To study the insecticidal activity and toxicity mechanism of spinosad,we assayed bioactivity of spinosad by leaf membrane method and its effects on the activities of detoxifying and protective enzymes in Malacosoma neustria testacea larvae by spectrophotometry.The results showed that spinosad had an extremely high toxicity against the 4th and 5th instar larvae.The glutathione S-transfer (GST) activity in 4th and 5th instar larvae was firstly inhibited,then induced,and finally inhibited,while the mixed-functional oxidase(MFO) activity was inhibited and then enhanced.The induced effect on carboxylesterase(CarE),the induced and inhibited effect on acetylcholinesterase (AchE) and peroxidase(POD),and the complicated effects on superoxide dismutase(SOD) and Catalase(CAT) were determined in 4th instar larvae with spinosad treatment.However,the obvious inhibition effects were found on the activities of CarE,AchE,SOD and POD in the 5th instar larvae whereas the catalase(CAT) activity was inhibited, then increased,and inhibited finally.Therefore, spinosad could effectively disrupt and interfere not only the detoxifying and protective enzymes but also normal physiological metabolism of M.neustria testacea and show extremely high toxicity against this pest. To study the insecticidal activity and toxicity mechanism of spinosad, we assayed bioactivity of spinosad by leaf membrane method and its effects on the activities of detoxifying and protective enzymes in Malacosoma neustria testacea larvae by spectrophotometry. The results showed that spinosad had an extremely high toxicity against the 4th and 5th instar larvae. glutathione S-transfer (GST) activity in 4th and 5th instar larvae was examined inhibited, then induced, and finally inhibited, while the mixed-functional oxidase (MFO) activity was inhibited and then enhanced. induced effect on carboxylesterase (CarE), the induced and inhibited effect on acetylcholinesterase (AchE) and peroxidase (POD), and the complicated effects on superoxide dismutase (SOD) and Catalase (CAT) were determined in 4th instar larvae with spinosad treatment. , the obvious inhibition effects were found on the activities of CarE, AchE, SOD and POD in the 5th instar larvae and the catalase (CAT) activity was inhibi ted, then increased, and encouraging finally.Therefore, spinosad could effectively disrupt and interfere not only the detoxifying and protective enzymes but also normal physiological metabolism of M.neustria testacea and show extremely high toxicity against this pest.