Evolution And Mechanisms Of Resistance To λ-cyhalothrin In Seasonal Migration Of Mythimna Separata

The Oriental armyworm,Mythimna separata（Walker）（Lepidoptera:Noctuidae）,is a notorious migratory pest of agricultural plants such as wheat,maize and rice in Asia,Europe,and Oceania.Pyrethroid insecticides have long been recommended as an effective insecticide to control M.separata.However,the development regularity and resistance mechanisms of M.separata migratory populations to chemical insecticides are unclear.In this study,we determined the toxicity and effects of LC₂₀and LC₅₀concentration ofλ-cyhalothrin on development and reproduction of M.separata.And seasonal changes inλ-cyhalothrin resistance in M.separata,that migrates between northern and northeast China seasonally,were monitored and the underlying mechanisms were characterized.The main results are as follows:1.Sublethal effect ofλ-cyhalothrin on the growth,development and reproduction to a susceptible strain in M.separataThe study investigated effects of LC₂₀and LC₅₀concentrations ofλ-cyhalothrin on the development,reproduction and life table parameters of the parents and offspring of M.separata by bioassay.Results indicate that under the treatment of LC₂₀λ-cyhalothrin,the oviposition period and net reproductive rate（R₀）of adults were significantly lower than that of the control group,and the hatching rate of eggs was significantly lower than that of LC₅₀group in the parents.Under the treatment of LC₅₀λ-cyhalothrin,the total number of eggs laid by single females of adults in M.separata increased significantly compared with the LC₂₀treatment group.The oviposition peak of adults was 2 days earlier than that of LC₂₀and control group.The daily maximum oviposition of single female was significantly higher than that of LC₂₀and control group.The survival period of female adults was significantly shortened compared with the control group and the LC₂₀treatment group.The biological parameters of net reproductive rate（R₀）and intrinsic rate of increase（r_m）were higher than those of the control group.In the offspring,the larval weight of the LC₂₀treatment group was significantly higher than that of the LC₅₀treatment group and the control group and the larval duration was significantly shortened,the larval duration was significantly extended under the LC₅₀treatment.Transcripts of vitellogenin（Ms Vg）and vitellogenin receptor（Ms Vg R）genes were suppressed at day 1 after emergence of moth which developed from the larvae exposed to LC₂₀and LC₅₀ofλ-cyhalothrin,but were significant elevated when the moth begin to lay eggs（day 4）,with a more remarkable induction by LC₅₀ofλ-cyhalothrin than those of LC₂₀ofλ-cyhalothrin.2.Resistance monitor of the migratory population of M.separata toλ-cyhalothrinThe vertically pointed searchlight traps were used to collect the migratory population of M.separata across the Bohai Bay from May to October in 2017 and 2018.The F₁and F₂third-instar larvae were tested for bioassay.The results show that there is a significant evolution of resistance in seasonal migration of M.separata toλ-cyhalothrin,with higher levels of resistance in the northeastward population in late May and the returning populations in late July and August.3.Mechanism of resistance of M.separata migratory populations toλ-cyhalothrinThe resistance mechanism of M.separata migratory population toλ-cyhalothrin was studied by synergistic tests,enzyme activity determination,target mutation detection and resistance gene frequency monitoring.The results showed that cytochrome P450monooxygenase（P450s）played an important role in resistance.P450 activity（ECOD）was significantly correlated with resistance levels of M.separata migratory population.The P450genes of the related insects were used as the query sequences to screen and identify the genes in the transcriptome of M.separata.A total of 92 P450 genes were identified,which comprised 6 families in Clan 2（6 genes）,9 families in Clan 3（42 genes）,4 families in Clan 4（34 genes）and 8 families in mitochondria（10 genes）.Correlation analysis of the expression levels and resistance levels of P450 genes to M.separata migratory populations in different months showed that CYP9A144,CYP9G40 and CYP6B79 are potential P450 genes related toλ-cyhalothrin resistance.