Studies On The Selective Degradation Of Indoxacarb Enantiomers On Rice And Their Differences In Toxicity To Silkworms

A detection method for analysis of indoxacarb enantiomers residues in rice plant,brown rice and rice hull using ultra-high-performance liquid chromatography-tandem high resolution mass spectroscopic（UPLC-HRMS/MS）.The field trials was conducted in 2016 and 2017 in Guizhou and Hunan.The enantioselective degradation behaviours and whether the enantiomorphic transformation of indoxacarb enantiomers happen or not in rice plant were investigated by the dissipation test of 15%indoxacarb suspension concentrate（SC,EF=0.71）and 150 g/L indoxacarb emulsifiable concentrates（EC,EF=1.0）in rice plant.Simultaneously,the optical purity of preparative indoxacarb enantiomer（evaluating by ee）were greater than 98.12%.And then the toxicity difference of the two enantiomers of indoxacarb on Bombyx mori were explored by acute toxicity test.Rice plant,brown rice and rice hull were extracted with acetonitrile,sodium chloride salting out,purified by primary secondary amine（PSA）and C18 after concentration,detected by UHPLC-HRMS/MS.Linearity range of R-indoxacarb and S-indoxacarb standard in rice plant,brown rice and rice hull were 0.00625⁰.625μg/mL and 0.01875¹.875μg/mL,respectively,obtaining correlation coefficient（R²ï¼‰>0.99 all.The limit of quantitation（LOQ）were 12.5×10^-3,6.25×10^-3 and25.0×10^-3 mg/kg and the limit of detecton（LOD）were 0.2×10^-3,0.07×10^-3 and0.8×10^-3 mg/kg in rice plant,brown rice and rice hull of each enantiomers of indoxacarb.The recoveries of R-indoxacarb and S-indoxacarb at three spiked levels in three matrixes were 75.03¹02.61%and 74.00⁸8.91%,respectively.And the relative standard deviation（RSD）were 2.77¹1.56%and 3.22¹1.28%,respectively.The accuracy,precision,and sensitivity can meet the needs of pesticide residue analysis and successfully apply to the real samples under field conditions.The enantioselective degradation behaviours and whether the enantiomorphic transformation of indoxacarb enantiomers happen or not in rice plant were investigated in this work.The application dosage of 15%indoxacarb SC and 150 g/L EC in degradation and terminal residue experiments were 51.5 and 36.0 g a.i./ha,respectively（The content of active ingredient S-indoxacarb are same）.After spraying15%indoxacarb SC in Guizhou and Hunan paddy field in 2016 and 2017,the half-live(T_1/2)of R-indoxacarb were 4.20⁶.60 d,and the T_1/2 of S-indoxacarb were3.45³.96 d,while the enantiomer fraction（EF）reduce from 0.72 to 0.49.The values of T_1/2 and EF indicated that enantioselective degradation was appeared in rice plant and preferential degradation of S-indoxacarb.Meanwhile,no R-indoxacarb was observed in rice plants in the whole experiment after spraying 150 g/L indoxacarb EC,indicating that enantiomertic transformation did not occur in rice plants.The initial concentration of S-indoxacarb in same times and regions was different,illustrating that they were affected by the formulation of indoxacarb.The EF values in three matries were 0.58⁰.66 after applying indoxacarb SC in the final residue experiment in Guizhou and Hunan in 2016 and 2017,verifying further the enantioselective degradation of indoxacarb in the rice.the order of residues of indoxacarb enantiomers was rice plant>rice hull>brown rice in both experimental areas whatever application indoxacarb SC or indoxacarb EC and the residues in brown rice far less than rice plant and rice hull.It was maybe caused by the slowly moving of indoxacarb in the plant,rarely pesticide liquid transformed from the upper to the middle and lower,so the indoxacarb residues in the rice husk and rice were far lower than the plants.Then,the parcel role of rice hull,which sharply decreased the indoxacarb transferred from rice hull to brown rice,lead to pindling residues in brown rice as well.150 g/L indoxacarb EC,used for prevention and control of Cnaphalocrocis medialis Guenee,spraied 2 times with the dosage of 36.0 g a.i./ha and7 d-interval.The rice sample was safe after 14 d of the last applicaton.15%indoxacarb SC,used for prevention and control of Cnaphalocrocis medialis Guenee,spraied 2 times with the dosage of 51.5 g a.i./ha and 7 d-interval.Obtaining the residues of indoxacarb in brown rice were 0.037⁰.127 mg/kg,greater than 0.1 mg/kg（the MRL of indoxacarb on brown rice set in China）.Thus,it was recommended that the harvest interval should exceed 14 d after the last application.Finally,the toxicity difference of the two enantiomers of indoxacarb on Bombyx mori were explored by acute toxicity test using leaf immersion method.The result indicated that the oder of acute toxicity on bombyx were enriched S-indoxacarb（2.33S/1R）>S-indoxacarb>R-indoxacarb,and R-indoxacarb showed moderate toxicity on bombyx while S-indoxacarb and enriched S-indoxacarb（2.33S/1R）high toxicity.There are acute toxicity differences between two enantiomers of indoxacarb on bombyx and the toxicity order was S-indoxacarb>R-indoxacarb.