The relationship between parasitoid size and fitness in the field, a study of Achrysocharoides zwoelferi (Hymenoptera: Eulophidae)

1. A knowledge of the relationship between body size and fitness is very important in many models of parasitoid behaviourial ecology. We estimated this relationship for Achrysocharoides zwoelferi (Hymenoptera, Eulophidae), a parasitoid of leaf mining moths (Gracillariidae) on willow (Salix). 2. The size distribution of female wasps emerging from their pupae was compared with the size distribution of wasps caught while searching for hosts. A parametric statistical approach was used to obtain a function relating size to fitness. 3. Laboratory estimates of the influence of size on a component of fitness, longetivity, were obtained under two different sets of experimental conditions. 4. In the field, fitness increased rapidly with size over the first two-thirds of the observed size range, but plateaued (or possibly declined) in the last third of the range. 5. One laboratory experiment showed no effect of size on fitness while the other did show an effect although much weaker than that observed in the field. This suggests caution in using results from the laboratory in parameterising behaviourial ecological models. 6. The results are compared with two other field estimates of the influence of size on fitness which show broadly similar patterns.