Elimination of a Recessive Allele Conferring Resistance to Bacillus thuringiensis from a Heterogeneous Strain of Diamondback Moth (Lepidoptera: Plutellidae)

We devised and tested a procedure for eliminating a recessive allele conferring resistance to Bacillus thuringiensis subsp. kurstaki from a laboratory strain of diamondback moth, Plutella xylostella (L.), composed of resistant and susceptible individuals. We established a homozygous susceptible strain (LAB-PS) from a heterogeneous strain (LAB-P) as follows: We obtained F₁ progeny from 7 single-pair families from the heterogeneous strain. Hybrid F₂ progeny were produced in 7 mass crosses, each of which involved 10 female F₁ progeny from a single-pair family and 10 males from a resistant strain (NO-QA). The hybrid F₂ progeny were tested in bioassays with a liquid formulation of B. thuringiensis toxin Cry1Ab at a diagnostic concentration that kills susceptible homozygotes and heterozygotes, but not resistant homozygotes. If the resistance allele occurred in either of the 2 parents that produced a particular F₁ family, the hybrid F₂ progeny derived from that family were expected to contain at least 25% homozygous resistant individuals that would survive exposure to the diagnostic concentration of Cry1Ab. Conversely, 0% survival of a set of hybrid F₂ progeny in the diagnostic bioassay would indicate that the single-pair family from which it was derived was homozygous susceptible. We found 0% survival in 1 set of hybrid F₂ progeny and used the F₁ single-pair family from which this set was derived to establish a homozygous susceptible strain. Subsequent bioassays showed that diagnostic concentrations of Cry1Ab or Cry1Aa killed 100% of larvae tested from this strain. The LC₅₀ of Cry1Ab at 5 d for the susceptible strain was 7-fold lower than that for the heterogeneous strain. The procedure can be adapted for other insects and other traits, such as resistance to other insecticides.