In eusocial insects, social parasitism—the exploitation of the host's brood care behavior for survival and reproduction—can occur either within or between species. Parasitic queens invade host nests and aggressively replace the resident queen. While the adoption of conspecific queens is a common feature of species with multiqueen colonies (polygyny), the origin of parasitic founding is not fully understood. Functionally monogynous ants, in which nestmate queens establish social and reproductive hierarchies through biting and antennal boxing, may provide a link between peaceful adoption and social parasitism. In this study, we investigated whether alien queens can usurp colonies of the functionally monogynous ant Leptothorax gredleri. Ovary dissection of queens from 33 nests showed that multiple queens with developed ovaries can occasionally co-occur in the same nest. Genetic analysis revealed frequent replacement of the dominant queens by relatives. Instead, alien queens rarely take over reproduction, suggesting a few occurrences of intraspecific social parasitism. However, the data suggest that alien queens without developed ovaries frequently invade nests without being eliminated. This suggests that alien queens are somehow prevented from reproducing and social parasitism is therefore limited in this species.