Mitigating an Epidemic of Resistance with Integrated Disease Management Tactics: Conflicting Management Recommendations from Insecticide Resistance and Epidemiological Models.

Insect-transmitted plant pathogens threaten crop production worldwide. Because a single feeding bout may be sufficient for a vector to transmit a pathogen that kills the plant, treatment thresholds for vectors of plant pathogens are low. For many vector species, overreliance on chemical controls has resulted in evolution of insecticide resistance. Analysis of complementary insecticide resistance and epidemiological models indicated that tactics for delaying resistance evolution conflict with tactics for limiting pathogen spread. Insecticide resistance models support maintaining untreated refuges that serve as a source of susceptible insects that reduce the likelihood of mating among rare resistant insects. In contrast, epidemiological models indicate that movement of vectors from untreated areas to insecticide-treated areas contributes to pathogen spread. Accordingly, epidemiological models support area-wide insecticide spray programs, although resistance models indicate that such an approach is likely to lead to rapid resistance. To mitigate risk of insecticide resistance, additional management approaches must be integrated into plant disease management strategies. The resistance and epidemiological models were used to evaluate effects of integrating application of insecticides with two additional management strategies: deployment of partially resistant plants (plants that are not immune to infection but have lower acquisition and inoculation rates than susceptible plants) and mating disruption (reduced vector birth rate in mating disruption-treated areas). Deployment of partially resistant plants reduced the risk that untreated areas served as a source of inoculative vectors. Mating disruption reduced the risk of resistance by suppressing growth of insecticide-resistant populations and benefited disease management by reducing vector abundance. Simulation results indicated that by targeting multiple aspects of the plant-pathogen-vector system, pathogen spread could be suppressed and resistance delayed. Implementation of such an approach will require innovations in vector control and sustained efforts in plant breeding.

Effects of four nematode species on fitness costs of pink bollworm resistance to Bacillus thuringiensis toxin Cry1Ac.

Evolution of resistance by pests can reduce the efficacy oftransgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis Berliner (Bt). In conjunction with refuges of non-Bt host plants, fitness costs can delay the evolution of resistance. Furthermore, fitness costs often vary with ecological conditions, suggesting that agricultural landscapes can be manipulated to magnify fitness costs and thereby prolong the efficacy of Bt crops. In the current study, we tested the effects of four species of entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) on the magnitude and dominance of fitness costs of resistance to Bt toxin CrylAc in pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae). For more than a decade, field populations of pink bollworm in the United States have remained susceptible to Bt cotton Gossypium hirsutum L. producing CrylAc; however, we used laboratory strains that had a mixture of susceptible and resistant individuals. In laboratory experiments, dominant fitness costs were imposed by the nematode Steinernema riobrave Cabanillas, Poinar, and Raulston but no fitness costs were imposed by Steinernema carpocapsae Weiser, Steinernema sp. (ML18 strain), or Heterorhabditis sonorensis Stock, Rivera-Orduño, and Flores-Lara. In computer simulations, evolution of resistance to Cry1Ac by pink bollworm was substantially delayed by treating some non-Bt cotton refuge fields with nematodes that imposed a dominant fitness cost, similar to the cost observed in laboratory experiments with S. riobrave. Based on the results here and in related studies, we conclude that entomopathogenic nematodes could bolster insect resistance management, but the success of this approach will depend on selecting the appropriate species of nematode and environment, as fitness costs were magnified by only two of five species evaluated and also depended on environmental factors.