Dissipation,accumulation and risk assessment of thiamethoxam and beta-cyfluthrin in greenhouse celery after continuous spraying
Thiamethoxam and beta-cyfluthrin are commonly used insecticides for aphid control in greenhouse celery.It is of great significance to reveal their dissipation and accumulation characteristics for evaluating their dietary intake risks and optimizing application methods.Based on this,gas chromatography-tandem triple quadrupole mass spectrometry was used in the multi-reaction monitoring mode of mass spectrometry,The dynamics of residue digestion of two insecticides under three repeated spraying methods(combination I:thiamethoxam+beta-cyfluthrin,alternating Ⅱ:thiamethoxam-beta-cyfluthrin-thiamethoxam,alternating Ⅲ:beta-cyfluthrin-thiamethoxam-beta-cyfluthrin)and assessed the risk of chronic dietary intake.The results showed under the three continuous spraying methods,the original deposition amount was the highest after the second spraying(22 d after planting)and the lowest after the first spraying;and the celery leaf area index may be an important factor causing this phenomenon.According to the first-order kinetic equations,the digestion half-life of thiamethoxam and beta-cyfluthrin were 2.95-3.63 d and 2.33-3.11 d,respectively.Comparative analysis of residues after the safety interval of 7 d after three repeated spraying,residue of thiamethoxam and beta-cyfluthrin in combination I was higher than maximum residue limit(MRL)except for the first spraying(thiamethoxam:1mg/kg;beta-cyfluthrin:0.5 mg/kg);However,the residue of beta-cyfluthrin in alternating Ⅲ exceeds the MRL after the last spraying.Since thiamethoxam could be metabolized to clothianidin,dynamic residue of clothianidin was determined.The highest concentration was obtained at 3-5 d after spraying.However,the residue of clothianidin was lower than MRL(0.04 mg/kg)after the pre-harvest interval(PHI);The results of risk assessment of dietary intake showed that in this standardized residue test in Shaoxing,Zhejiang Province,thiamethoxam and beta-cyfluthrin were combined and applied alternatly for three times.After the PHI of 7 d,the contribution rate of thiamethoxam and beta-cyfluthrin residues in celery samples collected to risk quotient(0.11％～3.76％)was lower than 100％,and the long-term dietary intake risk was at an acceptable level.The results showed that compared with combined application,pesticide residues could be reduced to some extent by alternate spraying,and the alternate spraying of thiamethoxam-bata-cyfluthrin-thiamethoxam showed the lowest residue level and contribution rate of dietary intake risk,which was the relatively best application strategy.
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