A Disjunct Argentine Ant Metacolony in Macaronesia and Southwestern Europe

The Argentine ant, Linepithema humile, originally from South America, is now a major pest in many parts of the world with Mediterranean-like climates. Earlier research indicated that southwestern European L. humile populations are segregated into two distinct supercolonies, the ‘Main’ supercolony extending through Portugal, Spain, southern France, and Italy, and the ‘Catalonian’ supercolony in eastern Spain. Both supercolonies are unicolonial, with workers showing no aggression towards members of the same supercolony, but severe aggression towards members of the other supercolony. Here we evaluated the behavioral relationships among non-native L. humile populations on the Macaronesian islands of Madeira and the Azores and non-native populations in southwestern Europe. We conducted aggression assays among L. humile workers from Madeira, the Azores, and the two southwestern European supercolonies. We found no aggressive interactions among any combination of workers from Madeira, the Azores, and the Main supercolony. However, workers from Madeira and the Azores always fought aggressively with workers from the Catalonian supercolony. Thus, the populations of L. humile in Madeira and the Azores appear to be unicolonial and act as if they belong to the Main supercolony of southwestern Europe. This set of geographically separated but mutually compatible supercolonies, which we term a ‘metacolony,’ appears to descend from one supercolony. Historical evidence suggests that the first L. humile population in the greater Mediterranean region was established in Madeira and that propagules from the dominant supercolony in Madeira gave rise to the dominant supercolonies in to other parts of the region.