Susceptibility of a field-derived, Bacillus thuringiensis-resistant strain of diamondback moth to in vitro-activated Cry1Ac toxin.

Resistant and susceptible populations of the diamondback moth (Plutella xylostella) were tested with crystalline, solubilized, and partially and fully activated forms of the Bacillus thuringiensis Cry1Ac delta-endotoxin. Fully activated toxin greatly reduced the resistance ratio (ratio of the 50% lethal concentration for the resistant population to that for the susceptible population) of the resistant population, suggesting that a defect in toxin activation is a major resistance mechanism.