Western corn rootworms, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), with resistance to crop rotation, frequently move between fields of corn, Zea mays L., and soybean, Glycine max L. Merril, laying eggs in both crops. By comparing the behavioral and reproductive responses of beetles experiencing mixed corn and soybean diets to those provided with a continuous corn diet, the cost of mixing diets was measured. Larvae were collected near Urbana, IL (rotation-resistant population), and Monmouth, IL (typical rotation-susceptible population), and reared to adults in the laboratory. Females from each population were assigned to alternating corn and soybean diets of different durations. Beetles were more likely to feed on corn if it followed soybean than if they had continuous access to corn. The cost of a mixed corn and soybean diet was minimal; the fecundity of females given alternating diets of corn and soybean was similar to that of beetles given daily access to corn diets. Females laid more eggs on days when they had access to soybean than on days with access to corn, suggesting dietary stress can induce oviposition. The only behavioral difference between the populations was a greater sensitivity to daily alternation of corn and soybean diets among beetles from the rotation-susceptible population. We found no evidence that rotation-resistant adult D. v. virgifera have any greater inclination or capacity to feed on soybean tissue than rotation-susceptible adults. Instead, periodic exposure to soybean results in nutritional stress that increases oviposition rate, lowers the threshold for feeding, and increases beetle activity.
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