Abstract:

 F3-28 is an insecticidal fraction which was isolated from the extract of Periploca sepium bark.  In order to understand the action mechanism of the insecticidal fraction F3-28, the effects of F3-28 on the activities of proteases, trypsin-like protease, chymotrypsin-like protease, lipase and amylase in the midgut of the 5th instar larvae of Mythimna separata and Agrotis ypsilon were studied. The results showed that after A. ypsilon larvae which are insensitive to F3-28 ingested this fraction, the activities of proteases, lipase and amylase in the midgut changed indistinctly. However, after M. separata larvae which are sensitive to F3-28 ingested this fraction, at 2, 4, 8, 10, 12, 24 and 48 h post treatment their midgut protease activity was 0.76, 2.53, 1.45, 1.88, 1.54, 1.46 and 1.70 times as high as that of the control, respectively, and their midgut trysin-like protease activity was 1.60, 1.75, 1.60, 1.12, 1.39, 1.16 and 1.15 times （BAPNA as the substrate） and 1.68, 1.95, 1.53, 1.26, 1.15, 1.13 and 1.14 times (TAME as the substrate ) as high as that of the control, respectively. Similarly, after M. separata larvae ingested F3-28, at 2, 4, 8, 10, 12, 24 and 48 h post treatment their midgut chymotrypsin-like protease activity was 0.50, 1.66, 1.44, 1.18, 1.54, 1.08 and 1.03 times as high as that of the control, respectively, and their midgut amylase activity was 1.60, 1.35, 1.27, 1.31, 1.23 and 1.20 times as high as the control, respectively. The increase in activities of proteases and trysin-like protease in the midgut was dependent on the concentration of F3-28, but that of chymotrypsin-like protease and amylase in the midgut was not. After M. separata larvae ingested F3-28, their midgut lipase activity changed inconspicuously. These results imply that the insecticidal mechanism of F3-28 may be related with the activation of proteases, especially trypsin-like protease, in the larval midgut.

Key words: Periploca sepium, plant extract, insecticidal activity, protease, trypsin-like protease, chymotrypsin-like protease, lipase, amylase