Research On The Sublethal Effects Of Thiamethoxam To Bactrocera Dorsalis (Hendel)

Oriental fruit fly,Bactrocera dorsalis（Hendel）（Diptera:Tephritidae）,has a wide range of hosts and strong reproductive capacity,and it is difficult to control.It is one of main measures to control B.dorsalis by insecticide.Thiamethazine is a kind of neonicotinoids insecticide,used for the control of B.dorsalis.Sublethal effect of thiamethazine on B.dorsalis.However,has not been reported.In this experiment,Toxicity of thiamethoxam to adults and larvae of B.dorsalis were determined by liquid feeding method and larval feed mixed poisoning method,respectively,and effects of sublethal doses of thiamethoxam on growth and reproduction of B.dorsalis were investigated.Sublethal effects and mechanism of reproduction of thiamethoxam to B.dorsalis were preliminarily studied.The main results were as follows:Toxicity of thiamethoxam to B.dorsalis were determined by liquid feeding method and larval feed mixed poisoning method,respectively.The results were as follows:After 24 hours of treatment,LC₅₀ of thiamethoxam to male and female adults,male adults,female adults,larvas of B.dorsalis were 12.43 mg/L,12.49 mg/L,9.91 mg/L and 0.33 mg/L,respectively.LC₁₀ were 2.08 mg/L,2.51 mg/L,2.20 mg/L and 0.15 mg/L,respectively,The LC₃₀ were 5.69 mg/L,6.48 mg/L,5.35 mg/L,and 0.22 mg/L,respectively.The results showed that adults and larvas of B.dorsalis were sensitive to thiamethoxam,and male adults were more sensitive than female adults.After treatment with sublethal doses of thiamethoxam on adults of B.dorsalis,activity of detoxification enzymesweres of B.dorsalis detected.The results showed that sublethal doses of thiamethazine could promote activity of multifunctional oxidase P450,and with the increase of sublethal concentration,there was no significant effect on carboxylesterase.LC₁₀ had a low stimulating effect on glutathione transferase in B.dorsalis,and effect of LC₃₀ was weaker than LC₁₀.The results showed that P450 may play a major detoxification role,when B.dorsalis was treated by thiamethazine.Treating with sublethal doses of thiamethoxam on adults and larvae of B.dorsalis.Larval duration,pupa calendar period,and eclosion rateand,fecundity per female et al of B.dorsalis were observed.Fecundity per female in the treatment group(LC₁₀ and LC₃₀)increased by 48.08 and 104.43,respectively,and the difference was significant.Age first mating on LC₃₀ was significantly longer,meanwhile LC₁₀ has no significant difference.Longevity of B.dorsalis was shortened and eclosion rate was decreased.Food consumption,larval duration,pupa duration,pupation rate and hatching rate couldn’t be affected when B.dorsalis were feeded at 2th instar larvae with sublethal doses of thiamethoxam.Egg hatchability couldn’t be affected when B.dorsalis were feeded at 4th instar adult with sublethal doses of thiamethoxam,but age of first mating and first laying significantly increased and fecundity per female in the treatment group(LC₁₀ and LC₃₀)increased 188.67 and 448.78,which significantly increased,longevity of male and female adults were shorted.Offspring:fecundity per female significantly increased,fecundity per female on offspring of(LC₁₀ and LC₃₀)in the treatment group increased by 39.30 and101.21 respectively,age of first laying was last and the other growth and development indexes of offspring had no significant effect.To sum up,thiamethazine stimulated reproduction of B.dorsalis in all stages,and the degree of stimulation decreased with its growth and development.In order to further study effect of thiamethazine on reproduction of B.dorsalis,juvenile hormone,ecdysone,vitellogenin and vitellogenin receptor content of female in different stages of female adults treated with sublethal concentration of thiamethazine were measured.After 24 hours,the contents of the four indexes increased significantly with the increase of the concentration of sublethal.On the third day after treating with sublethal concentration(LC₁₀)of thiamethazine there was no significant difference in the content of vitellogenin and ecdysone,feeding with sublethal concentration(LC₃₀)of thiamethazine was significantly increased,and the contents of vitellogenin receptor and juvenile hormone increased significantly in LC₁₀ and LC₃₀ compared to the control.The contents of juvenile hormone,ecdysone,vitellogenin and vitellogenin receptor on the 6th day after feeding with sublethal concentration(LC₁₀,LC₃₀)of thiamethazine were significantly increased compared to the control,The contents increased with the increase of sublethal concentration(LC₁₀ and LC₃₀)of thiamethazine.By using fluorescence quantitative PCR technique,expression of related genes of vitellogenin and vitellogenin receptor was determined.The results showed that gene expression of vitellogenin and vitellogenin receptor was up-regulated compared to tocontrol,which is similar to contents of vitellogenin and vitellogenin receptor.The results suggest that thiamethazine stimulates reproduction of B.dorsalis mainly by juvenile hormone regulation of vitellogenin and its receptor to increase fecundity of B.dorsalis.