Hosts and brood parasites are a classic example of conflict. Parasites typically provide no offspring care after laying eggs, imposing costs on hosts. Female subsocial wasps, Ammophila pubescens, alternated between initiating their own nests and an ‘intruder’ tactic of replacing eggs in nests of unrelated conspecifics. Hosts could respond by substituting new eggs of their own, with up to eight reciprocal replacements. Remarkably, intruders usually provisioned offspring in host nests, often alongside hosts. We used field data to investigate why intruders provision and to understand the basis of interactions. We found that intruders could not increase their fitness payoffs by using the typical brood parasite tactic of not provisioning offspring. Intruders using the typical tactic would benefit when hosts provisioned in their stead, but their offspring would starve when hosts failed to provision. Although some hosts obtained positive payoffs when intruders mistakenly provisioned their offspring, on average utilizing a conspecific nest represents parasitism: hosts pay costs while intruders benefit. Both females used the same tactic of egg replacement, but intruders more often laid the final egg. Selection should favour better discrimination of offspring, which could lead to repeated cycles of costly egg replacement.