| Crops producing toxins derived from the bacterium Bacillus thuringiensis (Bt) are widely planted to manage insect pests, including western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), a significant maize pest. The evolution of resistance would diminish the effectiveness of this technology; however, fitness costs can delay resistance. We quantified the level of resistance and tested for fitness costs of resistance in western corn rootworm with resistance to transgenic maize (Zea mays L.) that produces the Bt toxin Cry3Bb1. Fitness costs were measured in two experiments with non-Bt maize, the first used three commercial hybrids and the second used three inbred lines. The experiment with commercial hybrids compared resistant and susceptible strains and the experiment with maize inbreds also included heterozygous individuals. We tested whether entomopathogenic nematodes and fungi increased mortality of western corn rootworm, and whether these entomopathogens increased fitness costs of resistance to Cry3Bb1 maize. We exposed larvae to two species of nematodes and two species of fungi in two assay types: seedling mat and small cup. Survival on Cry3Bb1 maize was more than twice as high for resistant insects versus susceptible insects. The only non-recessive fitness cost detected was an additive fitness cost affecting adult size. Recessive fitness costs were observed for developmental rate, female survival, and egg viability. However, fitness benefits of higher fecundity, higher male survival, and greater longevity were detected for the resistant strain in the absence of Bt maize. Including all statistically significant fitness costs and benefits, the average difference between strains, in the absence of Bt maize, was a 5.3% fitness benefit for resistant insects. In the small cup assay, larval mortality increased with the concentration of the two nematode species and mortality from entomopathogens was significantly greater than zero for all entomopathogens. However, no fitness costs were observed in either assay type for any entomopathogen. The increased mortality of western corn rootworm larvae caused by these entomopathogens supports their potential use in biological control. These results suggest that resistance to Bt maize by western corn rootworm may not impose fitness costs, and consequently, may evolve quickly and persist once present. |
