Widespread pyrethroid resistance in Australian diamondback moth, Plutella xylostella (L.), is related to multiple mutations in the para sodium channel gene

Populations of Plutella xylostella, extending over 3800 km in southern Australia, show no genetic structure as assessed by microsatellite markers; yet outbreaks of pyrethroid resistance occur sporadically in cropping areas. Since mutations in the para voltage-gated sodium channel gene have been implicated in pyrethroid resistance, we looked for DNA sequence variation at this target among Australian moths. We found two resistance mutations previously reported for this species (L1014F and T929I), as well as a novel substitution (F1020S). Of the eight possible haplotypes formed by combinations of these three biallelic polymorphisms, only four were found in Australian populations: the wild-type allele (w), the kdr mutation allele (kdr) with only L1014F, the super-kdr-like combination of L1014F and T929I (skdrl), and the crashdown allele with only F1020S (cdr). Comparison of genotype frequencies among survivors of permethrin assays with those from untreated controls identified three resistant genotypes: skdrl homozygotes, cdr homozygotes and the corresponding heterozygote, cdr/skrdl - the heterozygote being at least as resistant as either homozygote. Spatial heterogeneity of allele frequencies was conspicuous, both across the continent and among local collections, consistent with reported spatial heterogeneity of pyrethroid resistance. Further, high resistance samples were sometimes associated with high frequency of cdr, sometimes high frequency of skdrl, or sometimes with a high combined cdr + skdrl frequency. The skdrl and cdr alleles explain a high proportion of the Australia-wide resistance variation. These data add to evidence that nerve insensitivity by mutations in the para-sodium channel gene is a common pyrethroid resistance mechanism in P. xylostella.