Seasonal oviposition of a western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), variant in east central Illinois commercial maize and soybean fields

Three different on-farm sites in east central Illinois (Iroquois County), each containing a commercial production maize and soybean field, were sampled for variant western corn rootworm, Diabrotica virgifera virgifera LeConte, eggs from June 1999 through September 2001. Egg densities of D. v. virgifera eggs were determined at the 0.0- to 10- and 10- to 20-cm depths in these fields. Total mean egg densities of D. v. virgifera were significantly greater in soybean (0.42 and 0.57 eggs/0.47 liter) than in maize (0.27 and 0.29 eggs/0.47 liter) in 1999 and 2001, respectively. However, in 2000, there was no significant difference in the numbers of eggs in maize (0.43 eggs/0.47 liter) and soybean (0.52 eggs/0.47 liter). Most D. v. virgifera eggs were found in the 0.0- to 10.0-cm depth in both maize and soybean in 1999, 2000, and 2001. Oviposition was ≈50% complete by 28 and 28 July and 1 August in maize and by 31 and 30 July and and 10 August in soybean fields from 1999 through 2001, respectively. Oviposition was ≈90% complete by 9, 17, and 14 August in maize and by 15, 12, and 26 August in soybean fields from 1999 through 2001, respectively. Although densities of female D. v. virgifera adults increased significantly in soybean and decreased in maize after maize reached the R2 stage of development, oviposition by variant D. v. virgifera is indiscriminate and occurs in soybean before maize reaches the R2 stage.