Effects Of Low Doses Of Insecticides On Cnaphalocrocis Medinalis And Its Egg Parasitoids

Rice Leaffolder(Cnaphalocrocis medinalis Guenee)is one of the important long-distance migratory insect pests of rice.It widely distributes in south and south eastern Asian countries,has strong adaptability to the environment.So far,chemical control remains one of the main measures for the control C.medinalis.After spraying of insecticides,many factors could lead to the exposure of insects to low dose of insecticides,such as degradation due to high temperature,dilution by rainfall and others.The exposure of insect to low dose of insecticides could influence the insects by different routines.In this study,insecticidal susceptibilities of C.medinalis from different geographical populations were determined,and the effect of low dose of insecticides on the folding behaviors,growth and development,and detoxified enzymatic activity of C.medinalis were investigated.And furthermore,the effect of low dose of insecticides targeted at C.medinalis was investigated on the growth,development and reproduction of two egg parasitoids Trichogramma japonicun and Trichogramma dendrolimi.Main results were shown as below.1.The insecticidal susceptibilities of C.medinalis from different geographical populations were determined by detached leaf dipping method in 2015 and 2016.Among 4 populations collected from Hepu(HP)and Nanning(NN)of Guangxi,Changsha(CS)of Hunan and Hangzhou(HZ)of Zhejiang in 2015,CS had lowest susceptibility to chlorpyrifos;NN population had the lowest susceptibility against chlorantraniliprole;HZ population had the lowest susceptibility against emamectin benzoate;CS population had the lowest susceptibility against spinosad.The susceptibility of C.medinalis varies 1-4 folds among different populations in 2015.Among 4 populations collected from Hepu(HP),Leizhou(LZ),Changsha(CS)and Hangzhou(HZ)in 2016,HZ population had the lowest susceptibility against chlorpyrifos;LZ population the lowest susceptibility against chlorantraniliprole,spinosad and emamectin benzoate.The susceptibility of C.medinalis varies 1-7.1 folds among populations in 2016.Compared with 2015,the decreases in susceptibilities of C.medinalis in 2016 were observed on CS population against 4 insecticides,HZ population against chlorpyrifos,chlorantraniliprole and spinosad,and HP population against chlorantraniliprole,emamectin benzoate and spinosad.2.The sublethal effects of four insecticides(chlorpyrifos,chlorantraniliprole,emamectin benzoate and spinosad)were investigated on the folding and spinning behaviors of the third to fifth instar C.medinalis larvae(L3-L5)after expoure to insectides in the second instar period.LC25 treatment of chlorpyrifos shortened selection time of L5 to rice leaf,and lowered binds per primary fold by L4 and L5,LC10 treatment of chlorantraniliprole increased binds per primary fold by L4 and head wings per bind by L5.LC10 of emamectin benzoate shortened the primary fold length by L5,decreased head wings per primary fold by L3 and L4,and head wings per bind by L3 and L5,while LC25 of emamectin benzoate shortened fold length per 24h by L5 and folding time of L3.The LC10 of spinosad decreased the fold length per 24 h,head wings per primary fold and head wings per fold by L5.LC25 of spinosad influenced the all observed folding characteristics in L5 including prolonged selection time to rice leaf,decreased primary fold,binds per primary fold,binds per fold and fold length per 24 h.3.The developmental growth of C.medinalis was observed after treatment with low concentration of insecticides(LC10 and LC25).Results showed that chlorpyrifos with low concentration prolonged the larval growth and development duration,shorten pupal duration,increased the activities of esterase and GST;chlorantraniliprole with low concentration shortened the longevity of female adults,enhanced the activities of esterase and GST;spinosad with low concentration did not influence the larval duration,but increased the activities of esterase and GST;and reduced the activity of MFO.Emamectin benzoate with low concentration did not influence the larval duration,declined the activity of MFO,while activities of EST and GST increased at LC10 treatment.4.The toxicity of chlorpyrifos,emamectin benzoate and chlorantraniliprole,the target agents to C.medinalis,on three egg parasitoids Trichogramma species were increased with the increased chemical concentration.Different low concentrations of spinosad did not significantly affect the mortality of Trichogramma species.Chlorantraniliprole had no effect on the development and reproduction of T.japonicum,T.chilonis and T.dendrolimi in FO and F1 were not significantly affected by the four chemicals with low concentrations.Emergence rate of T.japonicum and T.dendrolimi in F1 were reduced after treated by chlorpyrifos with low concentrations.Emamectin benzoate with low concentration imbalanced the sex ratio and reduced the parasitic capacity of offsprings of T.japonicum.And parasitic capacity of T.japonicum in FO was increased after treated by spinosad with low concentrations.