Sublethal concentrations of the bisacylhydrazine moulting hormone agonists, RH-5849, and tebufenozide (RH-5992) were fed to sixth (final) instar larvae of Spodoptera litura. Both RH-5849 and tebufenozide adversely affected the mating success of S. litura when the surviving treated males were crossed with normal females. The ecdysone agonists decreased the longevity of treated males and of untreated females when crossed with treated males. The number of eggs laid by untreated females mated to treated males was decreased, and the fertility (percentage of hatching success) of the resulting eggs was reduced. These effects on male reproductive success were at least in part explained by a reduction in the number of sperm transferred during mating. The adverse effects of tebufenozide on male reproductive function were qualitatively the same as those of RH-5849, but tebufenozide was active at lower concentrations. To understand the reason for these adverse effects on male reproduction, we investigated the effects of the insecticides on male reproductive physiology. Male reproductive tract development and testicular volume of resulting adult moths were adversely affected by sublethal larval exposure to the ecdysone agonists. Dose-dependent reductions occurred in the production of eupyrene and apyrene spermatozoa in the adult testes, and in the number of spermatozoa released from the testes into the male reproductive tract. The descent into the male tract of both eupyrene and apyrene sperm was found to start at the normal stage of development in both normal and treated insects, but the daily rhythm of sperm descent was subsequently disturbed in the insecticide-treated moths. This affected the numbers of sperm in the upper vas deferens (UVD), seminal vesicle (SV), and duplex (duplex). Injections of RH-5849 given to pharate adult or newly emerged adult S. litura also caused drastic reduction in the number of sperm in the upper regions of the male tract, when measured 24 It after injection. The possible importance of pest population reduction through the sublethal anti-reproductive effects of insecticides is discussed. (C) 2004 Elsevier Ltd. All rights reserved.