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Abstract: The onion thrips, Thrips tabaci (Thysanoptera: Thripidae), is one of the major greenhouse pest on sweet pepper in Iran. Lethal effects of abamectin, spinosad and chlorpyrifos were evaluated on three life stages (1st, 2nd larval instars and adult) by using two bioassay techniques, leaf dipping method and Thrips Insecticides Bioassay System (TIBS). All insecticides were significantly more toxic to first instar than 2nd instar larvae and adult but there were not significant difference between 2nd instar of larvae and adult. Comparison between two methods showed that abamectin was the most toxic insecticide in TIBS method and its lethal concentration values were significantly less than leaf-dipping method. Chlorpyrifos has the least toxicity in both methods and its toxicity in TIBS was higher than leaf dipping method. Spinosad was the most toxic among the tested insecticides in leaf dipping method and its toxicity was not significantly different in two methods. Results indicated that TIBS is an appropriate method for estimating spinosad toxicity. But this method can not be used for abamectin and chlorpyrifos toxicity evaluation.