Dissecting the contributions of plasticity and local adaptation to the phenology of a butterfly and its host plants

Phenology affects the abiotic and biotic conditions that an organism encounters and consequently its fitness. For populations of high latitude species, spring phenology often occurs earlier in warmer years and regions. Here we apply a novel approach to decompose spatiotemporal covariation between spring temperature and the phenology of two flowering plants, Cardamine pratensis and Alliaria petiolata, and a Lepidopteran herbivore, Anthocharis cardamines, across the UK, into the contributions of plasticity and local adaptation. All three species overlap in the time-window over which mean temperatures best predict variation in phenology and we find little evidence that the position of time-windows varies latitudinally, as expected if they were initiated by day-length. The focal species all show pronounced temperature-mediated phenological plasticity of similar magnitude. While we find no evidence for local adaptation in the flowering times of the plants, geographic variation in the phenology of the butterfly is consistent with countergradient local adaptation. The butterfly’s phenology appears to be better predicted by temperature than it is by the flowering times of either host plant and we find no evidence that coevolution has generated geographic variation in adaptive phenological plasticity.