Resistance Monitoring And Resisance Mechanism Of Mythimna Separata(Walker) To Beta-cypermethrin

The oriental armyworm,Mythimna separata（Walker）,is a serious pest of three staple crops in our country,which occurs in all provinces and regions in China except Xinjiang.Because of its polyphagous and gluttonous,together with round-trip migratory habits,it has brought serious economic loss to our country’s agricultural production.In recent years,with the change of climate and planting structure and other factors,oriental armyworm makes frequent outbreaks.So efficient,low toxic pesticides become the best choice of the field control to armyworm.Pyrethroid insecticides are widely used in agriculture and health pests in China,with broad-spectrum,high efficiency and low toxicity characteristics.But according to reports,long-term use of insecticides has contributed to the development of insecticide resistance in the target insects.To detect the mutations of voltage-gated sodium channel related to pyrethroids resistance,provide information about the development of insecticide resistance and to give suggestions to the design of appropriate resistance management strategies of M.separata.We collected 5 field populations of the pest from Shaanxi and Shanxi provinces of China and analyzed the regional susceptibilities of the field populations to 6 insecticides.And then we cloned the sodium channel gene sequence,at the same time,a beta-cypermethrin-resistant strain was set up,the biochemistry and the molecular biology mechanism of resistance was analyzed.1.The resistance monitoring of field populations of M.separata to six common insecticidesResistance of five field populations of M.separata to phoxim,chlorpyrifos,beta-cypermethrin,lambda-cyhalothrin,emamectin benzoate and chlorantraniliprole was evaluated by leaf dip method in laboratory.The results showed that the armyworm populations in those five areas showed different resistance level to all insecticides except for the emamectin benzoate.Especially to phoxim,Resistance was moderate to high level in all the tested populations（RR 19.4-70.1）except for the Sanyuan population.Pair-wise correlation analysis among different insecticides indicated that chlorpyrifos has a positive and significant correlation with emamectin benzoate.However,chlorantraniliprole has no significant correlation with emamectin benzoate,chlorpyrifos or phoxim.Therefore,emamectin benzoate,chlorantraniliprole and beta-cypermethrin were recommended for the control of oriental armyworm.Meanwhile,to delay the generation and development of insecticide resistance in Shaanxi and Shanxi Province,rotational application of insecticides between chlorantraniliprole and emamectin benzoate or chlorpyrifos is necessary.2.Cloning of voltage-gated sodium channel genes from M.separataThe cDNA sequence of sodium channel gene named MsNav was cloned through RT-PCR.The gene length of MsNav was 5977 bp,encoding 1992 amino acids,which included four homologous domains,each containing six hydrophobic transmembrane segments（S1–S6）of all the typical hallmarks of VGSC（Voltage Gated Sodium Channels）.In each domain,S4 segment contains four to seven repeated motifs of a positively charged amino acid residue followed by two hydrophobic residues.The MsNav shared highest similarity with other Noctuidae insects,and shared 80.8% similarity with para of Drosophila melanogaster（NP₀01188635）.It is suggested that sodium channel genes are highly conserved in different insects.3.The beta-cypermethrin resistance mechanisms underlying Taigu populationAfter 7 generations selection with beta-cypermethrin by group selection method,the resistant level of M.separata to beta-cypermethrin was 118.51-fold.Synergism tests showed that the carboxylesterase inhibitor（TPP）and oxidase inhibitor（PBO）has obvious synergistic effect,which resulted in 5.50 and 3.40-fold synergist ratios,respectively.Detoxification enzyme activity assay also confirmed that the esterase（EST）and mixed function oxidase（MFO）activity is the main mechanism of armyworm to beta-cypermethrin.The EST activity of resistant and susceptible strains had significant differences,with ratio was 2.45.There were highly significant differences between the MFO activity of the resistant and susceptible strains,the ratio was 1.70.There were not detected any mutations associated with knockdown resistance by comparing the sodium channel gene Ⅱ S4-S6 fragment of resistant and susceptible strains.The results showed that the detoxifying enzymes might play an important role in the resistance development of resistance of M.separata to beta-cypermethrin.