title: The impact of strain diversity and mixed infections on the evolution of resistance to Bacillus thuringiensis.
abstract: Pesticide mixtures can reduce the rate at which insects evolve pesticide resistance. However, with live biopesticides such as the naturally abundant pathogen Bacillus thuringiensis (Bt), a range of additional biological considerations might affect the evolution of resistance. These can include ecological interactions in mixed infections, the different rates of transmission post-application and the impact of the native biodiversity on the frequency of mixed infections. Using multi-generation selection experiments, we tested how applications of single and mixed strains of Bt from diverse sources (natural isolates and biopesticides) affected the evolution of resistance in the diamondback moth, Plutella xylostella, to a focal strain. There was no significant difference in the rate of evolution of resistance between single and mixed-strain applications although the latter did result in lower insect populations. The relative survivorship of Bt-resistant genotypes was higher in the mixed-strain treatment, in part owing to elevated mortality of susceptible larvae in mixtures. Resistance evolved more quickly with treatments that contained natural isolates, and biological differences in transmission rate may have contributed to this. Our data indicate that the use of mixtures can have unexpected consequences on the fitness of resistant and susceptible insects.
authors.affiliation: Mathematical Ecology Research Group, Department of Zoology, University of Oxford, Oxford OX1 3PS, UK. b.raymond@imperial.ac.uk
authors.forename: Ben
authors.initials: B
authors.lastname: Raymond
authors.forename: Denis J
authors.initials: DJ
authors.lastname: Wright
authors.forename: Neil
authors.initials: N
authors.lastname: Crickmore
authors.forename: Michael B
authors.initials: MB
authors.lastname: Bonsall
mesh_terms: D000818:Animals; D001413:Bacillus thuringiensis / Q000235:genetics / Q000502:physiology*; D005075:Biological Evolution; D005260:Female; D007305:Insecticide Resistance; D007814:Larva / Q000235:genetics / Q000382:microbiology; D008297:Male; D009036:Moths / Q000235:genetics* / Q000382:microbiology*; D010572:Pest Control, Biological; D012641:Selection, Genetic
keywords: Bt; Plutella xylostella; insecticide resistance; pest management; synergism; transmission