The harvester ant ( Pogonomyrmex badius ) midden: refuse or boundary?

Abstract. 1. P.badius Latreille, a harvester ant of the southern U.S.A., surrounds its nest mound with small bits of charcoal, previously believed to be incidental refuse, along with other components of the colony's midden. 2. Charcoal middens were removed from the nest mounds of colonies in northern Florida. These colonies collected new charcoal and replaced the middens within 7 days. 3. Activity rhythms, use of space, and interspecific aggression were compared in experimental and control colonies. When middens were removed, the frequency of invasions of the nest mounds by other species of ants increased. Foreign ants circled the nest mound and interfered with colony activities. Increased numbers of invading ants led to an increase in patrolling and defensive behaviour by the P.badius colonies. 4. The charcoal midden is not merely refuse. Instead, it acts as a boundary that deters ants of other species from coming onto the P.badizls nest mound. Ants may use charcoal bits as a repository of a chemical substance functioning as a territorial marker.