Host-searching and mating in an outbreeding parasitoid wasp

1. Female parasitoid wasps (Hymenoptera) must search for hosts to reproduce, but only require mates if their broods are to contain female progeny. In outbreeding species, females locate mates after dispersal from the emergence site, Unmated females may therefore face a trade-off between searching for hosts and searching for mates, if hosts and mates are spatially separated, 2. In the outbreeding parasitoid Bracon hebetor Say (Hymenoptera: Braconidae), males and females are spatially segregated in the field. Females are found primarily below the surface of stored corn where they search for hosts, whereas males are found on or above the surface, 3. Wasps placed in laboratory observation chambers designed to mimic B. hebetor's stored corn habitat distributed themselves in a manner consistent with field observations, Males remained on the surface of the grain, whereas females moved below the surface to attack hosts. 4. In the laboratory, female distribution was influenced by their mating status, the presence of males or hosts, and female age. Virgin females were more reluctant to move into the corn than were mated females, younger females foraged deeper than older females, and all females moved deeper into the corn when males were present. 5. 10% of all females did not mate even when males were present in the chambers, a percentage consistent with previous observations from the field, If B. hebetor faces a trade-off between host-searching and mate-searching, the trade-off seems to be part of 'split sex ratio strategies', with some females remaining constrained to producing only male offspring.