Argentine Ants: Appearances Can Be Deceiving

Take a look at Linepithema humile and you won’t be very impressed. This species, dubbed the ‘Argentine ant’ after its native home in South America, is modest indeed. They are tiny, skittish, and highly inconspicuous. These little bugs must be harmless, right? They are surely dominated by the countless ant species that are bigger, scarier, more ferocious… Well, that’s where you’d be wrong. The Argentine ant is a ruthless conqueror. In both form and function it stands out among its relatives for being incredibly flexible. And its success as a species reflects that flexibility-- Argentine ants have taken the world by storm and become one of the most notorious invasive species known to science.

Linepithema humile worker. Photo by Alex Wild [1]

As suggested by its name, the Argentine ant hails from the lush forests of South America. It is a terrestrial species that nests only on the ground, but, counterintuitively, its scrawny jaws and small size make it terrible at digging tunnels. You might be asking, “doesn't the inability to climb or dig limit its potential?” This makes perfect sense. But, in terms of ecological flexibility, the truth is quite the opposite. Since Argentine ants can only colonize places they find along the ground's surface, they are not at all picky in choosing nest sites. They require nothing special. A pile of loose leaf litter will work just fine. So will a cluster of stones, or some decaying wood, or a crack in a wall, or a gap in some floorboards. These ants are masters at “making do with what they’ve got”, and because of this they can thrive basically anywhere, including indoors and in urban areas.

Related to this, their dull coloration and tiny stature (only 2-3 mm long for a worker, and 4-6 mm for a queen) makes Argentine ants infamously furtive. It was likely their tendency to go unnoticed in cargo that caused their introduction to non-native habitats during the globalization of 19th century trade. Hitching a ride on produce and other commodities, these ants were inadvertently shipped from South America to all corners of the globe. And, much to the chagrin of native species, they proliferated wherever they were taken to. Argentine ants have undergone an explosion of population growth since their dispersal, and are now found in abundance on every continent except Antarctica. They can live happily in a variety of climate conditions, eat whatever is available, and require relatively small amounts of energy to sustain each individual. Combined with their stealthiness, the expansion of these ants has no end in sight.

Figure 1. Linepithema humile distribution across the globe. [2] 

The Argentine ant’s invasive success is also attributed to its colonial structure. The species is known for having a very high queen-to-worker ratio. Individual nests will sometimes have dozens of queens. Polygynous behavior not only allows Argentine ant colonies to grow quicker and outcompete other species, it also means that the death of a single queen does little to hinder that colony’s reproductive output. All of a colony’s queens must be killed for the colony to be eradicated... To make matters worse, Argentine ants create new colonies through budding. This means that a queen can take a group of workers from an existing colony and leave to create a new one (usually located close to the original). Only a handful of workers and one queen may be needed to successfully ‘bud’ a whole new colony. This often results in high densities of nests near each other because all the reproductive effort of the new queen gets added to the original colony. Budded colonies fuse as they expand to form a continuously growing population and a heavily fortified invasion front. Argentine ants are thus difficult to attack and difficult to defend against simultaneously.

Now comes the Argentine ant’s ‘trump card’ that truly distinguishes it from other organisms: their formation of supercolonies. A supercolony is a conglomerate of many, many individual colonies spread over a huge area. This extremely rare social organization occurs because, within a supercolony, Argentine ants have such a uniform genetic makeup that individuals from one nest can mingle in other nests without being viewed as a foreigner. The result is that different nests of Argentine ants in the same supercolony will seldom attack or compete with each other-- a phenomenon referred to as unicoloniality. Cooperation between nests links colonies into vast networks that effectively function as one gigantic colony. These networks can become so massive that only geographic borders limit them. Scientists have documented a single, united Argentine ant supercolony in California that extends for many hundreds of miles!

Figure 2. Linepithema humile population in California. The bottom red segment depicts the territory of a single, unified supercolony comprised of nearly one trillion individuals [3]

The social organization of these ants starkly contrasts with other ant species. Most ants will register ants from any other colony but their own as hostile, even if they are the same species. In effect, colonies tend to oppose one another, resulting in competition and even wars among the same species. The 'trust nobody' mentality shared by most ants is a negative feedback mechanism that prevents a colony of any single species from proliferating and obtaining a monopoly. No colony can ever become too big, for the bigger the colony grows the more enemies it makes, and the harder it is to keep growing. The cooperative behavior of Argentine ants, on the other hand, negates this rule completely. They can invade territories and spread cooperatively without inhibiting their own growth through intraspecies competition.

What are the consequences of a global Argentine ant invasion? For most other species on Earth, they are nothing good. Wherever they are introduced, Argentine ants displace native ants and can threaten native invertebrates that are not adapted to defending against such an aggressive species. This can reverberate and imperil other native species in that ecosystem, such as plants that depend on native ants for seed dispersal. Here where I live in Southern California, the recent plummeting of coastal horned lizard populations has widely been blamed on Argentine ants for decimating native ant species that the lizards feed on. They are a nuisance for humans too. They can enter homes with ease, cause infestations in urban areas, and indirectly destroy crops.

In short, Linepithema humile is a perfect example of how a species’ innocuous appearance can be deceiving. Despite how benign they may look, Argentine ants are unstoppable invaders that leave devastation in their wake. Even humans have been unable to effectively quell their spread. They are small but mighty-- menaces in miniature-- and they add new meaning to the old phrase, “never judge a book by its cover!”



Further Reading:

L. Passera, L. Keller, J. P. Suzzoni. 1988. "Control of brood male production in the Argentine ant Iridomyrmex humilis(Mayr)". Insectes Sociaux. 35 (1): 19–33. doi:10.1007/BF02224135

Tatiana Giraud, Jes S. Pedersen, Laurent Keller. 2002. "Evolution of supercolonies: the Argentine ants of southern Europe". Proceedings of the National Academy of Sciences. 99 (9): 6075–6079. doi:10.1073/pnas.092694199

Moffett, Mark (2010). “Adventures Among Ants: A Global Safari with a Cast of Trillions”. Berkeley and Los Angeles, CA: University of California Press. p. 218. https://archive.org/details/adventuresamonga00moff/page/218

Serge Aron, Luc Passera, Laurent Keller. 1994. “Queen-worker conflict over sex ratio: A comparison of primary and secondary sex ratios in the Argentine ant, Iridomyvmex humilis”. J. evol. Biol. 7: 403-418. https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1420-9101.1994.7040403.x



Media Credits:

(1) Photo by Alex Wild. https://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Linepithema/i-Bp4LJHR/A

(2) James K. Wetterer, Andrew Suarez, Nuria Roura. 2009. “Worldwide spread of the Argentine ant, Linepithema humile (Hymenoptera: Formicidae)”, Myrmecological News.12:187-194.
www.researchgate.net/publication/228654113_Worldwide_spread_of_the_Argentine_ant_Linepithema_humile_Hymenoptera_Formicidae

(3) Image from UC Riverside’s Center for Invasive Species Research. https://cisr.ucr.edu/invasive-species/argentine-ant