Abstract:More than ten populadon of masson pine caterpillar (Dendrolimus punctatus Walker) in Jiangxi Province were tested by the methods of the bioassay and bioohemisby. The results showed that the tolerance to pyrethroids was related to the activity of carboxylestense in masson pine carterpillar The activity of carboxylesterase was higher in population more tolerant to deltamethrin, fenvalerate and cypermethrin than in the population less tolerant Treated carboxylestemse of masson pine carterpillar with five synergists, SV1, and TPP showed strongly inhibition, and as the amount of SV1 or TPP increased, the activity of carboxylesterase decreased correspondingly. After applying topically massonpine carterpillar with the same amount of SV1 or TPP, the activity of carboxylesterase changed regularly.