Subsociality in halictine bees

To look for the occurrence and the significance of brood care in social evolution, I reared six eusocial halictine bee species in laboratory cages enabling the observation of intranest behaviour: Lasioglossum (Evylaeus) laticeps, L. (E.) pauxillum, L. (E.) nigripes, L. (E.) euboeensis, Halictus (Halictus) scabiosae and L. (E.) fulvicorne. All of them were subsocial, each mother caring for her brood. Brood cells were sealed after oviposition with earthen plugs; they were then reopened, visited and closed again. These observations plus the reports in the literature on eleven eusocial species indicate that seventeen species of eusocial halictine bees provide parental care, i.e. are subsocial. Brood care, subsociality, is strongly associated with eusociality. To study reversal from eusociality to subsociality, I have reared the non-eusocial form of two species within which there are or have been eusocial forms: Halictus (H.) rubicundus and Lasioglossum (E.) fratellum. They are secondarily solitary, having lost worker brood. However, both species still show brood care. This suggests that in transitions to eusociality, brood care antedated eusociality. To further examine this issue I reared two truly solitary species that are not derived from eusocial ancestors: Lasioglossum (E.) villosulum and L. (L.) quadrinotatum. Unlike secondarily solitary species, females of both these species close their brood cells after oviposition and ignore their progeny thereafter. This association strongly suggests that the subsocial route with maternal brood care is the route to eusociality in halictine bees.