Instability of Resistance to Bacillus thuringiensis

The continued efficacy of environmentally safe biopesticides derived from Bacillus thuringiensis (Bt) is threatened by the potential for development of resistance in pest populations. Instability of resistance is defined here as the tendency for the frequency of resistant genotypes to decrease in a population beyond effects directly attributable to immigration or emigration. Instability can be quantified as R, the average rate of change in the logarithm of the LC50 (concentration killing 50% of individuals) per generation, which is analogous to the average rate of response to selection per generation. In seven strains of Plutella xylostella, the first insect with field populations reported to be resistant to Bt, resistance declined when exposure to insecticide ceased (mean R = — 0.19). In four other pests, resistance to Bt declined slowly or not at all (mean R = —0.02) in the absence of exposure to Bt Reduced biotic fitness associated with resistance is the most likely cause of instability of resistance in P. xylostella. Understanding the instability of resistance may clarify why resistance to Bt has been relatively rare, and may also help to devise strategies for extending the usefulness of this ecologically benign insecticide.