Inheritance, stability, and lack-of-fitness costs of field-selected resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae) from Florida

A colony of Plutella xylostella (L.), established from crucifer fields in Florida, was used to investigate resistance to Bacillus thuringiensis Berliner subsp. kurstaki . From an initial level of >1,500-fold, resistance fell within 3 generations in the absence of selection to ≈300-fold compared with susceptible larvae. Unlike previous cases of resistance to B. thuringiensis in P. xylostella , resistance in our Florida colony was stable at ≈300-fold without additional selection in the laboratory. High levels of resistance (>1,000-fold) recurred after a single exposure to B. thuringiensis subsp. kurstaki in the 4th generation. High levels of resistance did not recur after a 2nd selection in the 8th generation. Cage studies and genetic analysis of F1 larvae and backcross progeny. where the resistant parents were characterized by stable levels of resistance, showed that resistance was an incompletely recessive, autosomal trait probably controlled by a single allele that did not confer detectable levels of reduced fitness in the absence of exposure to B. thuringiensis . As one of the few shldies to demonstrate stable resistance to B. thuringiensis subsp. kurstaki from insects that were collected from the field and not subject to further selection in the laboratory, our results clearly emphasize the need to develop specific resistance management strategies for B. thuringiensis before there is widespread evolution of resistance.