PROLONGED SELECTION AFFECTS STABILITY OF RESISTANCE TO BACILLUS-THURINGIENSIS IN DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE)

We studied effects of prolonged selection with the microbial insecticide, Bacillus thuringiensis Berliner, on diamondback moth, Plutella xylostella (L.). A laboratory colony of diamondback moth derived from a moderately resistant field population was selected with B. thuringiensis during 21 of 40 generations. Continued increases in resistance after repeated exposure to concentrations of B. thuringiensis that were high enough to kill 100% of putative heterozygotes suggest that resistance was not controlled solely by one locus with two alleles. After an apparent plateau in response to selection occurred, we evaluated the stability of resistance in die selected colony and in six isofemale lines derived from the selected colony. Resistance declined in most cases, but in one of the isofemale lines, resistance remained extremely high (resistance ratio = 5,800) after >20 generations without exposure to B. thuringiensis. These data show that at least one genotype conferring resistance to B. thuringiensis was not inherently unstable.