Nest membership determines the levels of aggression and cooperation between females of a supposedly communal digger wasp.

Females of communal and eusocial Hymenoptera differ greatly in the treatment of conspecifics: communal species lack the nest membership-bias treatment via nestmate (NM) recognition, which is typical of eusocial Hymenoptera. Therefore, an analysis of social interactions can suggest whether a given bee or wasp is communal or eusocial. For this reason, we observed females of the digger wasp Cerceris rubida in experimental dyadic encounters. Because this species had previously been labeled "communal," we expected to find little evidence of a NM bias with respect to cooperation or aggression when paired with NMs and non-nestmates (NNM). Contrary to expectation, females were highly cooperative toward NMs and highly aggressive toward NNMs. Variation in the intensity of aggression toward NNMs partially stems from the degree to which the paired females differed in size. From these results and quantitative comparisons with studies on solitary, communal, and eusocial bee species, it appears that C. rubida has a more complex social organization than previously believed and, in particular, that females of this species exhibit NM recognition, a trait associated with eusociality.