LIFE-HISTORY COSTS ASSOCIATED WITH THE EVOLUTION OF INSECTICIDE RESISTANCE

Strong directional selection for insecticide resistance in agricultural systems favours an increase in frequency of resistance allele(s). Provided that the resistance allele(s) has negative pleiotropic effects on life-history traits, such a change in gene frequency is expected to result in a progressive increase in fitness costs. Therefore, negative impacts on fitness components should increase with the degree of resistance across populations exposed to different insecticide regimes. The aim of this study was to assess whether selection for insecticide resistance resulted in such an evolutionary change in life-history traits in the obliquebanded leafroller, Choristoneura rosaceana. Within a local geographic area there was significant variation in pesticide resistance. Compared with unsprayed populations, which were highly susceptible to pesticides, resistant populations had lower 16 day live mass, smaller pupal mass, and longer development time. Thus, in C. rosaceana pesticide resistance is associated with life-history costs. This suggests that an alternation of insecticides to which the obliquebanded leafroller is not resistant could be a valuable strategy to manage this species. There were also significant across-population correlations between the fitness components and resistance. These relations may illustrate a mechanism that can limit evolutionary responses of populations by natural selection.