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Natasha Mothapo collecting Argentine ants in the field for aggression bioassays.

Argentine ants (Linepithema humile) are successful invaders of areas with a mediterranean-type climate similar to their home country in South America. They are thought to have been introduced into South Africa in the early 1900’s during the Anglo Boer war. In their introduced range, these ants form large interconnected colonies (unicolonies) that often form supercolonies. It has been shown that Argentine ants within these supercolonies show very low levels of aggression towards more distant colony members.

Using aggression bioassays, we set out to determine Argentine ant colony boundaries in the Western Cape. Aggression assays were conducted in the field across eight sites (Porteville in the north to Elim in the south). Aggression bioassays are used to indicate the level of acceptance/rejection between two ants from different nests. Only those ants from Elim showed high levels of aggression towards all other ants in this study – immediately pulling and biting the introduced non-familiar ant. These behavioural data suggest that a unicolony boundary exists between the ants from Elim and all those found to the north. These findings indicate that at least two supercolonies exist within the Western Cape study area, suggestive of more than one introduction into South Africa.