Systematic review of cuckoo bumblebee research reveals data gaps and understudied species.

Cuckoo bumblebees (Bombus subgenus Psithyrus) are social parasites that have lost the ability to establish their own nests, and instead usurp the nest of a bumblebee host to reproduce. Accordingly, they are entirely dependent upon a host species to complete their life cycle, and are therefore vulnerable to co–extinction. Despite this, the current consensus is that there remains much we do not know about the fundamental ecology of many cuckoo bumblebee species, which is crucial for conservation.To investigate data gaps for cuckoo bumblebees, we conducted a systematic review to quantify the peer‐reviewed literature that has been produced, including which species have been most studied and which disciplines have been explored.Over half of all Psithyrus species were represented in fewer than 10 publications each, including 2 species listed as Critically Endangered by the IUCN.Research on cuckoo bumblebees is heavily weighted towards Europe and European species, while Asian species were under‐represented.Key research areas such as ecology and behaviour have received little attention for the majority of cuckoo bumblebees, which may hamper conservation effortsOur results quantitatively support observations made in other publications on the scarcity of data on cuckoo bumblebees, and highlight the need for further research on many species for whom fundamental knowledge is missing—particularly in light of the vulnerability of this subgenus to extinction. [ABSTRACT FROM AUTHOR]