OIKOS 92: 309 – 314. Copenhagen 2001 Seasonal changes in female size and its relation to reproduction in the parasitoid Asobara tabida Jacintha Ellers, Minka Bax and Jacques J. M. van Alphen Ellers, J., Bax, M. and van Alphen, J. J. M. 2001. Seasonal changes in female size and its relation to reproduction in the parasitoid Asobara tabida. – Oikos 92: 309–314. The relation between female size and fitness was studied in female Asobara tabida throughout the field season. The size of A. tabida females varied considerably, with average size being smallest in the middle of the season. There was a positive correlation of realized fecundity with size, and the fitness advantage of larger females increased later in the season. A possible explanation for this can be found in the energy expenditure during the season. Regression analysis showed that fat use increases with size of the female, but also with temperature. Temperature was low early and late in the season, but high in the middle. We argue that the high temperatures may constrain fitness advantages of large females because of their increased metabolic needs. Variation in the form of the fitness function within the season may moderate directional selection for larger females. J. Ellers, Center for Conseration Biology, Dept of Biological Sciences, Stanford Uni., Stanford, CA 94305 -5020, USA ( jellers@bing.stanford.edu).– M. Bax and J. J. M. an Alphen, Inst. of Eolutionary and Ecological Sciences, Leiden Uni., P.O. Box 9516, NL-2300 RA Leiden, The Netherlands. The relationship between insect size and fitness has been the subject of many studies (Leather 1988, Van den Assem et al. 1988, Heinz 1991). In female insects, fecundity is often found to be strongly positively corre- lated with body size (Honek 1993, Godfray 1994, Visser 1994). However, the main body of evidence for this relationship comes from laboratory studies, mostly measuring potential fecundity under controlled condi- tions. The question is whether this relationship holds for realized fecundity in the field, where abiotic and biotic factors fluctuate. Variation in realized fecundity in nature may well be attributed to other factors than body size. So far, only a few studies have tested the fitness function of body size in the field and the results are variable. Some studies find a linear positive correla- tion between size and reproductive success (Tammaru et al. 1996, Ellers et al. 1998, Sopow and Quiring 1998), whereas others proposed a curved relationship with diminishing fitness advantage for larger individuals (Kazmer and Luck 1995, West et al. 1996). However, all of these studies assume the relationship between size and reproductive success to be a constant one, whereas the magnitude of the advantage of being large may vary through space and time. For example, for any species there is spatial and temporal hetero- geneity in reproductive opportunities encountered. If resources are scarce, large-sized individuals may be at an advantage due to their better dispersal ability (Ellers et al. 1998). On the other hand, small individuals may be more likely to escape predation and therefore se- lected for under high predation pressures. Therefore, to measure the relation between size and realized fecundity one has to take into account the different constraints acting on size and the way they change during the season. In this study we examine the relationship between size and realized fecundity throughout the season in female Asobara tabida (Nees) (Hymenoptera: Bra- conidae). Earlier work on this species showed that fitness strongly increases with size in the field. The two Accepted 17 September 2000 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 92:2 (2001) 309