Social parasitism is the best-studied cleptoparasitic strategy. It stands on the border of social behaviour, usurpation and brood parasitism. Social parasitism always includes a combination of these three behavioural patterns. Usurpation, as a component of social parasitism, is described above. The brood parasitism component is quite different from cuckoo behaviour, as social hosts are progressive provisioners and feed their brood directly from the crop (Michener 1974). In mass provisioning species (Halictidae) the biology of social parasites is unknown.

Social parasites have evolved independently from eusocial ancestors (except some Sphecodes) roughly sixteen times in two families (Michener 2007). The well-known group of socially parasitic bees of the family Apidae includes cuckoo bumblebees (Bombus sg. Psithyrus). There are two other Bombus species with possible obligate or occasional socially parasitic behaviour.

Psythirus rupestris

The largest number of origins of social parasitism has occurred within Allodapini (Apidae: Xylocopinae). There are from eight to ten independent origins of social parasitism in this group. The most problematic group is family Halictidae. Some cleptoparasitic species have been found inside the nests of eusocial species, but their biology is not clear. Three independent origins of social behaviour are possible within this family (Michener 2007).

Social parasitism at the intraspecific level has been documented in Bombus terrestris (Linnaeus). Male-producing reproductive workers enter unrelated conspecific colonies in which they then produce adult male offspring. These socially parasitic workers are significantly more reproductive and aggressive than resident workers that reproduce within their own colonies (Lopez-Vaamonde et al. 2004).