In 2002 and 2003, surveys were conducted to track the distribution of the variant western corn rootworm that lays its eggs in crops other than corn. This variant of the western corn rootworm lays eggs in soybeans, resulting in larval damage the following year in rotated corn. Since the early 1990s, the occurrence of this variant has been common in east-central Illinois, but its distribution has been spreading north and west across Illinois. For the 36-county survey in 2003, we extracted five roots from each of 10 randomly selected fields within each preselected county. We had no knowledge whether an insecticide had been applied or a transgenic Bt hybrid had been planted in any of the fields surveyed. All 1,800 roots were transported to Urbana, washed, and rated using the Iowa State 1-6 root-rating scale developed at Iowa State University. (The literature citation for this root-rating scale is Hills, T. M., and D. C. Peters. 1971. "A method of evaluating post-planting insecticide treatments for control of western corn rootworm larvae." Journal of Economic Entomology 64:764-765.) The root-rating scale is explained in the article "Root Ratings from 2003 Corn Rootworm Control Trials" in issue no. 22 (September 5, 2003) of the Bulletin and also in a video clip that you can access on the IPM Website.
Table 1 and table 2 show the root-rating information for each county included in the surveys in 2003 and 2002, respectively. Table 3 shows the ranges of root ratings from the counties surveyed in both 2002 and 2003. Other maps, data, and additional information regarding the survey can be viewed at the On-Farm Survey Results page on the IPM Website. County-specific information is featured at this site, including the relative location of each field within a county and the corresponding root ratings from that field.
Overall, results from the survey in 2003 revealed that the distribution of the variant western corn rootworm has spread farther northwest than was detected in 2002. The most severe rootworm larval damage in 2003 occurred east of a line extending northeast from Woodford County through DeKalb County. However, average root ratings in Lee, Marshall, and Stark counties were greater in 2003 than they were in 2002. Although Bureau and Ogle counties were not surveyed for rootworm larval damage in 2002, the average root ratings in these counties in 2003 were similar to the average root ratings in Lee, Marshall, and Stark counties.
The variant western corn rootworm still does not seem to be present in most counties west of the Illinois River. Average root ratings in Adams, Brown, and McDonough counties in 2003 were similar to average root ratings from similar surveys in these same counties in the mid- to late 1980s. (The literature citation for this comparison is Steffey, K. L., M. E. Gray, and D. E. Kuhlman. 1992. "Extent of corn rootworm larval damage in corn after soybeans: Search for the expression of the prolonged diapause trait in Illinois." Journal of Economic Entomology 85: 268-275.)
If producers in northwestern Illinois have not already begun to monitor for western corn rootworm adults in soybean fields, they should initiate monitoring in 2004. Guidelines for using yellow sticky traps to monitor for western corn rootworms in soybeans can be found on the IPM Website. Results from western corn rootworm monitoring efforts should allow growers to determine the best management strategy for each field. Producers within counties in which severe rootworm larval damage has been detected for some time should continue efforts to determine whether western corn rootworm densities in soybeans indicate that control of larvae in corn the following year is necessary. Remember, economic levels of rootworm larvae do not occur in every field.--Jared Schroeder, Sue Ratcliffe, and Kevin Steffey