Evaluating the effect of foliar insecticides on suspected Bt-resistant western corn rootworms in rotated soybean

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is the most economically significant insect pest of U.S. corn production and can inflict substantial yield loss. Historically, crop rotation was a successful management strategy because oviposition occurred predominantly in cornfields. However, beginning in 1995, crop rotation failed to adequately manage larval injury to first-year corn in east central Illinois and northwestern Indiana. Adoption of crop rotation had selected for female western corn rootworm beetles that laid eggs in both corn and soybean fields. Commercial Bt corn hybrids targeting this pest have been widely adopted and were an effective tool to manage the rotation-resistant western corn rootworm. Until recently, documentation of field-evolved resistance by western corn rootworm to Bt traits was associated with the cultivation of continuous corn expressing the same Bt toxin. In 2013, severe injury to rotated Bt corn was documented in Illinois, adding urgency to existing concerns about Bt resistance. Unexpected western corn rootworm injury to Bt corn hybrids in rotated cornfields and high beetle densities in corn and soybean have increased grower interest in adult management. In 2014, an experiment was initiated to determine how applying foliar insecticides to soybean affects patterns of western corn rootworm beetle abundance at trial sites where resistance to Cry3Bb1 was suspected and subsequently confirmed. Treatments were soybean foliar-applied insecticide applications and included: (1) an early application of Warrior II (lambda-cyhalothrin) applied during the tasseling stage of nearby corn, (2) a late application of Warrior II applied when nearby corn silks were brown, and (3) an untreated check. Adult western corn rootworm abundance in soybean was evaluated using unbaited yellow sticky traps and sweep samples. No significant differences were observed in beetle abundance across all treatments as measured with sticky traps and sweep samples for 2014 and 2015 with the exception of sweep samples from after the late spray. This exception did not correlate with sticky trap data for that time period. These results suggest that a single application of an insecticide in soybean timed to coincide with phenological events in adjacent cornfields does not have an effect on western corn rootworm beetle abundance throughout the growing season in soybean