Field-evolved resistance to Bt maize by western corn rootworm: Predictions from the laboratory and effects in the field

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

“…20% (Storer et al 2006, Petzold-Maxwell et al 2013a). These levels of survival are consistent with a Bt crop that does not produce a high dose of toxin against a target pest, as is the case for Bt corn targeting western corn rootworm (Gassmann 2012, Andow et al 2016. In cases where a baseline survival of 20% is present for a susceptible population, adding three-to sixfold resistance on top of this 20% survival may result in similar survival between Bt and non-Bt corn and high levels of feeding injury to Bt corn, as has been observed in the field when Cry3Bb1 resistance was present (Gassmann 2012).…”

“…The refuge strategy is expected to be most effective at delaying resistance when a Bt crop produces a high dose of Bt toxin, because highdose Bt crops will kill both susceptible and heterozygous individuals and render resistance a functionally recessive trait (Gould 1998). However, none of the currently commercialized Bt events targeting western corn rootworm are considered high dose (Gassmann 2012, Andow et al 2016). More recently, resistance management for western corn rootworm also has incorporated pyramiding of multiple Bt toxins.…”

“…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, US EPA recommends the use of plant-based bioassays for monitoring of Bt resistance by western corn rootworm (EPA 2013(EPA , 2016a.…”

“…Adaptation of western corn rootworm to Bt corn in the field was likely facilitated by the lack of a high dose produced by all commercialized Bt corn targeting western corn rootworm (Gassmann 2012, Andow et al 2016. High-dose Bt crops delay resistance by killing heterozygous resistant individuals, which in turn results in functionally recessive resistance (Tabashnik et al 2008, Carrière et al 2010.…”

“…Bt crops that produce less than a high dose of toxin permit some survival of heterozygous individuals, which can increase the rate of resistance evolution compared to high-dose events (Tabashnik et al 2008, Tabashnik andGould 2012). Additionally, low rates of dispersal by adult western corn rootworm prior to mating likely limited mixing between refuge individuals and Bt-selected individuals, especially when structured refuges (i.e., block refuges) were planted, which also will facilitate resistance to Bt corn (Spencer et al 2009, Gassmann 2012, Hughson and Spencer 2015, Andow et al 2016. Furthermore, high rates of adoption of Bt corn by farmers and a lack of compliance in planting of non-Bt refuges may further hasten resistance development (Jaffe 2009, James 2013.…”