Here in Southern California it’s the height of ant season, and while I hear a lot of bitching and moaning about ants on peoples’ counters and ruining peoples’ food, I don’t hear much about ‘common’ ‘household’ ants as an invasive species. And yet, if you live in the southern United States, they probably are.

Linepithema humile. Image care of BugGuide.net.

The little brown ants that appear in my house (and probably yours too) on hot or rainy days are properly termed Argentine ants, Linepithema humile (= Iridomyrmex humilis), and are native to Argentina and Brazil, where they are actually rare. Despite being imports from South America, they are now the most common ant in California, though they’re present throughout the southwestern and southeastern United States, Hawai’i, South Africa, and were recently discovered in New Zealand as well.

Areas infested by Argentine ants as of May 2000. Image care of UCSD Division of Biological Sciences. Click image for full-size version.

Not only do Argentine ants push out populations of other, native ants, but they also impact species that feed on native ants, such as birds and reptiles, and will even disrupt beehives and consume newly hatched bird chicks in the nest.

What’s really interesting about these little guys is the debate over why they’re so invasive. One factor that controls their population in Argentina is the fact that they can recognize Argentine ants from other colonies by the degree of genetic difference between ants of their colony and ants of other colonies. Upon sighting an ant from a different colony Argentine ants will literally tear the trespasser to shreds. For a number of years it was thought that this phenomenon simply didn’t occur in some of the U.S. populations of ants, especially those in California, which were purported to be part of a 600 mile long ‘supercolony’ caused by a genetic bottleneck in the introduction of Argentine ants to North America.

But more recent research suggests that this isn’t the case. Specifically, genetic heterogeneity seems to be greater than previously thought, and combat has been observed between ants that are part of the purported ‘supercolony.’ The most recent data suggest that aggressive behavior is more closely related to environmental factors (such as diet), which influence the scent of ant populations. These factors may be less geographically variable in environments that the ant invades than they are in the ant’s native South America.

The unfortunately conclusion of this most recent information is that the ants will be much more difficult to get rid of than they would have otherwise been – under the ‘supercolony’ theory we might have actually introduced other colonies of Argentine ants, thereby increasing diversity and inter-colony combat. The discovery that inter-colony combat already exists means that far greater steps would have to be taken to slow the spread of Argentine ants, let alone cleanse already infested regions. The two means of actually destroying Argentine ants in an invaded area are to (1) limit their food supply by destroying the honeydew-producing aphid on which they depend for food, or (2) coat the area in a pesticide which would likely kill all first-order consumers in the ecosystem.

Even if option one were desirable, it could probably only be accomplished via option two. The harsh reality of a serious biological invasion – especially when the invasion has been present for so long – is that removing the species is rarely feasible. The story of Argentine ants seems to be one in which we now have to sleep in the bed we’ve made for ourselves.