Abstract: A sublethal strain of western flower thrips (WFT),Frankliniella occidentalis (Sub) was obtained by selecting the relative susceptible strain of WFT (SS) with LC25 of spinosad.After 36 generations of selection,its sensitivity to spinosad decreased 5.2-fold compared to the SS strain.The second instar larvae of the SS and Sub strains were treated with LC10 or LC25 of spinosad and the specific activity of the detoxifying enzymes was determined after 1,6,12,24 and 48 h, respectively.The results showed that for the untreated control group the specific activity of the carboxylesterase (CarE) and the glutathione S-transferase (GSTs) in the Sub strain were higher than those in the SS strain except 48 h after treatment.In addition,the difference of CarE activity between the two strains after treatment for 6 h was significant,the difference was 1.37-fold.The activity of the mixed-function oxidases (MFOs) of the Sub strain were higher than those of the SS strain,and showed significant difference between 1 h and 6 h after treatment.The differences were 1.62-fold and 1.36-fold,respectively.For the spinosad treated groups at LC10 or LC25,the CarE activity in the Sub strain was higher than that in the SS strain.The GSTs and MFOs activity in the Sub strain was lower than that in the SS strain shortly after treatment by LC25 of spinosad,but turned to be higher with the time prolonged.The results indicated that after the SS strain was seletced with sublethal concentration of spinosad for more than 36 generations,the activity of the detoxification enzymes showed an increasing trend.The Sub strain had stronger ability of regulating the detoxifying enzymes than that of the SS strain when treated with sublethal concentration of spinosad.