We evaluated the potential effect of phenological differences in maize, Zea mays L., and soybeans, Glycine max L. Merr., on adult densities and oviposition of the western corn rootworm, Diabrotica virgifera virgifera LeConte. Maize and soybean varieties were planted at various planting dates in Urbana-Champaign (east central Illinois) and Monmouth (northwestern Illinois) during 2000 and 2001. Historically, maize is grown in rotation with soybean in east central Illinois and is not rotated (continuous maize) in northwestern Illinois. Results from our research in the Urbana-Champaign phenology trial indicated that adult captures increased in soybean plots after the maize developed past the R2 stage of development. Significantly more D. v. virgifera eggs were deposited in the soil of soybean plots than in early planted maize plots. However, maize was not abandoned as an egg-laying site. In fact, the soil in late-planted maize plots was a competitive sink for D. v. virgifera eggs compared with soybean treatments. Results from our research in the Monmouth phenology trial indicated adult captures increased in late-planted maize after the R2 stage of development. Significantly more D. v. virgifera eggs were deposited in late-planted and early planted maize as compared with soybean. Soybean plots were not an egg-laying target at the Monmouth experimental site. We believe this documents different field level responses in ovipositional behavior of D. v. virgifera populations that inhabit east central and northwestern Illinois. We also hypothesize the intensive selection pressure that resulted from decades of crop rotation in east central Illinois resulted in a variant D. v. virgifera that responds to the phenology of maize by expanding its ovipositional range of crops, most notably to include soybeans.
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