关键词: 菜青虫, 苍耳, 甾醇, 拒食活性, 酶活性, 血淋巴, 中肠组织

Abstract: Sterols are important secondary metabolites in plants with a variety of biological activities. In order to reveal the mechanism of sterols from plants against insect pests, the effects of sterol constituents (sterol A and sterol B) extracted from Xanthium sibiricum (Compositae) on feeding, enzyme activities in the hemolymph and midgut, and midgut tissues of 4th instar larvae of Pieris rapae treated by feeding on leaf-discs of crucifer Brassica oleracea were investigated. The results showed that the feeding of P. rapae larvae was obviously deterred by sterol A and sterol B with the AFC₅₀ of 0.0229 mg/mL and 0.0147 mg/mL, respectively, at 24 h after treatment. The activities of midgut amylase, protease and carboxylesterase (CarE) were significantly lower than those of the control within 36 h treatment, and sterol B showed stronger effects, which inhibited activities of midgut protease by 23.74% and 58.59%, and activities of midgut CarE by 49.01% and 83.03% at 24 and 36 h after treatment, respectively. Meanwhile, the content of hemolymph protein was reduced and the CarE activity in hemolymph was induced within the exposure time. Moreover, the microstructures of the tested larval midgut tissues exhibited such distinct pathological changes as the micro-rods of cylindrical epithelium dissolved and the goblet cells in the midgut epithelium deformed at 24 h after treatment. The results suggest that the inhibition of digestive enzyme activity and the damage of midgut tissues might be involved in feeding deterrence of the sterols from X. sibiricum against P. rapae larvae, and the proportion difference of plant sterols may affect their effects on insects.

Key words: Pieris rapae, Xanthium sibiricum, sterols, antifeedant activity, enzyme activity, hemolymph, midgut tissues