Abstract: 【Objective】In order to study the insecticidal activity and preliminary insecticidal mechanism of Artemisia divaricata against Mythimna separate (Walker),and provide theoretical basis for efficient and green prevention and con-trol of M.separate.【Method】The insecticidal activities of extracts from 10 species Artemisia plants and four parts extracts with different polarities from A.divaricata against the 3^rd instar larvae of M.separata were determined by the method of soaking insects and leaves.The main insecticidal methods of the petroleum ether extract of A.divaricata were determined by the leaf disc method,leaf soaking method,and insect soaking method respectively.The contact toxicity of the petro-leum ether extract,fractions,and main compounds of A.divaricata were determined by insect immersion method.The petroleum ether extract of A.divaricata was separated by column chromatography and analyzed by gas chromatographymass spectrum to determine its active components,and the four enzyme activities of the active components was deter-mined.【Result】Contact toxicity effect of petroleum ether extract of A.divaricata against the 3^rd instar larvae of M.separata was the best.The results showed that the lethal medium concentration(LC₅₀) of the petroleum ether extract of A.divaricata on the 7^th d of treatment for the 3^rd instar larvae of M.separata was 6.85 mg/mL.After silica gel column chromatography separation,21 fractions were obtained,and fraction 8 showed the best contact toxicity.According to gas chromatographymass spectrum,hexadecanoic acid ethyl ester (49.57%),tridecane (14.55%),octadecanoic acid ethyl ester (4.86%) and nonane (4.74%) were the main compounds in fraction 8.Hexadecanoic acid ethyl ester had the best contact activity among the four compounds,on the 7^th d of treatment,LC₅₀ on M.separata was 1.82 mg/mL.On the 7^th d of treatment with hexadecanoic acid ethyl ester LC₅₀,the scanning electron microscopy results showed that the wax layer particles on the surface of M.separata were loosely arranged and the gaps increased.When treated with hexadecanoic acid ethyl ester LC₅₀ for 1,3,5 and 7 d,the acetylcholinesterase(AChE) and Na⁺-K⁺-ATP of M.separata showed inhibition effect over time,carboxylesterase(CarE) showed activation first and then inhibition,and catalase(CAT) showed inhibition first and then activation.【Conclusion】A.divaricata has fine insecticidal activity against M.separate.Hexadecanoic acid ethyl ester of the petroleum ether extract from A.divaricata has the best contact toxicity and can be developed and utilized as a potential compound for biological control of M.separate.