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Intraspecific interference competition in the harvester ant, Messor aciculatus, was studied. Colonies of this species were found not to have territories. Some nests were located very close to each other, and the foraging areas of the neighbors usually overlapped. Even though the frequency with which alien and resident ants met was very high in the vicinity of the nest entrances, aggressive interactions between them rarely occurred. However, when hostile workers encountered each other, they exhibited a kind of ritualized combat and the winner ejected, but did not injure the loser. If any aliens entered the nest, some of them were pulled out, mainly by the residents.

Aliens roaming near a neighbor’s nest entrance ferociously attacked the residents carrying seeds in their mandibles and robbed them. On other occasions, aliens entered the nest and stole the collected seed. Although seed robbing and stealing occurred among neighboring colonies, there were remarkable differences in the frequency of their occurrence. The results of field observations and experiments suggest the existence of a dominance order among the neighbors. In one instance, extermination of an inferior colony by its neighbor was observed. The raider colony transferred the stored seeds from the nest of the inferior colony to its own and deposited the larvae and workers some distance away from the nest.

The influence of ritualized combat and food robbing on colony activities, and the ecological significance of this interference behavior in terms of spatial distribution and temporal persistence of the nest sites, is discussed.