“Say it ain’t so, Joe!” Suspected Bt resistance in the rotation-resistant western corn rootworm

Bt resistance in the western corn rootworm (Diabrotica virgifera virgifera LeConte,WCR) is a phenomenon that was thought to be associated with the year-after-year cultivation of corn rootworm (CRW)-Bt corn hybrids (continuous corn) expressing the same Cry toxin.  In late 2013, we investigated reports of greater than expected WCR injury to Cry3Bb1-expressing first-year, rotated corn across northeastern and east-central Illinois. These reports raised the possibility that WCR resistance to Bt may have evolved in an area where there is very little continuous corn. The affected area in Illinois coincides with the epicenter of WCR behavioral resistance to crop rotation.  CRW-Bt corn hybrids are routinely planted in rotated cornfields in this area to counter the persistent threat of WCR rotation-resistance. The presence of suspected Bt resistance amongst rotation-resistant WCR populations links two very significant pest management challenges. Bt resistance in rotation-resistant WCR populations would negate the utility of crop rotation as a best management practice for growers experiencing greater than expected injury in Bt cornfields. Single plant Bt-resistance bioassays are underway using larvae obtained from some of the 2013 problem fields.  During 2014-2016, we will work on-farm with Livingston and Kankakee Co. producers to evaluate several management approaches to suspected Bt-resistance in rotated Bt cornfields. Our evaluations and analyses of resistance bioassays will improve our understanding of WCR management tactics and support the development and deployment of more effectively-targeted management strategies. Results from some of those bioassays will be presented.