Evaluating multiple approaches for managing western corn rootworm larvae with seed blends

Corn rootworm, Diabrotica spp., larvae represent a significant and widespread economic threat to corn, Zea mays (L.), production in the United States, where control costs and yield losses associated with these insect pests exceed $1 billion annually. Preventing root injury and associated yield loss caused by corn rootworm larvae may be accomplished by the independent use of planting time soil insecticides or transgenic Bt hybrids. However, recent reports of both confirmed and suspected Bt resistance in corn rootworm populations throughout the Corn Belt have led to significant interest in the use of these two management tactics simultaneously. Although this approach has been investigated to some extent previously, information is lacking on how the use of a soil insecticide in tandem with a Bt seed blend-Bt and refuge (non-Bt) seed mixed into a single product-may affect root protection and yield. We describe an experiment including six trial sites conducted over a three-year period where various seed blends and soil insecticide/seed blend combinations were evaluated. The predominant species contributing to root injury across all sites was the western corn rootworm (Diabrotica virgifera virgifera LeConte). A weighted technique is presented for evaluating root injury for seed blends that offers a reliable estimate of product performance. The addition of a soil insecticide to the seed blend treatments never resulted in significantly improved root protection and failed to provide a consistent yield benefit. Our results suggest that a soil insecticide/seed blend combination approach is not warranted. Additionally, a subanalysis performed on individual refuge and nearby Bt root systems for seed blend treatments provides insight into the spatial characteristics of root injury in seed blend scenarios.