Abstract: Periplocoside P, which was isolated from the root bark of the insecticidal plant Periploca sepium Bunge, has strong insecticidal activity. To explore the insecticidal mechanism of periplocoside P, the classical determination methods for insect protease activity were used to compare the different effects on the activities of trypsin-like protease and chymotrypsin-like protease in the midgut of the 6th instar larvae of Mythimna separata and Agrotis ypsilon beween periplocoside P and periplocoside E which has no insecticidal activity. The results showed that periplocoside P displayed strong activation effect on the weakly alkaline trypsin-like protease activity in the midgut of M. separata larvae, which was 3.43 times as high as the control, and the activation time last up to 8 h; while periplocoside E did not show any activation effect. Significant differences at the 0.01 level between the effects of periplocosides P and E on the weakly alkaline trypsin-like protease activity of M. separata larvae were observed. At 2 h, 4 h and 8 h after treatment, the weakly alkaline trypsin-like protease activity in the midgut of M. separata larvae treated by periplocoside P were 15.4, 106.8 and 242.7 times as high as that in the midgut of M.separata larvae treated by periplocoside E. The results of protease activity determination also showed that the trypsin-like protease activity in the midgut of A. ypsilon larvae was relatively low, and the activation effect of periplocoside P on trypsin-like protease was also weak, compared to that on M. separata, which may be one of the factors that periplocoside P had insecticidal activity against M. separata, while A. ypsilon was insensitive to it. In addition, periplocosides P and E showed no obvious effect on chymotrypsin-like protease activity in the midgut of the tested insects. It is so inferred that the activation of the weakly alkaline trypsinlike protease in the midgut of M. separata may be one of the factors that periplocoside P causes toxicity to insects.

Key words: Mythimna separata, Agrotis ypsilon, botanical insecticide, periplocoside P, insecticidal action, trypsin-like protease, enzyme activity