Brood parasites that care: alternative nesting tactics in a subsocial wasp

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.