Effects of adult emergence timing on susceptibility and fitness of Cry3Bb1‐resistant western corn rootworms

Field‐evolved resistance by the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte to the Cry3Bb1 trait expressed in maize, has been documented in areas of Nebraska USA. Currently, only limited information is available on life‐history traits of Cry3Bb1‐resistant field populations. Therefore, the Gassmann on‐plant bioassay was used to investigate the potential variability among four Cry3Bb1‐resistant WCR field collections made in 2011–2012 by focusing on the key parameters: larval survival, developmental stage and weight with specific emphasis on the impact of adult emergence timing on these parameters in subsequent progeny. Key results: In three of four collections, the susceptibility of larval progeny from adults that emerged early or late within a generation from Cry3Bb1 plants was similar. Each of the three collections exhibited complete resistance; that is, survival on Cry3Bb1 plants was greater or equal to survival on non‐Bt isoline plants. Bioassays from an additional field collection from one site 2 years (2013) after the original collection (2011) (both from Cry3Bb1 maize) indicated that resistance to Cry3Bb1 was maintained over time at the site despite Bt trait rotation in 2012. In general, comparative WCR life‐history parameter data from Cry3Bb1 and isoline maize indicate that fitness of field collections exhibiting complete resistance was similar on each hybrid. The mean proportion of larvae in third instar and mean weight of larvae recovered in bioassays from progeny of early‐ and late‐emerged adults was not significantly affected by emergence period. This suggests that delays in development and associated mean adult emergence commonly observed in populations that are susceptible to Cry3Bb1 may become smaller as populations become resistant to Cry3Bb1. Results from this article will inform Cry3Bb1 resistance mitigation efforts and contribute to the development of sustainable WCR management programmes.