University of Illinois

No. 22/August 29, 1997

Survey Results for Western Corn Rootworm Beetles in Soybean Fields

Surveys of soybean fields in east-central Illinois counties continue to reveal the presence of western corn rootworm adults (Table 1). On August 25, densities of western corn rootworm beetles were very impressive in Iroquois County at two different sites (3.66 and 3.97 beetles per sweep), Livingston County (2.44 beetles per sweep), and McLean County (1.77 beetles per sweep). Although densities of beetles were not as great in Champaign County (0.50 and 0.72 beetles per sweep), we still have several weeks left this season for populations of rootworm adults to surge in certain areas. Several counties outside the "epicenter" of first-year corn problems also have western corn rootworm beetles visiting soybean fields, such as Coles County (0.21 beetles per sweep), Logan County (0.35 beetles per sweep), and Piatt County (0.55 beetles per sweep). Several counties did not have any western corn rootworms in soybean fields, or numbers of beetles per sweep were negligible (less than 0.1 beetle per sweep): in Clay, Cumberland, DeWitt, Effingham, Fulton, Jasper, Macon, Mason, Menard, Moultrie, Sangamon, and Shelby counties.

Table 1. Survey results for western corn rootworm (WCR) beetles in soybean fields.

County, Sample DateWCR Present
or Absent
WCR Total per
100 Sweeps
Champaign, August 13Present50
Champaign, August 21Present72
Clay, August 25Absent0
Coles, August 25Present21
Cumberland, August 25Present2
DeWitt, August 26Present4
Effingham, August 25Absent0
Fulton, August 13Absent0
Fulton, August 20Absent0
Iroquois (Site 1),
August 25
Iroquois (Site 2),
August 25
Jasper, August 25Present1
Livingston, August 21Present244
Logan, August 13Present35
Logan, August 20Present19
Logan, August 26 Present1
Macon, August 26Present9
Mason, August 13Present7
Mason, August 20Present1
Mason, August 26Absent0
McLean, August 13Present177
McLean, August 20Present85
Menard, August 26Absent0
Moultrie, August 25Present5
Piatt, August 25Present55
Sangamon, August 26Present1
Shelby, August 25Absent0

What do these numbers suggest? First of all, we can expect continued rootworm problems in first-year corn throughout east-central Illinois for the 1998 season. Secondly, western corn rootworms appear to be stretching their "sphere of influence" to the south and west in Illinois. Although densities of western corn rootworm adults in Coles, Logan, and Piatt counties were not as great compared with other counties in eastern Illinois, growers in these counties should be alert to the possibility that rootworms may pose a threat to first-year corn production during 1998. No scientifically established threshold for western corn rootworm beetles in soybean fields exists. Therefore, growers who are aware that western corn rootworm beetles are present in their soybean fields should assume that egg laying may be occurring. Under these circumstances, for 1998, the use of a soil insecticide applied during planting is the best option. This recommendation holds true despite the poor showing of soil insecticides this season.

We intend to continue with our survey efforts and will share results with our readership in future issues of this Bulletin. This has been a rootworm season to remember!

Mike Gray and Matt O'Neal, Extension Entomology, (217)333-6652