Density-dependent polyphenism of cuticle melanization is a widespread phenomenon in Lepidopteran caterpillars. In the sub-Arctic Fennoscandian birch forest, where the Winter Moth Operophtera brumata is expanding northward and exhibits 10-year cyclical outbreaks, crowding has been associated with the promotion of melanized larvae. No such striking polyphenism is reported at lower latitudes, possible due to lower densities. However, dark pigmentation does not seem to prevail either in populations currently invading North America despite outbreak densities, raising questions on the adaptive function of polyphenism in Fennoscandia. The evolutionary significance of melanism at high densities is not fully understood, but evidence is accumulating in a number of species that it can be associated with increased immunity, predator avoidance (camouflage, warning), or thermoregulation. In the winter moth, however, the camouflage and immunity hypotheses have been rejected, and melanization even appeared to increase enemy vulnerability. We hypothesized that melanization may be an intra- and inter-specific advantage over pale competitors during outbreaks if better absorption of solar radiation allows higher body temperature and earlier nymphosis (before total defoliation), especially at midnight sun latitudes. Such thermal melanism was tested by comparing metabolic rate and phenology among larval phenotypes. Metabolic rate was found to vary greatly but was 150% higher in melanized versus pale larvae under artificial light. The causal relationship with radiation absorption was further explored using respirometry with no light, and thermography. Preliminary results on the phenology of either phenotypes will be presented. Our findings suggest ecophysiological benefits of melanization that may offset its costs on enemy resistance during outbreaks. Sliding selection regimes caused by cold summers and cyclic dynamics at the northern front likely contribute to maintaining phenotypic heterogeneity, a parameter largely neglected in attempts to predict expansions and invasions.