Biology of a weakly social bee, Exoneura (Exoneurella) setosa (Hymenoptera: Apidae) and implications for social evolution in Australian allodapine bees

Australian allodapine bees provide excellent material for comparative appro aches to understanding social evolution. The subgenus Exoneurella occupies a cladistically basal position in the Australian Exoneura group and compris es only four species. We describe sociality in one Exoneurella species, E. serosa, and combine this with other data to infer some patterns of social e volution in allodapines. E. serosa rears a first brood solitarily, although staggered brood production and the production of a second brood in some ne sts leads to a situation where older, recently emerged brood have the abili ty to help rear their younger siblings and this overlaps with opportunities to lay eggs. This is similar to the situation for two other phylogenetical ly distal species of Exoneurella, as well als for members of the genus Brau nsapis, which is used as an outgroup for Exoneura. When combined with other studies, our results suggest that the opportunity for sib-rearing is a ple siomorphic trait for Australian allodapines and this has been largely lost in a distal subgenus, Exoneura sensu stricto. Instead, multifemale brood-re aring colonies in this latter group mostly comprise individuals of the same generation, and species exhibit large group size, univoltinism and kin cof ounding. This suggests that evolution can favour semisociality and quasisoc iality, even when eusociality has already arisen.