Responses of two potential host species (Formica gnava and Formica occulta) to pupae of the obligatory slave-making ant, Polyergus breviceps (Hymenoptera : Formicidae)

Queens of the slave-making ant, Polyergus breviceps, take over nests of adu lt Formica workers when establishing new colonies. Although naive to slave- maker brood, the usurped Formica rear Polyergus offspring and nests contain ing both host and parasite species forms. Host worker acceptance of parasit e brood has been attributed to the similarity of "brood tending signals" be tween these closely related taxa and/or the presence of an "attractive pher omone" in the slave-maker brood. By presenting single-species groups Of For mica occulta and Formica gnava (two host species of P. breviceps) with a ch oice of Formica pupae of both species or with a choice of P. breviceps pupa e from both types of mixed-species nests, it seems that neither close phylo genetic relatedness nor an attractive brood odor alone can account for the propensity of host workers to adopt slave-maker pupae. Significantly greate r numbers of P. breviceps pupae were adopted by enslaved workers than by fr ee-living workers, and within the enslaved groups and the free-living F. gn ava group greater numbers of P. breviceps pupae were adopted if they were f rom nests where the host species was conspecific to workers used in tests. When presented with F. gnava and F. occulta pupae, Formica workers adopted conspecific pupae almost exclusively and ignored or consumed pupae of the o ther host species. Taken together, these results imply that P. breviceps pu pae have both a species-specific odor and a general brood-tending pheromone , upon, which a host odor may be imposed. The disparate requirements of imm atures at different stages of development for cue specificity or generality in maintaining nest exclusivity and maximizing inclusive fitness are discu ssed.