Knockdown resistance allele frequency in field populations of Plutella xylostella in Korea

Abstract

Two separate PCR amplification of specific allele (PASA) protocols have been developed and optimized for the rapid detection of the T929I and L1014F mutations, known to confer pyrethroid knockdown resistance by a nerve insensitivity mechanism in the diamondback moth, Plutella xylostella. To investigate the current status of pyrethroid resistance mediated by the nerve insensitivity mechanism in Korean populations of P. xylostella, PASA genotyping was conducted with 10 representative regional field populations in conjunction with the bioassay using fenvalerate and cypermethrin. Moderate to high levels of pyrethroid resistance were determined in all the regional populations investigated as judged by LC 50 values. Individual genotyping by PASA revealed that the T929I and L1014F mutations exist en bloc as a haplotype, and the frequencies of the L1014F and T929I mutations were 100% and 70–100% in all examined field populations, respectively, correlating well with pyrethroid resistance levels. To investigate when the two mutations first arose and how quickly they spread, we examined the frequency of the mutations in dry P. xylostella adult specimens collected from 1974 to 1995. Unexpectedly, the L1014F was observed even in the specimens collected in 1974 long before the selection of P. xylostella population with pyrethroids, whereas the T929I mutations began to show in 1995 after extensive selection with pyrethroids. These findings indicate that the L1014F mutation occurred first, perhaps due to initial selection by DDT, whereas the T929I mutation was selected subsequently by pyrethroids as a second mutation. In summary, high frequency of the knockdown resistance alleles indicates that pyrethroid resistance by the nerve insensitivity mechanism is at an almost saturated level in the representative Korean populations of P. xylostella, demanding more intensive regulation of pyrethroid use for its control. Use of efficient genotyping techniques, such as PASA, should greatly facilitate the monitoring of P. xylostella resistance in the field.