Evaluating inheritance and fitness costs for western corn rootworm with laboratory-selected resistance to Gpp34/Tpp35Ab1 corn

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a serious pest of corn. Since 2003, this pest has been managed with corn hybrids that produce insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt). Bt corn reduces root-feeding injury by WCR larvae. The Bt-protein Gpp34/Tpp35Ab1 has been widely used for WCR management, but field-evolved resistance to Gpp34/Tpp35Ab1 has been found in some WCR populations. The refuge strategy is used to manage insect resistance to Bt crops in the US, with refuges of non-Bt host plants serving as a source of Bt-susceptible individuals, which in turn act to reduce the frequency of homozygous resistant individuals within a population. As such, the inheritance of resistance traits, and the extent to which heterozygotes survive on a Bt crop, will strongly influence the rate of resistance evolution, with the rate of resistance evolution increasing with the genetic dominance of a resistance trait. Additionally, selection for resistance by a Bt crop will lead to the accumulation of resistance alleles within a refuge population, thereby reducing the capacity of refuges to delay resistance. However, fitness costs act to remove resistance alleles from refuge populations and preserve the dynamic of refuges producing Bt-susceptible genotypes. Our objectives were to test for inheritance and fitness costs of resistance to Gpp34/Tpp35Ab1 by using a WCR strain with laboratory-selected resistance. This research will help improve insect resistance management by providing a better understanding of the risk of western corn rootworm evolving resistance to transgenic corn that produces Gpp34/Tpp35Ab1.