none Aphidius ervi; CHEMORECEPTION-; Response to hemipteran host cornicle secretion; HEMIPTERAN-HOSTS; Acyrthosiphon pisum; Response to host cornicle secretion Acyrthosiphon pisum; INTEGUMENTARY-GLANDS; Cornicle secretion; CHEMICAL-SIGNALS; Cornicle kairomone, behavioural response from hymenopteran parasite Females of the endophagous parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae) respond positively to the cornicle secretion of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae). The parasitoid response has been assessed in a Petri dish choice test by presenting an aphid dummy consisting of a glass bead coated with cornicle secretion along with an untreated bead, which acted as a control. Naive females responded similarly to the treated glass beads and aphids, while experienced females responded less to the treated beads than to aphids. The kairomonal activity of the cornicle secretion decreased as the wax dried. The behavioural response registered seems to be innate and not induced by associative learning. Foraging theory has largely concentrated on oviposition rate as the currency parasitoids use to maximize reproductive success. Female parasitoids foraging in a patchy environment face a variety of mortality risks that influence the survival of both themselves and their offspring. A foraging model is developed to suggest that patch residence times in parasitoids should be based on the trade-off between female and offspring mortality risks. The fitness gain of parasitoids that include all mortality risks in their calculation of optimal patch residence time is then compared to the fitness gain of rate-maximizing females, and to the fitness gain of females that ignore offspring mortality. It is shown that rate-maximizing foragers and foragers ignoring offspring mortality may suffer from substantial losses in fitness. The long-term average rate of enclosing adults produced and the lifetime number of eggs laid are therefore fitness currencies that are likely not to be selected for in insect parasitoids.