THE IMPORTANCE OF BEING LARGE - THE RELATIONSHIP BETWEEN SIZE AND FITNESS IN FEMALES OF THE PARASITOID APHAERETA-MINUTA (HYMENOPTERA, BRACONIDAE)

1. The relationship between the size of a female parasitoid and its fi tness is a major element of models of optimal clutch size, of host spe cies selection, of the evolution of siblicide in parasitoids and of so me sex allocation models. 2. In estimating this relationship, usually only size-dependent longevity and/or fecundity is considered, neglecti ng differences in searching efficiency for hosts, although this is an important component of fitness. The relationship between a number of c onstraints and female size was studied in the parasitoid Aphaereta min uta. Larger females were found to have more eggs available, have large r eggs, live longer and have a higher searching efficiency within patc hes than small females, but searching efficiency for patches and trave l speed were found to be independent of size. 3. The size-dependent co nstraints were combined into one fitness measure. To this end, the cha racteristics of a natural habitat were measured and, together with the relationships between female size and these constraints, incorporated into a dynamic programming model in order to calculate the fitness of females of different sizes. Fitness was found to increase linearly wi th female size. 4. Even from very accurate measures of the size-fitnes s relationship made under laboratory conditions it remains doubtful wh ether the relationship holds under field conditions. Three experiments to estimate the relationship between female size and fitness in the f ield were carried out, all showing an increase in fitness with female size. 5. Comparing the results from the field and the laboratory exper iments indicates that fitness increases much more rapidly with increas ing female size in the laboratory than under field conditions.