Abstract：The diamondback moth (Plutella xylostella (Linn.), DBM) is the most destructive insect pest of cruciferous vegetables throughout the world and has become highly resistant to pyrethroid insecticides. To furtherly elucidate the occurrence characteristics of knockdown resistance in DBM, by means of the nucleotide sequencing method, the knockdown resistance allele frequencies in both the laboratory and field strains were detected. Besides, the genotypes of the dead individuals after fenvalerate treatment were also determined. Only L1014F and T929I mutations were found. Same resistance allele frequencies of 98.75%~100% were found in the 2 mutation sites in the of 2nd to 4th instar larvae of laboratory strain. The resistance allele frequencies for L1014F and T929I mutations in field strains, collected from Fujian, Zhejiang, Inner Mongolia and Beijing, were of 90.00%~100% and 88.75%~100%, respectively. And that were both of 98.75%~100% in the 2 sites in the field strain collected from vegetable fields in Pudang, Fuzhou of during November, 2010 and November, 2011. The dead insects after fenvalerate treatment at different concentrations were all resistant homozygotes in 2 mutation sites. The results indicated that genotypes at the mutation sites were mainly resistant homozygous in DBM and knockdown resistance allele frequencies varied little with larval instar, occurrence seasons in the same locality and different collection localities in the same season. T929I mutation occurred only together with L1014F mutation, whereas, when the genotype was heterozygously resistant in L1014F site, then that would be homozygously susceptible or heterozygously resistant in T929I site. Genotype at mutation sites was not the sole factor that determined the sensitivity of DBM to fenvalerate. This study will help to betterly understand the evolution characteristics of knockdown resistance in DBM.
