Population characteristics of a western corn rootworm (Coleoptera : Chrysomelidae) strain in east-central Illinois corn and soybean fields

The recent failure of crop rotation to protect corn, Zea mays L., from rootworm, Diabrotica spp., larval injury has been extensive throughout east-central Illinois and northern Indiana. The widespread and long-term use of a corn and soybean, Glycine max L., crop rotation has been implicated in the selection of a new strain of western corn rootworm, Diabrotica virgifera virgifera LeConte, that has expanded its oviposition sites to include soybean fields. To understand better how western corn rootworm densities fluctuate in corn and soybean fields, we deployed Pherocon AM and vial traps in both crops. Also, sweep-net samples were taken in soybean fields and an alfalfa field. Twenty sites in 7 counties were monitored from late July to early September in 1996 and 1997. In soybeans across all trapping methods, >90% of the rootworm species collected were western corn rootworm. Counts of western corn rootworms and the percentage of females were greater on Pherocon AM and vial traps deployed in soybean than in cornfields. Although a greater (P < 0.05) percentage of females was collected on Pherocon AM traps deployed in a soybean field (73.5 plus or minus 1.6%) (mean plus or minus SE) than in a cornfield (42.3 plus or minus 3.4%), very few were gravid in either field. We suggest that yellow sticky traps may not be attractive to females searching for egg-laying sites. We discuss explanations of crop rotation failure by comparing our data with historical records of western corn rootworms trapped in both corn and soybean fields. Overall trapping data indicate that soybean fields are acceptable oviposition sites for western corn rootworms in east-central Illinois.