Don’t crawl for me, Argentina
Armies of alien ants are invading our homes and damaging our ecosystems, and scientist Neil Tsutsui wants to stop them
Perhaps Neil Tsutsui is a little crazy. That would explain the casual way he lifts the lid off the clear plastic bin, teeming with thousands of tiny brown ants, and places the bin, topless, on a long worktable.
The sight of the un-lidded ant colony plotting its escape is enough to make your skin crawl. But Tsutsui, undaunted, just flips open the top on another smaller, equally busy nest and peers inside the box.
These artificial colonies consist of Linepithema humile, a.k.a. the Argentine ant, or the sugar ant in its native Argentina. These local specimens spend most of their time living in boxes in an incubator on the fourth floor of Storer Hall at UC Davis, where Tsutsui is a researcher in UC Davis’ Evolution and Ecology Department.
Tsutsui’s ants nest in little boxes and test tubes placed randomly around the bottom of the bin. Scattered throughout, small hexagonal plastic cups hold tiny puddles of water, or piles of scrambled eggs mixed with a liquid protein supplement that has dried to a hard crust. If you’re an ant, this is the easy life.
Then there are a half-dozen or so small piles, each about as big around as a quarter, that look as though they might be dust and dirt. These are, in fact, ant graveyards, containing the corpses of dead ants along with assorted dust and other refuse.
It was the evolutionary biologist E.O. Wilson, whose 1990 book The Ants won a Pulitzer Prize, who first explained how ants go about building their graveyards. As with many social behaviors, the graveyard-building process is helped along by a host of chemicals that the ants produce and use to communicate very specific information.
Certain chemicals, for example, are used to mark trails, which is how ants manage those fantastically long and convoluted marching lines from their nests to that half-sandwich you carelessly left on the kitchen counter. But ants also use chemical tags to identify each other as friend or foe, and even dead ants give off a chemical cue that tells the others, “I’m dead. Take me away.” Wilson did experiments in which the “death odor” was washed from ant corpses, concentrated and then sprayed on living ants, who were then dutifully hauled off to mass graves, despite their animated protests of life.
Luckily for the rest of the lab community, there is little real danger of Tsutsui’s ants, be they dead or alive, escaping; he has prepared the boxes with a thin strip of fluon, a milky, almost translucent substance that is incredibly slippery, too slippery for the ants to traverse when attempting an escape. Another inch up the side is a ring of greenish-brown “tanglefoot,” to trap any ants that make it past the first line of defense.
Outside the lab, as readers may know, the Argentine ant lives just about wherever it wants: in backyards, farm fields, city parks—even in your house, where hordes of the six-legged menaces scour your kitchen counters and cupboards for food. What most folks don’t know is that this pesky ant only came to California by accident, about a century ago. Since then, it has become more than just a pest; it’s a highly successful invasive species that has crowded out native ants and is wreaking havoc on local ecosystems.
And, despite millions of dollars being spent to battle this alien invader, efforts to control the exotic, innumerable horde have failed. (In many parts of the state, Argentine ants have surpassed cockroaches and fleas in business generated for exterminators.)
Tsutsui and his colleagues have made some remarkable discoveries about why the Argentine ants have been so successful. Along the way, he’s found important clues as to how we may restore our own native ecosystems.
The Argentine ants first were detected in the United States in New Orleans in 1891. The colonizers probably stowed away in sacks of coffee or some other agricultural product, or perhaps they were present in soil being used as shipboard ballast that was dumped after a ship’s arrival in port.
From there, the ants quickly spread throughout the southeastern United States, and by 1907, they somehow had made the giant leap to California. Again, they probably had human help, perhaps by hitching a ride on freight cars into the state.
But they didn’t stop there. In the past century, the Argentines have built empires throughout the Mediterranean climes of Europe (both California and Argentina are mostly Mediterranean in climate) and places as far flung as Hawaii, Australia and even Easter Island. And everywhere they have been remarkably successful at taking over foreign lands.
Scientists still know very little about most invasive species, plant or animal, and why they are invariably so good at crowding out natives once they get established. One common factor is that invasives are untroubled by the predators and diseases with which they evolved back home. But Tsutsui and other researchers are finding it’s a bit more complicated than that.
