Evaluation of Field Variation in Flower Thrips, Frankliniella intonsa Trybom to Spinosoid Insecticide in Inner Mongolia, China

Flower thrips are a major vegetable and crop pest in Inner Mongolia, China. Due to the frequent application of insecticides for pest control, insecticide resistance in the species has been reported previously. Therefore, we evaluated the field variation in four populations due to the use of spinosoid insecticides, spinosad and spinetoram, by assessing stomach and contact toxicities. The LC50 value of the stomach toxicity for the DLT population for spinosad and spinetoram was 1.22–2.25 and 1.19–1.90-fold higher, respectively, compared with values in the other three populations. The LC99 value of stomach toxicity for the QBM population for spinosad was 9.22–161.57-fold higher compared with the other three populations, whereas the value for spinetoram in the HHG population was 3.28–6.77-fold higher compared with the other three populations. These results clearly showed that the field variation due to spinosoid insecticide use differed across the region. For spinosad, the LC99 values for stomach toxicity were higher than those for contact toxicity in all four populations, with action ratios ranging from 0.031 to 0.448. In contrast, for spinetoram, the LC99 values for stomach toxicity were lower than those for contact toxicity in all populations, with action ratios ranging from 52.64 to 1901.21. Sequence analysis of the FIα6 subunit of the nAChRα6 gene revealed no point mutations in any of the four populations. Therefore, we further investigated the activity of the detoxification enzyme, CYP450s, by analyzing the synergistic effect of piperonyl butoxide (PBO) on the different susceptibilities to spinosad in the populations. The LC99 value of spinosad was reduced 59.64-fold in the PBO-treated DLT population and 2.52-fold in the PBO-treated HLG population compared with the untreated population. These results clearly indicate that the field variation in flower thrips due to spinosoid insecticides in Inner Mongolia, China, may be due to the activation of the detoxification enzyme, CYP450s.