Last-instar larvae of the beet armyworm, Spodoptera exigua, fed on diet containing the nonsteroidal molting hormone agonist, tebufenozide, at 1 mg/liter, exhibited a premature induction of chitin synthesis. Treatment with tebufenozide apparently caused normal amounts of chitin to be formed in the cuticle. This effect complemented the precocious induction of larval molting (apolysis) that was observed in response to tebufenozide. Subsequently, normal ecdysis was inhibited and treated larvae died while still within their old cuticle.
In another series of experiments, 1-day-old pupae of the European corn borer, Ostrinia nubilalis, were injected with various doses of tebufenozide or 20E. Incorporation of N-[acetyl-1-14 C]glucosamine served as a measure of chitin production and was monitored in cultured day-4 and day-5 claspers. Tebufenozide at 100, 200, and 400 ng induced synthesis in day-4 claspers and incorporation was enhanced 8-fold as compared to control claspers. A similar stimulation of chitin synthesis was observed with 2,000 ng of 20E. For day-5 claspers, high doses of tebufenozide inhibited chitin synthesis. Treatment with tebufenozide had a deleterious effect on adult development and even at the lowest dose injected, 10 ng, adult eclosion was completely inhibited.
Our current results indicate that in both larvae and pupae/ pharate adults, tebufenozide can induce the premature synthesis of chitin as can 20E, but the effective dose of tebufenozide is much lower than that for 20E. The results support the view that tebufenozide, although nonsteroidal in nature, is able to mimic the activity of ecdysteroids in inducing chitin synthesis. However, development in treated insects is abnormal and the newly formed cuticle is imperfect and unable to survive a molt. Arch. Insect Biochem. Physiol. 41:33-41, 1999.