Cry3Bb1 resistant western corn rootworm populations: Potential effect of adult emergence timing on susceptibility to Cry3Bb1 and fitness of Cry3Bb1 survivors

Field-evolved resistance by the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte to Cry3Bb1 confirmed by single plant bioassays has been found in some locations of Nebraska.  To further characterize resistance in WCR populations from northeastern Nebraska, a study was conducted to evaluate the relative susceptibility and fitness of WCR progeny obtained from adults emerging from Cry3Bb1 expressing corn during the first and second half of the emergence curve. WCR populations were collected from four Cry3Bb1 problem fields that met the EPA definition of greater than expected injury (two populations each in 2011, 2012), then maintained to obtain F1 eggs. The F1 generation was reared on Cry3Bb1 expressing corn in the greenhouse. Adults were recovered daily, separated into early and later emerging cohorts and maintained to obtain F2 eggs. Single-plant bioassays comparing survival on Cry3Bb1 and non-Bt isoline were performed by infesting each potted plant with 12 F2 neonates at V5 growth stage and then recovering larvae in Berlese funnels after 17 days.  Proportion survival was significantly affected by an interaction between population and emergence period although 3 of 4 populations exhibited similar levels of survival between emergence periods.  Few fitness parameters differed between emergence periods.  Results increase our knowledge of resistance within and among populations and will inform resistance monitoring efforts.