1. The spread of Argentine ants, Linepithema humile (Mayr), in introduced areas is mainly through the displacement of native ant species owing to high inter-specific competition. In South Africa, L. humile has not established in the climatically suitable eastern and northern escarpments dominated by the African big headed ant, Pheidole megacephala (Fabricius), probably owing to local biotic resistance.
2. Inter-specific aggression, at the individual and colony level, and competition for a shared resource were evaluated in the laboratory.
3. Aggression between the two ant species was very high in all of the assays. Both species suffered similar mortality rates during one-on-one aggression assays, however, during symmetrical group confrontations, L. humile workers showed significantly higher mortality rates than P. megacephala workers. During asymmetrical group confrontations both species killed more of the other ant species when they had numeric advantage. Both ant species located the shared resource at the same time; however, once P. megacephala discovered the bait, they displaced L. humile from the bait through high inter-specific aggression, thereafter dominating the bait for the remainder of the trial.
4. The results demonstrate the potential of P. megacephala to prevent the establishment and survival of incipient L. humile colonies through enhanced resource competition and high inter-specific aggression. This is the first study to indicate potential biotic resistance to the spread of L. humile in South Africa.