Dormancy strategies, including diapause and quiescence, enable insects to evade adverse conditions and ensure seasonally appropriate life stages. A mechanistic understanding of a species' dormancy is necessary to predict population response in a changing climate. Climate change is influencing distribution patterns and population success of many species, including Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), the most important mortality agent of pines in western North America. Diapause is considered absent in D. ponderosae, and quiescence in the final larval stage prior to pupation (i.e., prepupal) is considered the main dormancy strategy. We evaluated if a facultative diapause in the prepupal stage, rather than a pupation threshold ∼15°C (i.e., quiescence), could describe pupation patterns in two latitudinally separated D. ponderosae populations in the western United States. We hypothesized that if pupation occurs at lower temperatures than previously described, and if significant prepupal developmental delays occur, diapause is a likely physiological mechanism. Although there was considerable variation within and between populations, pupation occurred below the previously established threshold suggesting a prepupal facultative diapause that is induced when late instars experience cool temperatures. Individuals that pupated at temperatures below 15°C also had developmental delays, relative to development at warmer temperatures, consistent with diapause development. Pupation patterns differed between populations wherein diapause was induced at cooler temperatures and diapause development was shorter in southern compared with northern D. ponderosae. Recognition of a facultative diapause that varies among and between populations is critical for making predictions about future population response and range expansion in a changing climate.