Science by Students
Imagine the ant hill in your backyard. Now imagine that ant hill extended across six continents, and you’ve begun to grasp the magnitude of the territory reigned over by the supercolony of Argentine ants. Their terrain covers more than 600 miles of coastal California alone; in the southern part of the state, the ant’s takeover is so complete that it is now the number one exterminated pest in the state. Argentine ants are just two millimeters in length. So how did such a small animal gain control over such a vast kingdom? By completely wiping out every other ant species they encounter, including some ten times their size.
Argentine fire ants originated in the floodplain between the rivers Rio Parana and Rio Uruguay in Northeast Argentina. When it rains and the rivers flood, the ants are driven out of their homes to seek higher ground. After the rain stops and the floodplain dries out, the ants return to their homeland and are forced to reconquer their territory. Argentine ants have emerged from thousands of years of fighting to the death to redraw the boundary lines of the floodplain every few weeks as the Spartan army of the ant world.
Argentine ants were first brought to the US in 1890 on trade steamboats carrying coffee, and were first sighted in New Orleans in 1891. They reached California in 1907, most likely making the journey by train since the Panama Canal was not yet open. Argentine ants spread deeper into California in the 1950’s, crawling into cars and travelling via the newly constructed freeways. They were well suited to suburban life: sprinkler systems forced them to move from place to place, much like the rain of their native floodplains.
If the Argentine ants found themselves in the midst of a foreign ant colony when they relocated, they would attack and claim the land for themselves, as they did at home. As Argentine ant colonies are generally set up like family structures, the ants possess an innate ability to determine how closely related they are to ants they come across. Ants give off a specific scent that other ants can detect with antennae. These pheromones basically serve as an ant’s “nationality”; it is specific to each colony and reveals an ant’s genetic makeup.
When American ants meet ants from other colonies, they will sometimes mate with them. When an Argentine ant meets an ant it detects as an outsider to the colony, the two ants will lock antennae, roll into a ball, and wrestle to the death. Argentine ants are genetically programmed to fight mercilessly, tearing off the legs and antennae of their opponent until they kill them. They take no prisoners, and after decimating all of the older ants in a foreign colony, they eat their babies.
Argentine ants are able to recognize other Argentine ants from different colonies across the globe as members of their own supercolony. According to the research of Australian entomologist Melissa Thomas, when confined in close quarters with one another, an ant from Japan and an ant from Italy will not fight, even though their home colonies are over six thousand miles apart. This is because the supercolony which spread so quickly around the globe was founded by such a small “immigrant” population.The ant’s propensity to kill ants from other colonies, rather than mate with them, kept the colony genetically pure, so that ants from Italy and Japan were able to recognize one another as relatives by smell despite the distance in origin which separated them.
When a closer study was conducted, it was found that the ant population in Argentina is twice as diverse as that of the global supercolony. Therefore, a native Argentine ant would recognize an ant belonging to a colony just a few meters away as an outsider and kill him, while an American-Argentinian ant would recognize a native Argentinian ant as a relative.
In 1995, Jill Shanahan was assisting UCSD entomologist Andy Suarez in his research on the Argentine ant population in San Diego when she found a colony of Argentine ants at the base of a tree in a park outside the city. When Jill introduced these ants to Argentine ants from her lab, they started fighting. The tree ants were members of a newly emigrated colony from Argentina, who considered members of the supercolony to be outsiders.
Melissa Thomas set out to find the border between the two colonies, and was able to trace it to the driveway of a home in Escondido, CA, a small suburb outside San Diego. The border of the supercolony’s empire lay on the left side of the driveway of 2211 Eucalyptus—clearly defined by a pile of dead ants which spilled out onto the curb, drawing a distinct boundary between the two sides of the driveway.
The newly emigrated colony of ants poses the biggest threat to the supercolony thus far. Researchers from UCSD are continuing to study the growth of these opposing Argentine ant colonies and the future of the supercolony. Is the reign of the supercolony coming to an end? The newly emigrated colony of ants poses the biggest threat to the supercolony thus far, putting the future of the world’s most powerful colony of super ants at stake. Only will tell whether the ants will learn to get along or whether they will destroy one another.
By Christine Georghiou, English, 2016
“Spartan Ant” credit: Kevin O’Leary