RESISTANCE RISK ASSESSMENT FOR SINGLE AND MULTIPLE INSECTICIDES - RESPONSES OF INDIANMEAL MOTH (LEPIDOPTERA, PYRALIDAE) TO BACILLUS-THURINGIENSIS

Criteria for comparing the risk of resistance development among single insecticides and between mixtures and sequences of two insecticides are described. The rate of development of resistance to an insecticide is proportional to the population's heritability (h2) of resistance to that insecticide. When cross-resistance is absent, a sequence of two insecticides is expected to be more durable than a mixture unless the population's h2 of resistance to the mixture is less than half of the mean of the population's h2 of resistance to the two individual components of the mixture. We applied these criteria to 11 previously reported selection experiments with the biopesticide Bacillus thuringiensis and Indianmeal moth, Plodia interpunctella (Hubner), a major pest of stored grain. The risk of resistance development did not differ significantly between the HD-1 strain of B. thuringiensis ssp. kurstaki and three other strains (HD-112, HD-133, and HD-198) of B. thuringiensis. Significant declines in realized h2 of resistance during individual selection experiments suggest that the initial frequency of resistance alleles was much higher than previously assumed. Our analysis also suggests that a mixture of HD-1 + HD-133 would not slow resistance development compared with a sequence of HD-1 followed by HD-133. Rapid evolution of resistance to the mixture of HD-1 + HD-133, which contained at least six different toxins, contradicts the claim that multiple toxins prevent or greatly retard resistance development.