Susceptibility of Nebraska Western Corn Rootworm (Coleoptera: Chrysomelidae) Populations to Bt Corn Events

Abstract

Transgenic plants have been widely adopted by growers to manage the western corn rootworm, Diabrotica virgifera virgifera LeConte, in field corn. Because of reduced efficacy in some Nebraska fields after repeated use of Cry3Bb1-expressing hybrids, single plant bioassays were conducted in 2012 and 2013 to characterize the susceptibility of western corn rootworm populations to the rootwormactive proteins Cry3Bb1, mCry3A, and Cry34/35Ab1. Results demonstrate that there are heritable differences in susceptibility of Nebraska western corn rootworm populations to rootworm-active Bt traits. Proportional survival and corrected survival data coupled with field histories collectively support the conclusion that a level of field resistance to Cry3Bb1 has evolved in some Nebraska populations in response to selection pressure and that cross-resistance exists between Cry3Bb1 and mCry3A. There was no apparent cross-resistance between Cry34/35Ab1 and either Cry3Bb1 or mCry3A. The potential implications of these results on current and future corn rootworm management strategies are discussed.

Evidence of Resistance to Cry34/35Ab1 Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae): Root Injury in the Field and Larval Survival in Plant-Based Bioassays

“…In general, we hypothesized that resistance to a specific Bt toxin would be present in a western corn rootworm population when corn with that Bt toxin suffered high levels of injury in the field (e.g., severe feeding injury to Cry3Bb1 corn would be associated with Cry3Bb1 resistance). In cases where resistance to Cry3Bb1 was present, we hypothesized that mCry3A resistance also would be present because of previous research indicating cross-resistance between these Bt toxins (Gassmann et al 2014, Wangila et al 2015. …”

“…Fields with Cry3Bb1-resistant western corn rootworm also were collected in Iowa during 2010 and 2011, and cross-resistance was identified between Cry3Bb1 corn and mCry3A corn (Gassmann et al , 2014. Similarly, resistance to Cry3Bb1 corn and crossresistance between Cry3Bb1 and mCry3A was identified for western corn rootworm populations from Nebraska (Wangila et al 2015), and resistance to Cry3Bb1 corn has been confirmed in Illinois (Schrader et al 2016). To date, however, western corn rootworm populations have not developed resistance to Cry34/35Ab1 corn (Gassmann et al 2011(Gassmann et al , 2014Wangila et al 2015).…”

“…Across multiple years and multiple states, severe injury to Cry3Bb1 corn has been associated with increased survival in plant-based bioassays (Gassmann et al 2011(Gassmann et al , 2014Wangila et al 2015;Schrader et al 2016). On-farm studies in fields where Cry3Bb1 resistance has been detected with plant-based bioassays have found elevated injury to Cry3Bb1 corn and elevated survival by western corn rootworm compared to known susceptible controls or compared to Cry34/35Ab1 corn (Gassmann 2012, Shrestha et al 2016.…”

“…Additionally, a co-occurrence of resistance to Cry3Bb1 and mCry3A corn was not found in all cases. Past work has found cross-resistance to mCry3A and Cry3Bb1 (Gassmann et al 2014, Wangila et al 2015, and that led to the hypothesis that resistance to either Cry3Bb1 or mCry3A should be accompanied by resistance to the other toxin. In general, for three out of four classes of field populations, this was the case, with significantly greater survival on both Cry3Bb1 corn and mCry3A corn observed among field populations compared to experimental controls.…”

Role of dispersal in resistance evolution and spread

“…Modeling indicates that the spatial scale over which resistance evolves depends in large part on the interplay between insect dispersal rate and the availability of refuge habitat in the landscape [60]. Multiple lines of evidence make it clear that D. v. virgifera resistance to various Bt toxins is evolving quickly and independently in local hotspots across a wide area [20,34,35]. Nevertheless, post-mating dispersal of D. v. virgifera can be extensive [36], and spatial spread from hotspots seems inevitable Codling moth, Cydia pomonella, dispersal seems to be mostly local, but with capacity for long-distance flights [37], causing uncertainty in whether regional variation in insecticide resistance is the result of independent evolution or spread [38]; most likely both processes are involved.…”