Forest defoliators of temperate regions have evolutionarily adjusted their phenology to cope with food resources availability and seasonal climate fluctuations. It is critical in such environments that reproductive and developments stages coincide with suitable environmental conditions. In the pine processionary moth Thaumetopoea pityocampa, changes in phenology can occur gradually, such as in the course of adaptation to climate with latitude and altitude, or abruptly, when mutations occur in one or more genes regulating life cycle events. Abrupt changes can either cause an eco-evolutionary dead-end, or on the contrary bring ecological opportunities, such as escape from predators, or adaptations to climate changes. Here, we report the ecological consequences of a phenological shift in a particular population of the pine processionary moth. In typical populations of this species, the adult emergence and oviposition occur in the summer, while the larval stages develop in the winter. In the shifted population, the adults mate in the spring and the larvae develop through summer. Our observations demonstrated ecological implications of the shifted life cycle and ongoing adaptations. Changes in the thermal biology, phenotypic morphological traits, fecundity and egg mortality are evidenced. Feeding constraints imposed by season and host plant physiology are also observed. Finally, competition between sympatric typical and "shifted" populations may lead to competitive exclusion. Altogether our results show a comprehensive understanding of the implications of phenology in the ecology of this pine defoliator.