Effects Of Sublethal Dose Of Emamectin Benzoate On Reproduction And Vitellogenin Gene Expression Of Spodoptera Frugiperda (Lepidoptera:Noctuidae)

Since the invasion in 2019,the occurrence area of Spodoptera frugiperda（Lepidoptera:Noctuidae）continues to expand.Due to long-term application of insecticides,the resistance of S.frugiperda to most conventional insecticides reached a relatively high level.Emamectin benzoate is a new type of high-efficiency semi-synthetic antibiotic insecticide,which is compatible with the environment and has strong toxicity to Lepidoptera pests in the field.Vitellogenin plays a decisive role in vitellogenesis and embryogenesis,and it can be used as one of the indicators to evaluate insect fertility.So far,the effect of insecticides on vitellogenin gene expression of S.frugiperda has not been reported.In view of this,this thesis studies the sublethal effects of emamectin benzoate on the growth,development and reproduction of S.frugiperda;based on the cloning and identification of Sf Vg,the m RNA expression level of Sf Vg gene after treatment with sublethal doses of emamectin benzoate were investigated.The main conclusions were as follows:1.Effects of sublethal dose of emamectin benzoate on growth,development and reproduction of S.frugiperda.The toxicity of the 3rd instar larvae of S.frugiperda was determined by leaf immersion method indoor,and the LC₅₀vaule was 0.018 mg/L after 72 h treatment,which indicated that emamectin benzoate was highly toxic to the S.frugiperda.After the sublethal doses of emamectin benzoate(LC₁₀and LC₃₀)were used to treat SS strain and RS strain of S.frugiperda,in SS strain,compared with the control group,the pupal weight,hatching rate and pupation rate of LC₁₀and LC₃₀ treatments were significantly reduced,and the hatching rates of the LC₁₀and LC₃₀ treatment group were respectively reduced by 13.28%and 15.87%（P<0.01）,the pupal rate of the LC₃₀ treatment was the lowest,only 66.78%of the control group（P<0.01）,the pupal period and pre-oviposition period were prolonged,and the egg production of the LC₁₀ treatment group decreased significantly,only 61.97%of the control group,indicating that the lethal dose of emamectin benzoate has adverse effects on the growth,development and reproduction of SS strain.In the RS strain,compared with the control group,the pupal rate of the LC₁₀and LC₃₀ treatment groups was reduced by 17.21%and 29.19%（P<0.01）,and the egg production was increased by 11.14%and 18.80%（P<0.01）,respectively,This may be because the RS strain reduces the number of emergence adults by reducing the pupation rate,thereby improving its fertility.Compared with SS strain,RS strain showed stronger adaptability,which suggested that it might have developed a certain resistance to emamectin benzoate.In the emamectin benzoate(LC₁₀ and LC₃₀)treatment groups,the emergence rate of RS strain was significantly higher than that of SS strain,and the pupation rate was also significantly increased.Under LC₁₀ treatment group,the pupation rate of RS strain was 18.45%higher than that of SS strain.Compared with the SS strain,the pupa weight,fertility,and hatching rate of the RS strain were significantly increased,and the pupal stage and the lifespan of the male adult were significantly prolonged（P<0.01）.It shows that RS strain shows stronger adaptability,and it is speculated that it may have developed a certain resistance to emamectin benzoate.2.Analysis of the characteristics of the Sf Vg.The full-length Sf Vg c DNA was amplified for the first time,Gen Bank accession number is MW728096.Sf Vg has a full length of 5,680 bp,an ORF of 5,247 bp,encoding 1,748 amino acids,the molecular weight of Sf Vg was 199.098 k Da with a proposed isoelectric point of 8.77.Sf Vg contains three domains:Vitellogenin＿N,DUF1943 and VWD,which are ubiquitous in insect Vg.The results of phylogenetic analysis showed that S.frugiperda was closely related to Spodoptera litura and Spodoptera exigua,the similarity of Vg amino acid sequence was 89.27%and 86.93%,respectively,indicating that the Vg of Noctuidae insects evolutionarily conservative.3.Effect of sublethal dose of emamectin benzoate on vitellogenin gene expression of S.frugiperda.The m RNA expression levels of Sf Vg under different developmental stages and under the sublethal doses of emamectin benzoate in different strains of S.frugiperda were studied by RT-q PCR.The results showed that Sf Vg gene only expressed in female pupae and female adults,and reached the highest value on the third day after emergence of female adults;In the SS strain,compared with the control group,sublethal doses of emamectin benzoate(LC₁₀)significantly reduced the expression level of Sf Vg（P<0.05）,which is consistent with the significant reduction in oviposition,indicating that emamectin benzoate might reduce the oviposition by inhibiting the expression of Sf Vg.In the RS strain,compared with the control group,the expression level of Sf Vg was up-regulated after sublethal emamectin benzoate(LC₁₀ and LC₃₀)treatment,and in the LC₃₀ treatment group was up-regulated by 2.6 times（P<0.01）,which was consistent with the increase of oviposition under this treatment,indicating that the strain had developed resistance to emamectin benzoate,and sublethal dose of emamectin benzoate could promote its reproduction.In conclusion,the toxicity of emamectin benzoate to S.frugiperda is strong,but long term use of sublethal dose of emamectin benzoate can develop its resistant and promote reproduction,the result can provide a theoretical basis for optimizing emamectin benzoate application to S.frugiperda,and successfully implementing effective integrated pest management（IPM） strategy.