Butterflies have competing demands for flight ability depending on e.g. mating system, predation pressure, localization of host plants and dispersal needs. The flight apparatus, however, are costly to manufacture and therefore trade-offs are expected since resources are limited and must be allocated between flight ability and other functions, such as reproduction. Trade-offs between flight and reproduction may be difficult to reveal since they interact with other factors and be confounded by differences in resource consumption. Previous studies have shown that adults of the summer generation of Pieris napi have relatively larger thoraxes compared to the spring generation. To study whether difference in thorax size results in a trade-off between flight ability and reproduction among the two generations we conducted a split-brood experiment under common garden conditions. Our results show, that summer generation adults have a higher dispersal capacity, measured as flight duration in five different temperatures. Reproductive output differed between the two developmental pathways; spring generation females laid a significantly higher output of eggs compared to summer generation females. We suggest that this is a consequence of a resource allocation trade-off made during pupal development implemented by different demands for flight between the spring and summer generations. The significance of this finding is discussed in relation to reproduction and mobility in butterflies.