Fate of Insecticide Spinetoram and Its Metabolites in Minor Crop Young and Small Radishes during Cultivation

Abstract

This study was aimed to optimize a residual analytical method of insecticide spinetoram (XDE-175-J and XDE-175-L) and its metabolites (N-demethyl-175-J/L, N-formyl-175-J/L) in minor crops (young and small radishes) using a QuEChERS (quick, easy, cheap, effective, rugged and safe) method combined with liquid chromatography tandem mass spectrometry (LC-MS/MS). For field trial, both minor crops were grown in separate greenhouse cultivation. The pesticides were applied to the plants with 7-day interval. Plants in plot 1 were treated with pesticides twice at 21 and 14 days before harvest and plot 2 were treated three times at 30, 21 and 14 days before harvest. Plants in plot 3 were also treated three times at 21, 14 and 7 days before harvest and plot 4 were treated three times at 14, 7 and 0 days before harvest. The samples were prepared with an optimized QuEChERS method for the LC-MS/MS analysis. Total concentration of spinetoram and metabolites in young radish was 0.40 mg L-1 for plot 1, 0.37 mg L-1 for plot 2, 0.61 mg L-1 for plot 3 and 0.96 mg L-1 for plot 4. Total concentration of spinetoram and metabolites in small radish were 0.69 mg L-1 for plot 1, 0.80 mg L-1 for plot 2, 1.22 mg L-1 for plot 3 and 1.86 mg L-1 for plot 4. For method validation, recovery tests were performed at two spiking levels (10MLOQ and 25MLOQ) in both radishes and limit of quantification (LOQ) and method limit of quantitation (MLOQ) were also determined. Average recoveries for young radish samples ranged 79.2 - 115.7% with <10% RSD (relative standard deviation). For small radish samples, average recoveries ranged 74.1 - 114.9% with RSD <10%. For test of storage stability, recoveries were obtained at one fortification level (25MLOQ) in young radish samples (ranged 78.7 - 114.2%) and small radish samples (ranged 74.5 – 115.4%) with RSD <10%, regardless of any sample type. LOQ for spinetoram was 0.005 mg L-1 (S/N>10) and MLOQ was 0.01 mg L-1. Based on the results from this study, pesticide registration status of spinetoram in young radish was established by Rural Development Administration (6th January 2016). The status specifies that, to control beet armyworm in young radish, the commercial insecticide product containing spinetoram should be diluted 2,000 times and can be applied twice to young radish anytime 7 days before harvest.