Abstract: The effects of fourteen active compounds previously isolated from Rhododendron molle G. Don on the cuticle components of Spodoptera litura larvae and their structure-activity relationship were studied to illustrate the mode of action and optimize rhodojaponins structure. The structures of three new compounds, rhodomolin A, B and C, were elucidated based on IR，UV，NMR and MS spectroscopic data. When the fourth instar larvae of S. Litura were treated with 50 mg/L of compounds rhodojaponin Ⅲ, rhodomolin A，rhodomollein Ⅰ，rhodomolin B，rhodomolin C，rhodomollein ⅩⅧ, grayantoxin Ⅲ and azadirachtin, an known botanical insect growth regulator, respectively, the weight of larvae and pupae and the emergence rate were significantly less than CK, which showed greater insect growth inhibition activities than other tested compounds from R. molle. After 120 h treatment, all the cuticle chitin contents of larvae, both treated with compounds and CK, were 28.15% -35.18%, which show no significant difference. When treated with compounds rhodojaponin Ⅲ， rhodomolin A， rhodomolin B， rhodomolin C， rhodomollein ⅩⅧ and azadirachtin, however, the contents of total protein of larvae cuticle significantly increased and those of total fats decreased than those of CK， respectively. Furthermore, the contents of water soluble protein, hydrogen bond combined prote in and covalent bond combined protein in the cuticle decreased significantly, while the content of weak bond combined protein increased significantly and the content of electrovalence bond combined protein was not significantly different from that of CK when larvae were treated with compound rhodojaponin Ⅲ and azadirachtin. It was inferred that the action mode of insect growth inhibition in rhodoja ponins is not to inhibit chitin syntheses, but to disturb internal secretion, and significantly decreasing the content of water soluble protein in cuticle would be one important mechanism. Preliminary analysis of their structure-activity relationship showed that substitute group structures of C-2,3-epoxy, C-6, C-10 and C-14 were most important to these grayanoid diterpenids with insect growth inhibition activity. The difference of model of inhibition action between rhodojaponins and azadirachtin was also discussed.

Key words: Rhodojaponins, Spodoptera litura, cuticle component, structure-activity relationship