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 approximate to 300-fold compared with susceptible larvae. Unlike previous cases of resistance to B. thuringiensis P. xylostella, resistance in our Florida colony was stable at approximate to 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 nd selection in the 8th generation. Cage studies and genetic analysis of F-1 larvae and backcross progeny, where the resistant parents were characterized by stable levels of resistance, showed that resistance mas 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 studies 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.