Social and genetic structure in colonies of the social wasp Microstigmus nigrophthalmus

Microstigmus (Hymenoptera: Crabronidae) is a genus of social apoid wasps which represents an origin of sociality independent from vespoids, but which has so far received little attention. Though group-nesting is widespread in Microstigmus, genetic relatedness has so far been studied in only one species, M. comes. We report on the social biology of M. nigrophthalmus, drawing from behavioural observations and molecular genetic analyses of relatedness and kinship. There was no evidence of distinctive behavioural suites that distinguished reproductive and non-reproductive individuals. Females could mate more than once, but mating frequency was low. Mean relatedness within nests was high, particularly between females (close to 0.5), but pairwise relatedness values were very variable, as nestmates displayed a wide range of relationships. Such high levels of relatedness should be a factor promoting social nesting and cooperative brood care in this species, as females gain only a slight genetic advantage through rearing their own offspring rather than those of nestmates. This study provides the finest analysis of genetic structure so far in an apoid wasp, and indicates that the form of sociality varies greatly between species of Microstigmus.