The facultative diapause of Ascogaster quadridentata depends upon the photoperiod experienced by its host. A host exposed to short day lengths (12L:12D) ceases development in its fourth stadium, spins a cocoon, and remains dormant throughout the winter. A first stadium parasitoid larva overwinters in a true diapause, within its dormant host.Dormancy of both animals is due to a failure of the host's endocrine system. Upon transfer of the host from dormancy-maintaining (OL:24D, 4°C) to dormancy-terminating conditions (DTC = 16L:8D, 25°C) a host cephalic factor is released. The cephalic factor blocked by host ligation within 48 h of transfer to DTC was circumvented by an injection of the host with 20-hydroxyecdysone. Farasitoid larvae that were either isolated within the abdomen of 20-hydroxyecdysone-injected hosts or held in vitro with the molting hormone initiated apolysis.An increased titer of hemolymph juvenile hormone, which was present 48 h after the host was transferred to DTC, may have had a prothoracicotropic effect upon the hostlparasitoid unit. A topical application of 1 ng of methoprene, concurrent to host transfer to DTC, accelerated parasitoid development. No parasitoid development was evident if the host was neck-ligated at the time of its methoprene treatment and transfer to DTC. These data suggest that the prothoracicotropic effect of methoprene was via the host head, rather than directly on the host prothoracic glands, or the parasitoid itself.Key words: Braconidae, Tortricidae, methoprene, juvenile hormone INTRODUCTIONAscogaster quadridentaka Wesmael (Braconidae) is an egg/larval parasitoid of the codling moth, Cydia pornonella (Tortricidae) L. Cox [l] described the life