1. Spatio-temporal variation in the degree of melanism is often considered in the context of thermal adaptation, melanism being advantageous under suboptimal thermal conditions. Yet, other mutually non-exclusive explanations exist. Analysis of geographical patterns combined with laboratory experiments on the mechanisms of morph induction helps to unveil the adaptive value of particular cases of polyphenism. 2. In the context of the thermal melanism hypothesis and seasonal adaptations, we explored an array of environmental factors that may affect the expression and performance of non-melanic vs. melanic larval morphs in different latitudinal populations of the facultatively bivoltine moth Chiasmia clathrata (Lepidoptera: Geometridae). 3. Geographic variation in larval coloration was independent of average temperatures experienced by the populations in the wild. The melanic morph was, however, more abundant in dry than in mesic habitats. In the laboratory, the melanic morph was induced especially under a high level of incident radiation but also at relatively high temperatures, but independently of photoperiod. Melanic larvae had higher growth rates and shorter development times than the non-melanic ones when both temperature and the level of incident radiation were high. 4. Our results that melanism is induced and advantageous in warm desiccating conditions contradict the thermal melanism hypothesis for this species. Neither has melanism evolved to compensate time constraints due to forth-coming autumn. Instead, larvae solve seasonal variation in the time available for growth by an elevated growth rate and a shortened larval period in the face of autumnal photoperiods. The phenotypic response to the level of incident radiation and a lack of adaptive adjustment of larval growth trajectories in univoltine populations underpin the role of deterministic environmental variation in the evolution of irreversible adaptive plasticity and seasonal polyphenism.