The Argentine ant (Linepithema humile, formerly Iridomyrmex humilis) is an ant native to northern Argentina, Uruguay, Paraguay, Bolivia and southern Brazil. This invasive species was inadvertently introduced by humans on a global scale and has become established in many Mediterranean climate areas, including South Africa, New Zealand, Japan, Easter Island, Australia, Europe, Hawaii, and the continental United States. Argentine ants are significant pests within agricultural and urban settings, and are documented to cause substantial harm to communities of native arthropods, vertebrates, and plants within their invaded range.
Linepithema humile is a small-bodied (2.2–2.6 mm) ant species, dull light to dark brown in color. Within the invasion zone, ant colonies are large and include many workers and multiple queens.
Argentine ants are opportunistic with regard to nesting preferences. Colony nests have been found in the ground, in cracks in concrete walls, in spaces between boards and timbers, even among belongings in human dwellings. In natural areas, they generally nest shallowly in loose leaf litter or beneath small stones, due to their poor ability to dig deeper nests. However, if a deeper nesting ant species abandons their nest, Argentine ant colonies will readily take over the space. Because the native habitat for this species is within riparian floodplains, colonies are very sensitive to water infiltration within their nests; if their nests become inundated with water, workers will collect the brood and the entire colony will move to dry ground.
Austrian entomologist Gustav L. Mayr identified the first specimens of Hypoclinea humilis in the vicinity of Buenos Aires, Argentina in 1866. This species was shortly transferred to the genus Iridomyrmex, and finally to Linepithema in the early 1990s.
The native range of Argentine ants is limited to riparian habitats in the lowland areas of the Paraná River drainage, which stretches across northern Argentina, Uruguay, Paraguay, and southern Brazil. Within South America, this species has spread into parts of Chile, Colombia, Ecuador, and Peru. Linepithema humile thrives in Mediterranean climates, and over the past century it has spread to across the globe by human-mediated transport. The species has become established to every continent except Antarctica and includes many oceanic islands.
They have been extraordinarily successful, in part, because different nests of the introduced Argentine ants seldom attack or compete with each other, unlike most other species of ant. In their introduced range, their genetic makeup is so uniform that individuals from one nest can mingle in a neighboring nest without being attacked. Thus, in most of their introduced range, they form supercolonies.The Very Large Colony, which covers territory from San Diego to beyond San Francisco, may have a population of nearly one trillion individuals.(p 204)
Conflict does occur between members of different supercolonies. In 1997, UC San Diego researchers observed fighting between different Argentine ants kept in lab, and in 2004 scientists began to map out the boundaries of the different supercolonies that clashed in San Diego. On the border of the Very Large Colony and the Lake Hodges Colony thirty million ants die each year, on a battlefront that covers many miles. While the battles of other ant species generally constitute colony raids lasting a few hours, or skirmishes that occur periodically for a few weeks, Argentine ants clash ceaselessly; the borders of their territory are a site of constant violence and battles can be fought on top of hundreds of dead ants. Fights may be halted by adverse weather such as rain.(p 203–205)
In contrast, native populations are genetically more diverse and form colonies that are much smaller than the supercolonies that dominate the introduced range. Colonies living in close proximity are territorial and aggressive toward one another. Argentine ants in their native South America also co-exist with many other species of ants, and do not attain the high population densities that characterize introduced populations.
In a series of experiments, ants of the same colony were isolated and fed different diets. The hydrocarbons from the diet were eventually incorporated into the cuticle of the subjects. Those that had the same diet appeared to recognize one another as kin. Those who had at least some overlap in dietary composition also appeared to react non-aggressively to one another. These interactions contrasts drastically with the groups that fed on completely different sources, such as those who lived off flies and those that fed on grasshoppers. The groups appeared to have incorporated hydrocarbons that were not similar to the others and created an unfamiliar identity cue. These groups reacted violently towards each other. This suggests that dietary factors affect the recognition cues for colony members.
