INHERITANCE OF RESISTANCE TO BACILLUS-THURINGIENSIS IN DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE)

We tested the offspring of various crosses of Plutella xylostella (L.) to determine the mode of inheritance of resistance to a commercial formulation of the microbial insecticide Bacillus thuringiensis. The progeny of reciprocal F1 crosses (resistant male x susceptible female and resistant female x susceptible male) responded alike in bioassays, indicating autosomal inheritance. The LC50 of F1 progeny was not significantly greater than the LC50 of a susceptible strain, showing that resistance was recessive. Responses of progeny from a backcross (F1 x resistant strain) corresponded more closely to predictions from a single-locus model than to predictions from models with 2, 5, or 10 loci. Deviations between observed and expected outcomes increased as the number of loci in the model increased. Estimation of the minimum number of effective factors (sensu Lande 1981) also suggests that the number of loci influencing resistance was not large. In summary, our results suggest that resistance was controlled primarily by one or a few loci.