Effects of entomopathogens on fitness costs of Bt resistance in western corn rootworm

Resistance of insects to transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) has the potential to cut short the benefits these crops provide.  Bt corn provides effective control of western corn rootworm, Diabrotica virgifera virgifera Le Conte (WCR), which is a significant pest of corn.  Fitness costs arise in the absence of Bt toxin when insects that harbor Bt resistance alleles have lower fitness than Bt-susceptible insects.  Fitness costs are important because they can help delay the evolution of resistance to Bt toxins.  Entomopathogens can influence both magnitude and dominance of fitness costs.  We tested for the presence of fitness costs using a WCR strain that had been selected in the laboratory for resistance to Cry3Bb1.  We reared Bt-resistant and Bt-susceptible insects on seedling mats of non-Bt corn in the presence of four entomopathogens and a control with no entomopathogens.  We measured larval survival and larval developmental rate.  These data address the potential for entomopathogens to affect fitness costs of Bt resistance in western corn rootworm.