A brutal conflict between rival ant supercolonies has raged around San Diego for decades, killing millions upon millions of ants. Although the battles unfold on a tiny scale, their violence, fixed territorial boundaries, and staggering casualties have drawn comparisons with human warfare.

The combatants are Argentine ants, scientifically known as Linepithema humile. Native to South America rather than California, the species is highly territorial and uses antennae and chemical signals to distinguish related ants from outsiders.

Globalization and international trade dramatically expanded the reach of this complex social system. During the 19th century, and perhaps earlier, Argentine ants accidentally traveled aboard cargo shipped from South America, establishing populations in places ranging from Europe to Japan.

Those distant populations began with relatively small numbers of ants, leaving them with less genetic diversity. Because members of separate nests carried similar chemical signatures, they recognized one another as relatives rather than enemies and stopped attacking each other.

That cooperation allowed nests with their own queens to merge into enormous supercolonies spread across broad areas. The arrangement became far more volatile when supercolonies were transported again and encountered other populations that had developed separately for generations.

A 2006 paper published in the journal Molecular Ecology reported that southern California contained five rival Argentine ant supercolonies. These vast communities shared little of the cooperation seen among nests belonging to the same supercolony.

The dominant supercolony controlled a loosely connected network of nests extending roughly 1,000 kilometers, or more than 600 miles, along coastal California. At least three of the four smaller supercolonies bordered this territory, with each holding an enclave roughly the size of a town.

Scientists from UC San Diego found that territorial boundaries extending for miles could be remarkably precise. In some places, only a few meters divided opposing populations, yet contact between ants from different supercolonies regularly triggered violent fighting.

Researchers recorded battles during 40% of their site visits. Each observed confrontation produced an average of around 15,000 deaths, turning seemingly small border encounters into a relentless source of casualties.

The toll became astonishing when measured across months rather than individual clashes. During a study lasting six months, researchers estimated that at least 15 million worker ants died in a single border conflict near Lake Hodges in north San Diego.

That episode alone suggested the potential scale of the wider conflict across decades and throughout the full territory. The article noted that it was not difficult to imagine billions of ants dying across the region over such an extended period.

Despite the carnage, the borders observed by researchers barely moved. Boundaries sometimes shifted back and forth from one week to the next, but neither side consistently gained territory, leaving the supercolonies trapped in a continuing war with immense losses and no clear victor.

The stalemate resembled the Western Front during the First World War, where Allied and Central Powers occupied trenches and inflicted devastating losses while the front line moved little over several years. For the Argentine ants, however, there may be no equivalent endpoint.

Writing for Scientific American in 2011, biologist Mark W. Moffett explained that Argentine ants lack the dear enemy effect. Under that effect, territorial animals become less aggressive toward familiar neighboring rivals than toward unfamiliar strangers after prolonged periods of conflict.

Reduced aggression can eventually create a zone along a boundary where fighting no longer occurs. Under suitable circumstances, the neighboring territories can pull away from each other, bringing the killing to an end.

Moffett argued that Argentine ants do not receive that relief. In their native South American floodplains, rival colonies frequently fight until rising water interrupts the battles, preventing the conflict from reaching a lasting resolution.

According to that explanation, the ants are effectively conditioned to resume fighting rather than settle their territorial struggle. Their San Diego war therefore continues through repeated clashes, vast casualties, and borders that resist meaningful change.

The San Diego ant wars apparently have not received academic study for around 20 years. That leaves the current locations of the borders, the balance among the rival supercolonies, and the possibility of any reduction in violence unclear.