Duration of prepupal summer dormancy regulates synchronization of adult diapause with winter temperatures in bees of the genus Osmia

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 should develop more slowly than late-flying populations and thus avoid the negative consequences of long pre-wintering periods. In this study we compare the development under natural and laboratory conditions of phenologically-distinct populations in two Osmia species. 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 early-flying populations. In contrast to the larval and pupal stages, the prepupal stage showed a non-linear response to temperature, was strongly affected by thermoperiod, and exhibited minimum respiration rates. Coupled with other lines of evidence, these results suggest that the prepupal period in Osmia corresponds to a summer diapause, and its duration may be under local selection to synchronize adult eclosion with the onset of winter temperatures. We discuss the implications of our results relative to current expectations of global warming.