Maintenance of body-size variation and host range in the orange-tip butterfly: evidence for a trade-off between adult life-history traits

1. The evolution of host range and preference in phytophagous insects is driven by a female's oviposition choice impacting her offspring's fitness. Analysis of the fitness of progeny on different host plants has commonly been restricted to the performance of immature stages. However, since host use can affect adult size, it is important to measure the ongoing effects of host choice on the resulting imagines. 2. The orange-tip butterfly, Anthocharis cardamines, shows a strong preference for two host plants in Britain, Alliaria petiolata and Cardamine pratensis, which affect body size. Whilst females exhibit a strong positive sizefecundity relation, the impact of body-size alteration is unknown in males. In this study, fitness effects of host plant choice for male A. cardamines were examined. 3. Males reared on C. pratensis were smaller and emerged earlier than those reared on A. petiolata, and early-season males were smaller than late-season ones in the field. Interestingly, regression analysis indicated that the earlier emergence of small males was a host-mediated rather than a size-mediated effect. Small size was associated with reduced male dispersal in a semi-isolated wild population over a 3-year period. 4. It is proposed that the earlier emergence associated with C. pratensis has evolved in response to depressed dispersal in isolated/semi-isolated populations associated with this patchily distributed host. We suggest that adult life-history traits are important for the maintenance of host range in this species, and offer a critique of Courtney's earlier hypothesis that host range is maintained by time-limited oviposition behaviour.