ford, Grundy, Iroquois, Kankakee, Living-ston, McLean, Piatt, Vermilion, Will, and Woodford. If you are in a moderate- or high-risk county, we recommend monitoring your soybean fields to predict root injury in first-year corn using the Pherocon AM sticky traps. To view the 2000 WCR monitoring data by county, township, and section number, visit our Web site at publications/rootworm-2000/index.htm.

If you are interested in determining presence or absence of western corn rootworms in your soybean fields, use the 4-trap monitoring protocol. However, if you plant to use the data to determine if a soil insecticide is needed in first-year corn, we recommend the 12-trap method. Pherocon AM (unbaited) sticky traps are available from Great Lakes IPM, 10220 Church Street NE, Vestaburg, MI 48891, (517)268-5693 and Gempler's, 211 Blue Mounds Road, P.O. Box 270, Mt. Horeb, WI 53572, (800)332-6744.--Susan Ratcliffe

Author: Susan Ratcliffe

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

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