Diapause has been rarely considered in the ecophysiology of the pine processionary moth (Thaumetopoea pityocampa) despite its acknowledged importance in pest establishment and dynamics. A review study indicated a U-shaped relationship between incidence of prolonged diapause and winter temperature, and a negative correlation between overall pupal survival and the proportion of diapausing individuals. Based on observations of anatomical changes in diapausing pupae, it has been proposed in the literature that the mechanism of diapause termination occurs at a key period not only in univoltine (emerging the same year) individuals, but also in those that will eventually enter prolonged diapause . By using two separate and real-time measures of metabolic activity, body temperature and O2 consumption, the existence of a transient restoration of activity (termination) has been confirmed in both univoltine and prolonged diapausing pupae. Both methods clearly detected simultaneous diapause termination through increased metabolic rate in both types of pupae before any morphological or behavioural changes could be observed, but univoltine individuals were characterized by a continuous increase until emergence while prolonged diapause individuals later returned to previous low activity levels. This finding is a starting point for the study of diapause development in the pine processionary moth from an ecological point of view. In addition to pupal diapause, there is evidence of a prepupal dormant stage implicated in synchronizing emergences. Through a weekly sampling spanning a two-month procession period, prepupae were found to differentially regulate their development time in such a way that adult emergences were concentrated and synchronized in less than one month. The new findings on diapause and their significance in synchronizing emergences may improve phenological models in a climate change perspective.