Abstract: 【Aim】 Isolated from the rootbark of Diatamnus dasycarpus Turcz., fraxinellone exhibits stomach toxicity against pests. This study aims to clarify the insecticidal mechanism of fraxinellone by investigating the effects of fraxinellone on the peritrophic membrane of the 6th instar larvae of Mythimna separata.【Methods】 Through biochemical methods and SEM observation, the content of carbohydrates, the contents and components of proteins and the surface structure of the peritrophic membrane of the 6th instar larvae of M. separata after in vivo and in vitro treatments with fraxinellone were studied. 【Results】 In vivo treatment of fraxinellone (20 mg/mL) resulted in a drop of the protein content in the 6th instar larvae of M. separata. However, after M. separata larvae were subjected to in vivo (20 mg/mL fraxinellone) and in vitro (8 mg/mL fraxinellone) treatments with fraxinellone, the contents of carbohydrates of both treatments increased, which were 1.75 and 2.17 folds as high as those of the control, respectively. SDS-PAGE result showed that partial proteins were hydrolyzed in the 6th instar larvae of M. separata subjected to in vivo and in vitro treatment with fraxinellone. SEM observation showed that the surface structure of peritrophic membrane changed after the 6th instar larvae of M. separata were treated with fraxinellone in vivo and in vitro. 【Conclusion】 These results showed that fraxinellone affected the contents of carbohydrates and the contents and components of proteins, and altered the surface structure of the peritrophic membrane of M. separata larvae. This study provides a basis for further research on the insecticidal mechanisms of fraxinellone against pests.

Key words: Mythimna separata, fraxinellone, peritrophic membrane, carbohydrate content, protein content, scanning electron microscopy