none High titers of juvenile hormone (JH) maintain developmental arrest in Manduca sexta larvae parasitized by Cotesia congregata. Parasitized hosts exhibit up to 9.5 times greater amounts of total hemolymph JH (from 0.6 [plus or minus] 0.09 to 2.51 [plus or minus] 0.43 ng/ml) compared to non-parasitized controls. Elevated titers are observed throughout the fifth instar, even beyond egression of the parasitoids on day 5. GC-MS analysis revealed that in hemolymph of unparasitized control larvae, JH I is the major homolog and levels of JH III are negligible; in parasitized individuals the amounts of JH I, II, and III rise, and JH III predominates. Neck ligation ensured separation of M. sexta's corpora allata from the posterior section, which contained most of the parasitoids in the infected insects. When the posterior region was sampled, JHs were not detected in the non-parasitized larvae, but in those parasitized, JH III was found (1.98 [plus or minus] 0.29ng/ml, 24 h post-ligation). JH III was the only homolog produced and secreted by the parasitoid in in vitro culture. This is the first report stating that a parasitoid secretes JH III and may contribute, at least in part, to the circulating titer in the host hemocoel, concurrently promoting host production of JH I and II. Circulatory system; Endocrinology; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Manduca sexta (Sphingidae); Haemocoel; Hormones; JH; Hymenopteran parasites; Cotesia congregata; Parasitoid host endocrine relations Cotesia congregata (Braconidae); Hormones; JH; Lepidopteran hosts; Manduca sexta; Parasitoid host endocrine relations