Eastern larch beetles, Dendroctonus simplex LeConte (Coleoptera: Curculionidae: Scolytinae), are monophagous, phloem-feeding herbivores of eastern larch (tamarack), Larix laricina (Du Roi) Koch (Pinaceae). Recently dead or moribund trees are preferentially colonised. Outbreaks of eastern larch beetles are generally localised and short-lived, although a large outbreak has been occurring in the Great Lakes region of North America since 2000. The beetle is reported as univoltine, with a single, spring-emergent, reproductive parent generation establishing one to three sibling broods per year. Some progeny emerging during summer or fall re-enter the tree bole close to ground level to overwinter, while remaining brood adults overwinter within pupal chambers in situ. Owing to these behaviours, eastern larch beetles have been suggested to possess an obligate overwintering reproductive diapause. However, studies have not confirmed this hypothesis. We tested the reproductive viability of non-overwintered progeny in three laboratory experiments. Non-overwintered progeny were reproductively viable, suggesting that a portion of the population may exhibit a facultative adult overwintering diapause. Progeny that emerged naturally from the host (i.e., putative fall-emergers) demonstrated reproductive rates almost six-fold those of manually extracted insects demonstrating a propensity to remain in situ (i.e., putative spring-emergers). These results shed new light onto the reproductive behaviour of eastern larch beetles, and suggest that future population dynamics may be influenced by a warming climate.