Absence of aggression but not nestmate recognition in an Australian population of the Argentine ant Linepithema humile

Intraspecific aggression is rare within introduced populations of the Argentine ant Linepithema humile, and colonies exhibit a structure known as unicoloniality, in which aggression among nests is atypical. We document a similar form of colony structure in an introduced population of Argentine ants in Victoria, Australia, in which aggression is extremely rare among nests ranging over hundreds of kilometres. However, using a highly sensitive behavioural bioassay we found that workers display subtle differences in their behaviour towards non-nestmates and nestmates. In particular, non-nestmates consistently engage in antennating behaviour with greater frequency than nestmates, perhaps providing a mechanism for homogenization of nest odour. Further, we found that non-nestmates at seaport sites (where populations may derive from multiple introductions) antennate each other with greater frequency than their counterparts from non-seaport sites. These data suggest that the Victorian population of L. humile may comprise multiple independent introductions.