For much of this century, little was known about the Argentines, other than their voracious nature and aggressiveness toward different ant species and other insects. But one of the early things scientists noticed was that Argentines rarely fought each other, unlike many native ants, which tend to fight little range wars with neighboring nests of the same species.
“People thought, ‘Well, that’s just the way that Argentine ants are.’ We were the first to really look at them in their native habitat,” Tsutsui said.
It turned out that Argentine ants behave very differently in Argentina than in California and other lands the ants have invaded. In their native habitat, Tsutsui said, “you can take an individual from one patch of ground and walk no more than 20 yards, and he will be instantly attacked” by other members of his species.
That is because, in Argentina, L. humile live in small, fairly self-contained colonies. And each clan is genetically distinct from neighboring groups.
It is the genetic makeup of the ant that determines the combination of chemicals it gives off. So, ants of the same colony who are genetically similar will recognize each other easily by their special chemical fingerprint. By the same token, outsiders “smell” like outsiders and will be attacked.
But when the Argentines came to California, they passed through what scientists call a “genetic bottleneck.” A small group of colonists were genetically very similar, and their generations of offspring have remained similar during the last hundred years. In California, lack of genetic variation has given rise to a supercolony that stretches from San Diego all the way to Ukiah. That is why the Argentines rarely fight each other; they all “smell” like one big family.
Local ants, on the other hand, who don’t share the single-colony structure, spend much of their time fighting each other as well as fighting other species. This gives the Argentines a great advantage in competing with fragmented, non-cooperative populations of native ants. “They function as a single family, competing with local native ants,” Tsutsui said.
Not surprisingly, in areas where there were once two dozen or more species of native ant, it is now common to find only one or two species, “and one of them will be the Argentine ant,” said Tsutsui.
There are gaps, places here and there in California where the Argentines have not taken hold and where native species still flourish. Those gaps are generally where the landscape is undisturbed by human development, irrigation and fragmentation of the natural ecosystem. “But they are filling in the gaps, at roughly the same rate as human development. Human development goes hand in hand with the expansion of Argentine ants,” Tsutsui said.
By suppressing local biodiversity, Argentines can affect other animals higher up the food chain, as well. Indeed, the interest in the Argentines started when some of Tsutsui’s colleagues began to investigate the mysterious decline of the coastal horned lizards in Southern California.
It turned out that the lizard is what is called a “native ant specialist,” meaning it feeds on one or two varieties of local ant species, and they quickly were being supplanted by the Argentines. The lizard doesn’t recognize the Argentines as food, so it starves to death. It’s now a candidate for protection under the Endangered Species Act.
Tsutsui believes that the new knowledge of L. humile’s genetic makeup and social structure may provide a strategy for controlling the exotic pest one day and could help to restore the native ant species that have been supplanted.
One strategy is simply to wait them out. The lack of genetic diversity in the Argentine supercolony may one day prove to be its downfall. Without variation, the ants are likely to be susceptible to a disease or pathogen that could come along and quickly wipe out most of them. The problem with the wait-and-see approach is that it could take a very, very long time or could not happen at all.
Another possible solution is to introduce some genetic diversity into the invasive population. The idea of importing more Argentine ants may sound backward at first, but it’s possible that introducing slightly different genetic strains of L. humile could differentiate the population enough that the supercolony would break down. Then there might be enough competition among the Argentines to give native ants a chance to come back. But this approach has tremendous dangers. A genetically diverse population might end up being more sustainable in the long run and make the ants impossible to dislodge.
“I think there is a middle way,” said Tsutsui, alluding to another tactic that takes a page from E.O. Wilson’s “death odor” experiment.
Tsutsui believes that if researchers can isolate and understand the specific suite of chemicals the ants use to recognize members of their own colony, they might be tricked into fighting each other, just as they can be tricked into carting live ants off to the graveyard.
Chemicals from a genetically different colony could be collected, concentrated and then sprayed onto large areas of ants, causing the supercolony to erupt into civil war.
“It would be very species-specific,” said Tsutsui, and would avoid large amounts of poison, which kills ants and a lot of other things. The elegance of this approach, he said, is that “we can use the ant’s own communication system against itself.”