Osmia ribifloris, the best little orchard bee in the West

Managing megachilid bees often involves establishing small genetically uniform populations within agricultural settings very different from the bees’ native habitat. Among select populations of Osmia ribifloris used in our blueberry pollinator program, mtDNA (COI) markers show two very distant populations from different subspecies actually shared the closest phylogenetic kinship. In fact, the fittest adult females originated from Texas and California. These bees, when kept in captivity, provisioned the largest and healthiest broods using pollen from southern blueberry cultivars. In contrast, Osmia ribifloris from northern Utah, their Mississippi progeny, along with a floral generalist from east Texas (O. lignaria) produced comparatively fewer brood. Overall, fitness of O. ribifloris also depends on a female’s reproductive status. Namely, mated bees out-produced unmated bees by nesting 2 weeks earlier and producing broods twice as heavy. Unmated bees in fact could be counter-productive to nest growth, as 22% of them reduce the fitness of conspecifics through aggressive nest parasitism, which killed 67% of brood in affected nests, a huge loss of potential blueberry pollinators. Such high rates of infanticide as well as a steepening rate of male production indicate declining growth for such a small captive bee population. Thus, prompt releases of wild O. ribifloris into blueberry fields with ample forage and nests are preferable to long-term captivity and its risk of unstable secondary sex ratios and facultative nest parasitism.