Technical Abstract: Osmia (Osmia) bees are strictly univoltine and winter as diapausing adults. In these species, the timing of adult eclosion with the onset of wintering conditions is critical, because adults exposed to long pre-wintering periods show increased lipid loss and winter mortality. Populations from warm areas fly in February-March and are exposed to longer growth seasons than populations from colder areas, which fly in April-May. Given their inability to produce an extra generation, early-flying populations must develop slower than late-flying populations to avoid the negative consequences of long pre-wintering periods. The objective of this study is to understand how phenologically-distinct populations of two Osmia species adjust their development to the duration of local growth seasons. We monitored development under natural and laboratory conditions, and analyzed the response of larval, prepupal and pupal stages to temperature. Early-flying populations took ~2 months longer to develop than late-flying populations. Differences between populations in larval and pupal period duration were very small, whereas the prepupal period was much longer in the early-flying populations. In contrast to the larval and pupal stages, the prepupal stage showed a non-linear response to temperature and was strongly affected by thermoperiod. In addition, respiration rates reached minimum values during the prepupal period. We conclude that the prepupal period in Osmia corresponds to a short summer diapause, and that it represents the main mechanism by which populations adjust adult eclosion to the onset of winter temperatures. We discuss the implications of our results in the face of impeding global warming.