This year, the average number of western corn rootworm adults in Illinois soybeans was almost 10 times lower than in 1997. During late July and August 1998, we took sweep-net samples from soybean fields in 43 counties in Illinois, including all counties at risk for western corn rootworms laying eggs in soybeans. In most counties, two fields were sampled by taking 100 sweeps at each of three sites per field. Only soybeans with cornfields nearby were sampled. Figure 1 depicts the average numbers of adult western corn rootworms collected per 100 sweeps with a 15-inch-diameter sweep net. If a county was also sampled last year, the average number found then is shown in parentheses below the 1998 average.
Figure 1. Numbers of adult western corn rootworms per 100 sweeps in soybean fields in Illinois, 1998 (and 1997, in parentheses).
Of the counties sampled both years, the incidence of adult western corn rootworms in soybeans was lower in all counties but one. (Moultrie County was the exception: 1997, 0.0 per 100 sweeps; 1998, 2.0 per 100 sweeps.) The greatest densities of western corn rootworms this year occurred in areas previously identified as being at highest risk. Western corn rootworm adults were not detected in soybeans in 14 of the counties sampled, suggesting that no significant expansion of the "variant" of western corn rootworms occurred in Illinois in 1998. Densities of western corn rootworms in soybeans continue to be very low or undetectable in counties west of the Illinois River and south of Interstate 70.
We had anticipated potentially devastating rootworm problems in 1998, based on our 1997 sweep-sample data, Pherocon AM trap captures, and other measures, coupled with the mild winter. Wet conditions during egg hatch and larval establishment early in the summer likely played a role in the much reduced densities. Captures of adult western corn rootworms in vial traps baited with cucurbitacin + Sevin and placed in corn and soybean fields in Champaign County revealed the decline in both corn and soybean fields. Throughout the 1997 growing season, vial traps in corn and soybeans captured an average of 70 to 80 adult western corn rootworms per day. In 1998, the average numbers in corn and soybeans were only 3.1 and 6.1 per day, respectively.
This picture is encouraging for growers who are concerned about the potential for rootworm problems in 1999. However, these data should not be interpreted as a blanket forecast for any entire county. Pockets of higher abundance of western corn rootworms existed in some regions in 1998, even in counties with only moderate densities of rootworms. Management decisions in 1999 should be based on local observations; use scouting procedures developed for western corn rootworms in soybean fields.