Ant Wars: California's Ant Conflict

Imagine a war so large that millions of soldiers die every year, yet the front line is only a few feet wide. No tanks, no treaties, and no end in sight. This war is real, and it is happening in Southern California. The fighters are Argentine ants, and they are smaller than a grain of rice.

Argentine ants are native to South America, but they reached the United States by steamer ship in the 1890s. Since then, they have spread across the state and built something remarkable. Most ants live in colonies of a few thousand to a few million. Argentine ants in California build supercolonies, where huge numbers of nests act like one giant family. Ants from different nests of the same supercolony rarely show aggression toward each other.

The biggest of these is known as the Very Large Colony. It covers territory from San Diego to beyond San Francisco and may hold around one trillion ants. That is roughly 125 times the number of humans on Earth, all cooperating as if they were one team.

So who could possibly challenge an empire like that? A rival supercolony near Lake Hodges, north of San Diego. When ants from these two groups meet, there is no friendly sniffing of antennae. They fight. Researchers from UC San Diego studied the border and estimated that skirmishes around the Lake Hodges colony killed at least 15 million workers over just six months. Other estimates put the toll at as many as 30 million ants per year. The battle lines are sharp, often separated by just a few meters, even though the borders stretch for miles.

Both supercolonies are the same species, so how do the ants know who to attack? Two things: chemistry and genetics. Ants recognize each other by scent, and the UC San Diego team found that these ants appear to use genetic differences to tell friend from foe. Think of it like a school uniform that you can smell instead of see. Ants across the huge Very Large Colony are genetically very similar, so they smell like family. Ants from Lake Hodges smell different, which makes them intruders. Scientists found that ants across one supercolony's wide range were very genetically similar, but distinct from ants in neighboring supercolonies.

You might expect a war like this to have a winner by now. Instead, it is a stalemate. The supercolonies lose enormous numbers of ants but gain no new territory. Each side keeps sending workers forward, and each side keeps losing them. It is a bit like a tug of war where both teams are pulling equally hard and nobody moves an inch.

Why should anyone care about ants fighting over dirt? Argentine ants not only fight each other, but they push out native species, including the ants that some local wildlife depends on. Horned lizard populations fell as their preferred prey, native harvester ants, were displaced. They also invade homes, which is why a 2007 survey found that 85% of urban pest control services in San Diego were aimed at this single insect. Understanding how supercolonies hold together, and what keeps them apart, could help scientists find ways to manage them.

There is also an open question. The San Diego ant wars have not been studied academically for around 20 years, so nobody knows for sure whether the borders have shifted or the balance of power has changed. The next chapter of this tiny war is still waiting to be written, maybe by a future scientist who is reading this article right now.

Sources

Seethaler, S. (2006, November 30). Invasive ants territorial when neighbors are not kin. UC San Diego. https://biology.ucsd.edu/about/news/article_113006.html

University of California, San Diego. (2006, December 1). Invasive ants territorial when neighbors are not kin. ScienceDaily. https://www.sciencedaily.com/releases/2006/12/061201110024.htm

As many as 30 million ants die each year in a war between two California ant colonies. California Sun. https://www.californiasun.co/as-many-as-million-ants-die-each-year-in-a-war-between-two-california-ant-colonies/

Supercolonies of ants in San Diego are locked in a violent forever war, killing millions upon millions every month. IFLScience. https://www.iflscience.com/supercolonies-of-ants-in-san-diego-are-locked-in-a-violent-forever-war-killing-millions-upon-millions-every-month-84963

Genocide rules ant warfare. Live Science. https://livescience.com/1177-genocide-rules-ant-warfare.html

Argentine ant. Wikipedia. https://en.wikipedia.org/wiki/Argentine_ant

Moffett, M. W. (2010). Behavioral ecology of supercolonies (excerpt). https://www.doctorbugs.com/wp-content/uploads/2019/04/Moffett-Behavioral-Ecology.pdf