Anti-juvenile hormone activity of azacoprostane depends on inhibition of molting

Abstract

Administration of the anti-ecdysteroid azasteroid 25-azacoprostane (AZCP) to larvae of the tobacco hornworm, Manduca sexta, often inhibits molting without curtailing growth. As a result, AZCP-treated larvae may attain weights 2–3 times greater than normal during the first four instars. This may explain the paradoxical anti-juvenoid activity of AZCP evident in the precocious metamorphosis of AZCP-treated fourth-instar larvae, which was noted only after those larvae attained unusually large weights over 2 g. The weight interval of 2–3 g has been previously identified as a critical threshold for initiation of metamorphosis by normal final (fifth) instar larvae. The premature attainment of this weight threshold by AZCP-treated fourth-instar larvae probably activates the same premetamorphic sequence of events that normally occurs in the fifth instar at this threshold, including activation of potent endogenous anti-juvenoids. Anti-juvenoid activity limited to the penultimate instar is likely to be a general feature of compounds that block molting without inhibiting growth. © 1997 Wiley-Liss, Inc.