The failure of crop rotation to protect corn from larval western corn rootworm, Diabrotica virgifera virgifera LeConte, injury has become common in regions of Illinois and Indiana, and is apparently spreading east into Ohio and Michigan. The extensive use of a corn-soybean rotation is considered to have selected a variant of the western corn rootworm that has expanded its ovipositional range to include soybean fields. Laboratory and field observations suggest that suspected variant western corn rootworm adults have a greater acceptance for soybean foliage as an adult feeding site than that of wild type adults. We attempted to identify variant western corn rootworm populations based on their propensity to feed on soybean foliage and what factors influence the consumption of soybean foliage. Feeding on soybean and corn leaves was quantified in laboratory feeding assays. There was no significant difference in amount of soybean leaf area eaten by western corn rootworm from Illinois versus those from Nebraska or Michigan, both regions were rotation failures have not been reported. To identify what factors influence western corn rootworm feeding on soybean, we first demonstrated that western corn rootworm feeding on corn foliage was influenced by corn phenology. Corn phenology also influenced the consumption of soybean leaves; more soybean leaf area was consumed in the presence reproductive stage corn leaves than younger, vegetative stage corn leaves. A phenology effect was observed also with corn silks; soybean consumption was reduced in the presence of corn silks compared with leaves. Given that western corn rootworm acceptance for soybean increases in the presence of older corn, we propose an explanation for western corn rootworm oviposition in soybeans based on corn phenology.