Local Specialization and Landscape-Level Influence on Host Use in an Herbivorous Insect

We studied host plant abundance, host use, and oviposition preference in metapopulations of the butterfly Melitaea cinxia within an area of 3500 km2in the Aland islands, southwestern Finland. In the study area, M. cinxia has ∼ 400 small local populations on dry meadows with the larval host plants, Plantago lanceolata and Veronica spicata. Plantago lanceolata occurs in practically all meadows otherwise suitable for the butterfly, whereas the distribution of V. spicata is largely restricted to the northwestern part of the study area. Based on observations of 6500 prediapause larval groups during 1993-1996, we document spatial variation in host plant use in relation to their abundance (electivity). The fraction of larval groups found on V. spicata increased disproportionally with the relative cover of V. spicata in the habitat patches. Additionally, the probability of Veronica use in a population increased with increasing number of larval groups found on Veronica in the surrounding populations but decreased with increasing use of Plantago in the neighborhood. This regional effect on host use at the scale of migrating butterflies could be caused either by spatial variation in the insect (in either preference or performance) or by spatial variation in plants (in resistance to attack by the butterflies). To study the first possibility, we conducted oviposition preference experiments using butterflies from five metapopulations located 2-55 km from each other and characterized by differences in host plant availability and host use. We found clear genetic differences in oviposition preference between the five metapopulations consistent with the observed host use patterns in the field. We conclude that the spatial host use patterns of M. cinxia in the study area are driven both by direct effects of local host abundance and by indirect effects mediated through metapopulation-level adaptation to the regionally more abundant host plant.