Most social insect lineages contain socially parasitic cheater species that, rather than produce their own workers, infiltrate the nests of closely related social species and force the hosts to rear their offspring. These parasites have often lost social traits, like the ability to rear and produce workers, while retaining abilities for reproductive control and exhibiting novel parasitic innovations to capitalize on host resources. Given their close relationships with their hosts, social parasites are particularly informative to understand antagonistic coevolution and the essential components of sociality. Bumble bee social parasites are well suited to inform such evolutionary questions as they exhibit a gradation from facultative to obligate parasitism in their three independent origins of social parasitism, while also exhibiting a diverse obligately socially parasitic lineage, the subgenus Psithyrus Lepeletier, that varies across species in host use and invasion strategies. Despite the insights it can provide, cuckoo bumble bees, like most social parasites, are rare to encounter, and as such represent some of the most poorly understood bumble bee lineages. In this review, we bring together the state of our knowledge on the ecology and evolution of these rare cuckoo bees, to set a framework for further study, while also highlighting our current gaps in knowledge. In particular, we describe patterns of host breadth, geographic range, behavioral and morphological innovations, and social invasion strategies utilized across these bees to varying success. Considering their rarity, we highlight the pressing need to study these social parasites given conservation threats posed by host species declines.
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