none Manduca sexta; NEUROTRANSMITTERS-; Octopamine levels in hymenopteran parasitized host haemolymph; Relationship with decreased activity & comparison with unparasitized hosts; FEEDING-BEHAVIOUR; Inhibition prior to hymenopteran parasite emergence; ACTIVITY-PATTERNS; Behavioural inhibition before hymenopteran parasite emergence; Correlation with haemolymph octopamine rise; HYMENOPTERAN-PARASITES; Cotesia congregata; Host activity & octopamine levels, comparisons with unparasitized hosts Cotesia congregata; LEPIDOPTERAN-HOSTS; Manduca sexta; Host behavioural suppression & haemolymph octopamine levels The parasitoid wasp Cotesia congregata lays its eggs within the body of its host, the larval form of the tobacco hornworm Manduca sexta. Host behavior appeared normal until approximately 8 h prior to the emergence of the parasitoids from their host at which time M. sexta feeding and locomotion declined irreversibly. This change in host behavior may be to the advantage of the wasp since unparasitized M. sexta presented with wasp pupae ate them. Despite the decline in feeding and locomotion, hosts with emerged parasitoids had normal reflexes and showed no other signs of debilitation. Concomitant with the change in host behavior, octopamine concentration measured using high-performance liquid chromatography with electrochemical detection (HPLC-ED) increased from 22.2+-2.1 pg mu-l-1 to 143.7+-7.8 pg mu-l-1 in the hemolymph of the host. In unparasitized M. sexta, however, increased octopamine levels were correlated with increased activity. We discuss possible explanations for the co-occurrence of high hemolymph octopamine levels and low behavioral arousal in parasitized M. sexta.