IN-VIVO AND IN-VITRO EFFECTS OF THE NONSTEROIDAL ECDYSTEROID AGONIST TEBUFENOZIDE ON CUTICLE FORMATION IN SPODOPTERA-EXIGUA - AN ULTRASTRUCTURAL APPROACH

To evaluate the ecdysteroid-like mode of action of tebufenozide (RH-59 92), the effects on the fine structure of the integument in last- and third-instar larvae of the beet armyworm, Spodoptera exigua, and on cu ticle formation in cultured imaginal wing discs, were studied. After 3 h of treatment with tebufenozide, the first signs of a normal moult w ere observed in treated larvae. A few hours later, ecdysial space form ation and secretion of a new epicuticle were started. Furthermore, the new cuticle was incomplete in treated larvae; the new procuticle was absent or contained only a very low number of lamellae. In addition, e pidermal cells showed many vacuoles and symptoms of degeneration with increase in time. Only a few lamellae of the old procuticle were diges ted, and normal ecdysis was inhibited which led to the presence of a d ouble cuticle within 24-48 h after treatment. Similarly, cultured disc s were stimulated to deposit a new cuticle within 12 h after cultivati on in a medium containing tebufenozide. Our observations in treated S. exigua larvae on the one hand and in imaginal discs cultured with teb ufenozide on the other hand are indicative of a hyperecdysteroid actio n, and confirm that the moult accelerating mode of action of tebufenoz ide resulted in a forced, untimely synthesis of cuticle by activation of epidermal or epithelial cells, and that its ecdysis inhibitory acti vity is mediated by its effect on post-apolysis processes. (C) 1996 Wi ley-Liss, Inc.