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.
Highlights
Bacillus thuringiensis (Bt) has become one of the most important sources of insect control agents in modern agriculture
We aimed to understand how altering the diversity of Bt strains affected the evolution of resistance, and we sought to do this with (i) biopesticide strains that have been combined in mixed-spray applications, and (ii) with a range of wild-type strains that might co-occur in the field in a multi-generation selection experiment
Btk-resistant larvae did not appear to be susceptible to infection from cadavers, as mortality rates were indistinguishable from controls
Summary
Bacillus thuringiensis (Bt) has become one of the most important sources of insect control agents in modern agriculture. Microbial sprays cause mortality in conjunction with a naturally occurring bacterial community This natural population of Bt is extremely widespread and can be locally highly abundant, so that diverse populations of bacteria can persist patchily on single leaves shortly after the application of biopesticides [19,20]. Perfect execution of the recommendations of the theoretical models of mixed insecticides can have predictable consequences These recommendations may often be set aside, either for short-term practicalities, or because growers are desperate or uninformed and combine products ad hoc [23,24]. We aimed to understand how altering the diversity of Bt strains affected the evolution of resistance, and we sought to do this with (i) biopesticide strains that have been combined in mixed-spray applications, and (ii) with a range of wild-type strains that might co-occur in the field in a multi-generation selection experiment. Following the results of our selection experiment, we explore hypotheses that could explain the poor efficacy of diverse applications in slowing the evolution of resistance and the different consequences of applying strains of wild-type and biopesticidal origin
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