Examination of the F2 screen for rare resistance alleles to Bacillus thuringiensis toxins in the diamondback moth (Lepidoptera: Plutellidae)

A synthetic laboratory population of the diamondback moth, Plutella xylostella (L.), was used to test the F-2 screen developed for detecting the frequency of rare resistance alleles to Cry1Ac and Cry1C toxins of Bacillus thuringiensis (Bt). Of the 120 single-pair matings set up, 106 produced enough F-2 families for screening of Cry1Ac or Cry1C resistance alleles using both transgenic broccoli and an artificial diet overlay assay with a diagnostic dose. When using Bt broccoli plants as the F-2 screen method, only one F-2 family was detected for Cry1Ac resistance and no family was detected for Cry1C resistance. Six families were detected for either Cry1Ac or Cry1C resistance using the diet assay. The survivors in the diagnostic diet assay were crossed with the resistant individuals to confirm their resistance genotypes. Four F-2 families were confirmed to contain one copy of an allele resistant to Cry1Ac in the original single-pairs and four other F-2 families contained an allele resistant to Cry1C. Our results suggest that using transgenic plants expressing a high level of a Bt toxin in an F-2 screen may underestimate the frequency of resistance alleles with high false negatives, or fail to detect true resistance alleles. The diagnostic diet assay was a better F-2 screen method to detect alleles, especially for the Cry1Ac resistance with monogenic inheritance in the diamondback moth. The estimated probabilities of false positives and false negatives were 33 and 1%, respectively, for detecting Cry1Ac resistance at the allele frequency of 0.012 using the diagnostic diet assay. Careful validation of the screening method for each insect-crop system is necessary before the F-2 screen can be used to detect rare Bt resistance alleles in field populations.