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

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.

壁蜂属（Osmia）的多个物种为兼性隔年发育（parsivoltine），即同生群个体需经历1个或2个冬季的滞育后，方可羽化为成虫。单个巢内的内容物常出现混杂，同时包含一年型与两年型个体，且二者分布通常无规律。本研究旨在探究一年型、两年型个体的发生率以及混合巢是否与海拔及平均气温下降存在关联；同时还将描述巢内子代的性别与年型的排布格局。研究样本采自犹他州北部9个海拔1553–2436 m的点位，通过木质诱捕巢进行饲养繁育，周边气象站提供了气温记录数据。本次调查共发现9种兼性隔年发育的壁蜂属物种。针对其中两个最常见的物种——科罗拉多壁蜂（O. coloradensis）与山地壁蜂（O. montana），两年型个体的占比随海拔升高与平均气温降低呈显著上升趋势。气温最剧烈的陡降发生在1800–1930 m区间，该海拔跨度同样对应两年型个体占比的大幅提升。混合巢的发生率在中等海拔区域达到峰值；子代性别与年型存在关联（雌性为两年型的概率显著高于雄性），但该现象仅见于科罗拉多壁蜂。对于所有物种而言，巢内子代的雌性占比从最内侧的第一枚巢室到最外侧的最后一枚巢室呈逐渐下降趋势。本研究探讨了以多样化风险分摊策略解释兼性隔年发育现象的合理性，并提出新的解读视角：该策略可通过两种不同的滞育阶段，降低越冬死亡率——尤其是生长季后期产生的未成熟个体的越冬死亡率。