none Heliothis virescens; ENZYMES-; HAEMOLYMPH-; JH-esterase suppression; HORMONES-; JH-esterase suppression in haemolymph, hymenopteran parasite effects; DEVELOPMENT-; JH-esterase suppression in haemolymph & hymenopteran parasite relationships; HYMENOPTERAN-PARASITES; Microplitis demolitor; JH-esterase suppression in haemolymph by calyx fluid, teratocyte effects Microplitis demolitor; EGG-; Teratocyte effects on lepidopteran hosts; DUCTS-OF-FEMALE; Oviduct; EGG-LAYING; Oviposition; MUTUALISM-; Polydnavirus; Calyx fluid, suppression of lepidopteran host JH esterase; LEPIDOPTERAN-HOSTS; Heliothis virescens; JH-esterase suppression in haemolymph by calyx fluid, teratocyte effects Microplitis demolitor is a polydnavirus-carrying braconid wasp that parasitizes the larval stage of Heliothis virescens. Previous studies indicated that parasitized hosts exhibit developmental arrest whereas larvae injected with either M. demolitor calyx fluid or polydnavirus (MdPDV) form larva-pupal intermediates. Here we investigated the role of teratocytes in altering host development and the effects of calyx fluid on juvenile hormone esterase activity in H. virescens. Injection of superphysiological doses of teratocytes or coinjection of a physiological dose of teratocytes and calyx fluid affected fourth-stadium host development similarly to parasitism, but parallel injections had lesser effects in fifth-stadium hosts. Juvenile hormone (JH) titers in parasitized, fifth-stadium H. virescens were 12-fold higher by 60 h post-parasitism than those in controls. Unparasitized H. virescens exhibited two peaks of JH metabolic activity in the last instar, one at 60 h after larval ecdysis and the other just before pupation (132 h post-ecdysis), whereas JH metabolism was completely suppressed in parasitized and calyx fluid-injected hosts. The effect of calyx fluid on JH metabolism was independent of the presence of teratocytes and, based on general esterase assays, was specific for JH esterase. Experiments using hemolymph from parasitized larvae suggested that the reduction in host JH esterase activity was not due to a factor in the hemolymph.