Resistance Mechanisms To Fipronil In Diamondback Moth, Plutella Xylostella (L.)

The diamondback moth, Plutella xylostella L. (Lepidoptera: Yponomeutidae), is one of the most destructive insects of cruciferous plants throughout the world. The diamondback moth is the first crop pest in the world to develop resistance to DDT, and now the diamondback m oth h as develop r esistance t o n early every c lass o f s ynthetic i nsecticides used against it in the field in many countries. It is also the first insect that had developed resistance to the bacterial insecticide Bacillus thuringiensis in the field.Fipronil is a member of the novel phenyl pyrazole insecticide class that is active at the neuron-inhibitory GABA-gated chloride channel, the same target of cyclodiene insecticides (such as dieldrin, endosulfan). The role of the detoxification enzymes and GAB A receptor Rdl gene involved in fipronil resistance was investigated in the present study.1. Cross-resistance of fipronil-selected strain of diamondback mothA field population collected from Pingshan town of Shenzhen in Dec 2002 was divided into two subpopulations. One subpopulation was reared in the laboratory without exposed to any insecticides and named as PS strain. The PSF strain was derived from another subpopulation after 20 generations of continuous selection with fipronil. The PSF strain developed 352-fold resistance to fipronil and showed obvious cross-resistance to two cyclodiene insecticides dieldrin and endosulfan compared with the PS strain. No cross-resistance to avermectin and spinosad was detected in the PSF strain. Neither piperonyl butoxide (PBO, microsomal oxidase inhibitor) nor S, S, S-tributyl phosphorotrithioate (DBF, esterase inhibitor) reduced the level of resistance to fipronil in the PSF strain.2. Biochemical characterization of fipronil resistance in the diamondback moth.Detoxication enzyme assays revealed that there were no difference in activities of Glutathione-S-transferases (DCNB, CDNB), general esterase and monooxygenases (ECOD, MCOD, PNOD) among the susceptible Roth, PS and PSF strains. The results suggested that fipronil resistance observed in the PSF strain was most likely attributed to decreased targetsite sensitivity.3. Cloning and sequence analysis of a gene fragment encoding the GAB A receptor (Rdl)Two Rdl gene fragments Rdll and Rdl2 were cloned from both genomic DNA and cDNA of the diamondback moth by PCR and RT-PCR. The predicted amino acid sequence shares high similarity (>99%) with GABA receptor homologous from various insects.Gene fragment Rdll has two alleles, Rdlla and Rdl Is. cDNA sequences and amino acids of Rdlla and Rdlls alleles are identical except for an AlaGCC to SerTCC substitution (homologous to Ala302->Ser mutation in Drosophild). Rdl2 has the same amino acids with Rdlls, but their codon usage is different. Only Rdlla and Rdl2 were detected in the susceptible ROTH strain, however the Rdlla -> Rdlls mutation and Rdl2 were found in both PS population and PSF strain. Rdlla -> Rdlls mutation may play a role in the fipronil resistance of P. xylostella.