Illinois Natural History Survey - University of Illinois

Monitoring the Spread of Western Corn Rootworm Beetles Infesting Illinois Soybean Fields

Historically, adult western corn rootworm beetles (WCR, Diabrotica virgifera virgifera) have exhibited a strong fidelity to cornfields for both feeding and egg-laying. Because WCR larvae can survive only on corn roots (and those of a few other grasses), growing a non-host, like soybeans, where WCR eggs were laid the year before meant WCR larvae emerged where they had no food source and quickly died. Corn planted where soybeans were produced the previous year could be grown without use of a WCR-targeted soil insecticide because WCR did not lay eggs outside of corn.

Around 1986, a WCR strain appeared in several cornfields in Ford County, Illinois, that could circumvent crop rotation by laying some eggs in the non-host crop rotated with corn. By 1995, serious economic damage to corn rotated with soybeans was documented in nine east-central Illinois counties ("WCR problem area"). At present, WCR with behavioral resistance to crop rotation have spread to other areas in Illinois, Indiana, Ohio, and Michigan. Growers in and around the WCR problem area can no longer rely on crop rotation to manage rootworms in corn grown after soybeans (or other crops), and must use soil insecticides costing approximately $15 per acre to control WCR in first-year corn.

As part of our continuing work to understand this problem and its spread, each year we conduct a sweep sample survey of WCR abundance in Illinois soybean fields. During July and August of 1998, we used 15-inch diameter sweep nets to sample WCR populations in weed-free soybean fields that were adjacent to cornfields. We surveyed 140 locations in 44 counties and two fields were sampled at most locations. A total of 271 samples (100 sweeps/sample) were collected and preserved on dry ice. A similar procedure was followed in 1997, when insects were collected in corn and soybeans in 29 counties.

In 1997, WCR populations were extremely high in soybeans throughout east-central Illinois. Where WCR were particularly abundant, it was not unusual for an individual 100-sweep sample in soybeans to contain several hundred WCR adults (at one site in Vermillion County, Indiana, Purdue scientists reported 1,081 WCR collected in 100 sweeps!). In addition to being very abundant in soybean fields, dissection of female WCR from the 1997 samples revealed that a greater proportion of those from problem area soybean fields were carrying mature eggs than females in nonproblem area soybeans or females in corn from any county.


Compared to 1997, the average abundance of WCR in Illinois soybeans appears to have declined by almost tenfold in 1998 (see maps). WCR abundance in soybeans declined in all but one of the counties sampled in both 1997 and 1998 (Moultrie County). We believe early summer rains drowned many WCR before they became established on corn roots. Even with a much lower population density statewide, WCR were still significantly more abundant in the region previously identified as being at highest risk for egg-laying by WCR in soybeans (WCR problem area) than in areas north, south, or west of that region. WCR were not detected in 14 of the sampled counties.

These data suggest there was no significant westward expansion of the WCR problem in 1998. WCR continue to be present at low or undetectable levels in soybean fields west of the Illinois River and south of Interstate 70. These data should not be interpreted as a blanket forecast for any location in a county. Pockets of higher abundance exist in regions with only moderate county averages. WCR management decisions for 1999 should be based on local observations and use of scouting procedures developed for WCR in soybean fields.


Joseph L. Spencer, Eli Levine, and David Onstad, Center for Economic Entomology; Scott A. Isard,Department of Geography, University of Illinois; and Mark Joselyn, Center for Wildlife Ecology.

Illinois Natural History Survey

1816 South Oak Street, MC 652
Champaign, IL 61820

Terms of use. Email the Web Administrator with questions or comments.

© 2021 University of Illinois Board of Trustees. All rights reserved.
For permissions information, contact the Illinois Natural History Survey.

Staff Intranet