Multi-Year Surveys Reveal Significant Decline in Western Corn Rootworm Densities in Illinois Soybean Fields

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is the primary insect pest threatening corn, Zea mays L., production in many areas of the United States and is capable of inflicting substantial yield loss. In Illinois and bordering states, managing this pest is complicated by populations exhibiting resistance to crop rotation, historically an effective strategy to prevent larval injury caused by this corn specialist. Rotation resistance is characterized by ovipositional infidelity to corn accompanied by various adaptations promoting prolonged residency in soybean, Glycine max (L.) Merr. We conducted an analysis using survey data to estimate densities of western corn rootworm adults in Illinois soybean fields and compared those estimates for two discrete time periods: (1) 1997–2003 and (2) 2011 and 2013–2015. Annual surveys during each time period were conducted from late July through late August by visiting a number of randomly selected soybean fields in counties distributed throughout Illinois. Adults were sampled using sweep nets. Mean densities were determined for each United States Department of Agriculture crop reporting district and interpolated values were estimated to produce statewide contour plots. Our findings reveal a substantial decline in adult abundance in soybean fields throughout much of Illinois. Where significant differences were observed between the two survey periods, recent densities were 88% lower on average than those observed during 1997–2003. Potential factors contributing to the declines we observed and implications for managing this adaptive pest of corn are discussed.