| The common cutworm, Spodoptera litura (Fabricius), was a worldwide distributed agricultural pest. Frequent application of chemical insecticides against Spodoptera litura had led to the development of insecticide resistance. Methoxyfenozide (RH-2485) was a novel insecticide belonging to a new group of insect growth regulators, bisacylhydrazines, which mimic natural insect moulting hormone by binding competitively to ecdysteroid receptors and induce a premature larval moult. Methoxyfenozide showed excellent insecticidal activity against larval Lepidoptera and high degree of safety to nontarget organisms, and is now used as an ideal candidate to substitute organophosphate and pyrethroid insecticides. In this paper, the susceptibility of common cutworm field populations to several insecticides was determined and the biochemical mechanisms of resistance were studied. Toxicological characteristics of methoxyfenozide, including sublethal effects, resistance risk, selective insecticidal mechanisms, were also studied.The resistance level of S. litura field populations, which were collected from Tongzhou, Taixing and Yangzhou, of Jiangsu province, respectively, was determined using leaf-dipping method. Compared with a susceptible population maintained in the laboratory, the resistance ratio of S. litura field populations to organophosphate and pyrethroid insecticides varied from 4.8 to 12.5 and 5.5 to 9.1 times, respectively. All three populations showed little resistance to hexaflumuron, fipronil, methoxyfenozide, chlorfenapyr, emamectin-benzoate and chlorantraniliprole, while low resistance to indoxacarb was observed in these three populations.The activities of detoxification enzymes in three S. litura field populations were also analyzed. The activity of mixed function oxidase (MFO) in all field populations and esterase (EST) in Tongzhou and Taixing populations increased sifnificantly compared to the suscepitible population, while no significant change of the activity of glutathione S-transferase (GST) was observed. Furthermore, the Vmax value of acetylcholinesterase (AChE) in Yangzhou population increased significantly. These results suggested that the EST, MFO and AChE played important role in the development of resistance of S. litura field populations to pyrethroids and organophosphate insecticides.Considering that nonlethal impacts of insecticides on various life history parameters may affect pest population dynamics, we investigated the sublethal effects of methoxyfenozide on S. litura. Based on the bioassay results, 3rd instar larvae of S.litura were treated with methoxyfenozide at concentrations corresponding to LC30 and LC10 by artificial diet incorporation method. Higher rate of deformed pupa and lower eclosion rate were observed, and the fecundity and egg hatchability were significantly decreased compared with control, suggesting that methoxyfenozide at sublethal concentrations had a negative effect on population development of S. litura. The sublethal effects of methoxyfenozide on detoxification enzymes were also studied. After exposure to methoxyfenozide at LC10 for 24 h and LC30 for 48 h and 96 h, the activities of esterase (EST)were significantly increased compared with control. The activities of microsomal multifunctional oxidase ( MFO ) and glutathione S-transferases (GST)were also significantly enhanced when 3rd instar larvae were treated with methoxyfenozide at LC10 for 24 h and LC30 for 96 h, respectively. These results indicated that detoxification enzymes of S. litura could be induced by methoxyfenozide depending on the concentration and the time after treatment. Finally, the importance of sublethal effects of insecticides on development, reproduction and detoxification enzymes of pests was discussed.Common cutworms were artificially selected in the laboratory for resistance to methoxyfenozide, and the realized resistance heritability and resistance risk were evaluated. The results showed that after selection with methoxyfenozide 9 times during 12 generations, S. litura exhibited 3.95-fold less susceptibility against methoxyfenozide compared with unselected parent strain, indicating slow development of resistance to methoxyfenozide in this important agricultural pest. According to the relatively low realized resistance heritability (h2=0.1199) and assuming selection pressure being 50%-90%, it requires 8-18 generations for S. litura to development 10-fold resistance to methoxyfenozide. These results suggested that S.litura may have limited resistance risk to methoxyfenozide.In toal of 50 ligand-bind domain (LBD) amino acid sequences of ecdysone receptor (EcR) including 38 insects and 12 other arthropods and vertebrates had been used for multiple sequence alignment. Three amino acid residues unique to Lepidoptera were found corresponding to Asp510, Asn511 and Glu531 in Drosophila melanogaster EcR (Genebank accession number AAF57278). Both of Asp510 and Asn511 located at the seventhα-helix, while Glu 531 located at the eighthα-helix, the difference of these three amino acid residues among Lepidoptera and other insects may play a role in the selective insecticidal mechanisms of bisacylhydrazines. Phylogenetic analysis with Neighbour-Joining method showed that all Mecopterida insects constitute a highly supported (100% bootstrap) monophyletic group separated from all other insects including holometabolous Coleoptera, Hymenoptera and Mecoptera, in which Diptera, Mecoptera and Trichoptera were located in one clade with 89% bootstrap value. Surprisingly, Bemisia tabaci was located out of the clade of Myzus persicae and Acyrthosiphon pisum, which were all Homoptera insect species.
|
PDF Full Text Request