Synergism between entomopathogenic nematodes and Bacillus thuringiensis crops: integrating biological control and resistance management.
The past decade has witnessed a continual increase in the use of crops genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This presents the challenge of designing agricultural systems to manage pests and the evolution of resistance to Bt. We tested whether entomopathogenic nematodes might act synergistically with Bt crops by killing pests in non-Bt refuges and by increasing the fitness costs of resistance to Bt. We also tested whether insect mortality and fitness costs were affected by the cotton phytochemical gossypol. The entomopathogenic nematode Steinernema riobrave increased the fitness cost of Bt resistance, indicating that its presence in refuges may slow pest adaptation to Bt crops. No effect on fitness costs was detected for the nematode Heterorhabditis bacteriophora. Gossypol did not alter nematode-imposed fitness costs. Simulation modelling supported the hypothesis that nematodes in refuges may slow resistance evolution. The effects of gossypol on insect mortality from nematodes and nematode reproduction differed between nematode species. Gossypol increased insect mortality caused by H. bacteriophora but did not affect mortality caused by S. riobrave. Gossypol enhanced reproduction of H. bacteriophora and decreased reproduction of S. riobrave. Synthesis and applications. Our results point to the value of developing integrated pest management strategies for Bt crops that include non-Bt refuges in which entomopathogenic nematodes are used as a pest-management agent. Because entomopathogenic nematodes can magnify fitness costs of Bt resistance, their presence in refuges may delay resistance by pests to Bt crops. Moreover, entomopathogenic nematodes can serve as biological control agents thereby decreasing dependence on conventional insecticides to manage pest populations in refuges.