Exploiting pathogens and their impact on fitness costs to manage the evolution of resistance to Bacillus thuringiensis

Authors


Ben Raymond, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK (fax +44 1865 271281; e-mail benjamin.raymond@zoo.ox.ac.uk).

Summary

  • 1Sustainable insect control requires effective management of the evolution of resistance to pesticides. Resistance management may ultimately depend on a range of diverse strategies. We explored how combining the use of a pathogen with an integrated pest management (IPM)-compatible pesticide, Bacillus thuringiensis (Bt), could affect the evolution of resistance.
  • 2We used fitness and laboratory selection experiments to explore whether the use of a nucleopolyhedrovirus could alter the rate of evolution of resistance to a Bt toxin. These results were incorporated into simulation modelling to investigate how pathogens could best be exploited in resistance management.
  • 3Simultaneous exposure to virus and Bt toxin (mixed sprays) reduced the fitness of Bt-resistant insects compared with treatments with toxin alone. Moreover, Bt resistance incurred additional fitness costs, in terms of egg fertility, in the presence of virus.
  • 4In a selection experiment with caged insects, spraying toxin-free refugia with virus slowed the evolution of resistance relative to unsprayed refugia, confirming the results of two fitness experiments that indicated that virus would increase the fitness costs of resistance. The impact of virus-mediated costs was explored further in simulation models.
  • 5Simulations showed that large virus-mediated fitness costs (a reduction in fecundity of 35%) with partially dominant inheritance could lead to effective resistance management with a Bt/virus rotation. Modest fitness costs could, however, markedly improve a rotation strategy with a pathogen that could replicate post-application.
  • 6Synthesis and applications. Mixed sprays of virus and resistance-prone pesticides such as Bt have the potential to slow the evolution of resistance within an integrated pest management context. However, the efficacy of mixed-spray strategies depends upon precise dosages and/or the presence of a spray-free refuge. In contrast, rotations of pesticides with pathogens that could replicate after application slow the evolution of resistance over a wider range of conditions and control insects cost-effectively. This efficacy is dependent on the co-occurrence of persistent pathogen and pesticide reducing the relative fitness of resistant individuals.

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