Parasitization by the gregarious larval endoparasitoid Glyptapanteles liparidis induces a dramatic increase in the hemolymph juvenile hormone (JH) titer (especially JH III) of its host larva, Lymantria dispar. Here, we investigated the role of the parasitoid larvae in JH synthesis and release by in vitro and in vivo experiments. GG-MS analyses confirmed that the rising hemolymph JH titer coincided with the time at which the parasitoids molt to the second larval instar. Peak values in host hentolymph titers were observed prior to parasitoid emergence, and titers dropped to negligible,levels within 24h after parasitoid emergence. Whole body extracts from excised second instar parasitoids yielded JH III and trace amounts of JH II. The in vitro secretory activity of the corpora allata (CA) of L. dispar larvae was not enhanced by parasitization. When the host's CA were separated by neck ligation, we found elevated JH III titers, but no JH 11 in the hemolymph of the posterior section, which contained the parasiioids. Parasitoids that were kept in in vitro culture produced and released only JH III. The parasitoids' ability to secrete JH and to molt independently from their host's molting cycles indicates that at least. second instar parasitoids are hormonally self-reliant. [copyright] 2004 Elsevier Ltd. All rights reserved. Lymantria dispar (Lymantriidae) : Host Glyptapanteles liparidis (Braconidae) : Parasite Circulatory system; Endocrinology; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Glyptapanteles liparidis; Lepidopteran hosts; Parasitism induced effects on host JH titer; role of parasitoid larvae; Lymantria dispar Lymantria dispar; Haemolymph; Hormones; JH titer; Hymenopteran parasites; Parasitism induced effects on host JH titer; role of parasitoid larvae; Glyptapanteles liparidis none