A field study of size-fitness relationships in the parasitoid Asobara tabida

1. Many evolutionary models of parasitoid behaviour assume a positive correlation between size and fitness. In this paper we study the size-fitness relationship in the laboratory and in the field using females of the solitary parasitoid Asobara tabida (Hymenoptera: Braconidae). 2. In the laboratory, fecundity, fat reserves and longevity without food were positively correlated with size. 3. Release-recapture experiments in the field showed that dispersal diminishes fat reserves. Dispersal ability is size-dependent: larger females, with larger fat reserves, disperse over larger distances than smaller females. 4. The form of the relationship between size and fitness in the field was estimated in two ways: one based on a comparison of the size distribution of released and recaptured females; the other based on the egg load and fat reserves of wild-caught females. Both showed an accelerating increase of fitness with size. 5. The majority of females appeared to be time-limited. Therefore, the increase in fitness with size is predominantly due to a larger dispersal ability and not to a higher egg load.