A peaceful zone bordering two Argentine ant (Linepithema humile) supercolonies

In invasion areas, the Argentine ant (Linepithema humile) forms huge supercolonies with free exchange of individuals among nests. Two continental supercolonies on the French Mediterranean coast, the Main European supercolony and the Corsican supercolony, are known to contain workers showing moderate to high levels of inter-supercolony aggression. We performed chemical and behavioral assays using workers from eight coastal sites in southeastern France covering a geographical zone that includes nests of the two supercolonies, to determine the nature of worker interactions in a supercolony boundary zone. We examined how the chemical and behavioral clines vary across the borders of the Main and Corsican supercolonies. Our results on Giens peninsula and Porquerolles Island populations bordering the Main supercolony reveal the existence of a peaceful border zone with no aggression between workers of the Main supercolony and the Corsican supercolony. The chemical results, however, exhibited qualitative similarity to those observed in the Corsican supercolony and revealed both quantitative and qualitative chemical differences from those observed in the Main supercolony. These peaceful populations may result from fusion and/or hybridization of the two supercolonies, and we hypothesize that such peaceful border zones are instrumental in the evolution of supercolonies.