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Several species of  Osmia  bees are parsivoltine, i.e., individuals of a cohort diapause for either one or two winters before emerging as adults. Frequently, contents of single nests are mixed, containing both one- and two-year morphs, commonly in an unpatterned array. We asked if the incidence of one- and two-year morphs, and mixed nests, was associated with elevation and decreasing average temperature. We also describe the arrangement of gender and year-morph of progeny within nests. Populations were sampled and reared from wood “trap-nests” at 1553–2436 m elevation at nine sites in northern Utah. Nearby weather stations supplied temperature records. Nine parsivoltine  Osmia  species were found. For the two most common species,  O. coloradensis  and  O. montana , the percent of two-year morphs increased significantly with elevation and a decrease in average temperature. The most precipitous drop in temperature occurred between 1800 and 1930 m, the elevation span also associated with large increases in percent two-year morphs. The incidence of mixed nests was highest at intermediate elevations, and gender and year-morph were associated (females were significantly more likely than males to be two-year morphs) but only for  O. coloradensis . For all species, the percentage of females declined from the first (innermost) to the last (outermost) brood cell in nests. We consider the merits of a diversified bet-hedging strategy as an explanation for parsivoltinism and offer a new interpretation that emphasizes the reduction in overwintering mortality, particularly of immatures produced later in the growing season, conferred by their ability to enter diapause at two distinct stages.

Mixed diapause duration in cohorts of four species of Osmia bees (Megachilidae) along an elevation and temperature gradient in Northern Utah (USA)