Study On Joint Toxicity And Sublethal Effect Of Methoxyfenozide And Lufenuron To Spodoptera Exigua

Beet armyworm,Spodoptera exigua Hübner?Lepidoptera:Noctuidae?,is an important omnivorous agricultural pest worldwide.Currently,chemical control is the major strategy to counter S.exigua,but this pest has developed high levels of resistance to some conventional pesticides in many parts of the world.Insect growth regulator?IGR?is a biological control approach that targets pest by inhibition of pest growth or by interference with reproduction.However,resistance can arise in response to excessive use of IGR.The use of pesticides with different mechanisms of action is one of the important means to enhance virulence and delay the rise of resistance.The purpose of this study was to examine the possible synergistic effect of a mixture of methoxyfenozide and lufenuron,and to analyze the changes of detoxification enzymes and chitin metabolism related genes of S.exigua by transcriptomes and qRT-PCR.The results of this research could may help to reveal the mechanism of joint toxicity of methoxyfenozide and lufenuron to S.exigua.The main findings are as follows:The combined toxicity of methoxyfenozide?Met?with lufenuron?Luf?,chlorantraniliprole,and cyhalothrin on the 3^rd instar larvae of S.exigua was determined by co-toxicity factor method.The results showed that the co-toxicity factors of methoxyfenozide and lufenuron were greater than 20,which had synergism,while the other combination of co-toxic factors were less than 20,which had antagonism.Therefore,the optimal ratio of methoxyfenozide and guanidinium was determined by the co-toxicity coefficient method to be 2:3,the LC₅₀ was 25.20 ng/cm² and the CTC was 165.71 at 72 h,and the synergism was significant.Treatment of S.exigua with a sublethal dose of Met?LC30=21.00 ng/cm2?,Luf?LC30=27.13ng/cm2?and the mixture?The ratio of 2:3.The same below?(MML,LC₃₀=16.50 ng/cm²),respectively.MML has significantly improved the quick-acting effect of S.exigua.Ingestion of MML by larvae significantly prolonged the larval developmental duration,inhibited larval and pupal weights,and reduced pupation rate and single female egg production of S.exigua compared to individual treatment with of Met and Luf individually.No significant difference were observed in adult emergence and egg hatching were observed for different treatments.The MML-treated S.exigua exhibited significantly lower activities of S.exigua PPO and Chi than levels treated separately with Met or Luf.AChE,CarE,GST and CYP450 activities in S.exigua treated by Met,Luf and MML were decreased.By analyzing the trancriptomes of S.exigua treated with sublethal doses Met,Luf and MML,62,765 transcripts and 45,797 unigenes were obtained.Compared with the control,there were 3,868,4,684 and 2,213 differentially expressed genes under Met,Luf and MML treatments,respectively.The results of gene ontology?GO?functional annotation of differentially expressed genes showed that these genes were annotated into 4 dominated process,including catalytic activity and binding activity in the molecular functional process,metabolic process and cellular process in the biological process.After the treatment of Met,Luf and MML,there were 18,5,1 and 1 down-regulated genes of S.exigua P450,GST,CarE and AchE families,respectively,and only one up-regulated gene?P450?.One of the TPS,G6PI,GFAT and UAP genes,two HK and CHS genes,three TRE genes and 10Chi genes were identified from the transcriptomes of S.exigua.The expression levels of chitin metabolism-related genes were changed after feeding different concentrations(LC₁₀,LC₃₀ and LC₅₀)of Met,Luf and MML by qRT-PCR.The results showed that the expression of SeTRE1,SeCHS1 and SeCHS2 were increased,while the expression of SeChi1 was decreased after treatment with Met for 24h;Luf significantly inhibited the expression of SeTRE2,SeCHS1,SeCHS2 and SeChi7;Under the treatment of MML,the expression levels of SeTRE1,SeTRE2,SeCHS2 and SeChi1 were decreased,while the expression levels of SeCHS2 was increased.In summary,the joint toxicity and sublethal effect of methoxyfenozide and lufenuron to S.exigua had a synergistic effect,affected the detoxification enzyme activity and the expression of chitin metabolism related genes.The results of this study will be helpful to study the synergistic mechanism of ecdysone analogues and chitin synthesis inhibitors.Thus,this work will provide a basis for theoretical research and field application.