Residue and dissipation of dimethoate and omethoate in Vigna unguiculata at different periods of planting

To evaluate the residues of dimethoate in Vigna unguiculata after its application during different growth periods, pesticide residue was determined in three scenes, including seedling stage, fruiting stage with application twice and fruiting stage with application thrice. 40% Dimethoate emulsifiable concentrate was sprayed at different active ingredient dosages (540, 600 and 900 g/hm2) and samples were collected at intervals. Residues were extracted with acetonitrile, cleaned by dispersed purification with C18 and determined with UPLC-MS/MS. The results showed that LOQ of dimethoate and omethoate were both 0.01 mg/kg. At the spiking level of 0.01-2 mg/kg, the average recoveries ranged from 77% to 101% and the RSD was 3.1%-17%. During the seedling stage and 10 days after the final application, the highest residue of dimethoate was 0.043 mg/kg, which was lower than MRL value. However, other than the 540 g/hm2 treatment group, the residue of its metabolite omethoate at 14 days after the application was over the MRL value (0.02 mg/kg). The metabolite was still 0.013 mg/kg at 18 days after the application. In terms of fruiting stage with application twice and thrice, the residues of dimethoate were reduced to below 0.5 mg/kg 3 days and 5 days after the application, respectively. Meanwhile, the detected concentration of metabolite omethoate was still as high as 0.036 mg/kg after 10 days, and only the residue of the lowest dosage treatment could meet the requirement of MRL value. Based on these results, it could be concluded that the risk of over MRL value was mainly due to the metabolite omethoate. As a consequence, the application during fruiting period should be prohibited, while seedling period application should be exactly controlled.