Weaver ants attain colony sizes of a half million; certain driver ants, possibly twenty million or more. But we close this book with the Argentine ant, whose dominion is the granddaddy of them all—colonies that can span hundreds of square kilometers. Knowing that as colonies grow larger, their inhabitants tend to become more aggressive, I had anticipated that colonies this large would have an almost unlimited capacity for bloodshed, and I was on the brink of witnessing their battles.
It was the fall of 2007, and I was in southern California with David Holway, an energetic associate professor from the University of California, San Diego. With us was Melissa Wells, my partner in adventure since we had met nearly a year earlier, at the counter of Pearl Oyster Bar in Manhattan. A redhead with a swimmer’s strong body, Melissa had a hunger for exploration (and for oysters) that matched my own. Though she proclaimed a preference for elephants, she thought chasing ants was pretty nifty, too. I had taken her to Laos and Cambodia, where she produced a video of me with the weaver ants, and now we were en route to what, I assured her, was the largest battlefield on Earth. Our visit would lead me to conceive of a new kind of superorganism, one not limited by the corporeal boundaries of time and space or the ordinary lines between individual, society, and species.
But at first Melissa and I must have looked doubtful as David drove past the suburbs along Del Dios Highway in San Diego County. He turned off in Escondido, a secluded coastal development north of Del Mar. Navigating through well-tended streets, he finally parked along a curb and proclaimed, “This is it!”
Dead bodies of Argentine ants piling up along the battle lines between the Lake Hodges Colony and the Very Large Colony in Escondido, California.
We were surrounded by tidy homes and hedges. Melissa shot me a quizzical look, but David urged us to our knees at the curb, and there, in a bare patch of dirt and continuing over the concrete, we saw a finger-wide, chocolate-brown belt of tiny dead bodies, piled up by the thousands. The heap of corpses continued out of view, hidden by the undergrowth. From what I’d read, the battlefront could extend for miles. David explained that thirty million ants die each year in border skirmishes between the Very Large Colony and the Lake Hodges Colony. That’s a casualty every second.
“Last night’s rain shower may have broken up the fighting,” David warned us.
That may have been the case, but already the battle lines were reforming. Trails of ants, converging from all directions, led the troops over the remains of the dead. Scanning the action through my camera, I gave Melissa and David the blow-by-blow on dozens of fierce confrontations. Most started one-on-one, with a slow and meticulous approach followed by a thrust-and-grab. Atop the corpses, pairs of workers pulled on each other, indefatigably, for minutes (and for all I knew, hours) on end. Here and there, a third or fourth worker joined in. I focused my camera on a group of three ants pulling on another that was already missing an antenna. As I watched, a hind limb tore free. The worker who wrenched it off stood for a moment as if surprised at her success, the leg hanging from her jaws, before dropping it and inspecting her adversary’s stump.
This intimate view was reminiscent of the warfare of species such as the marauder ant and the weaver ant, where workers use a similar rack spreading technique. But while marauder ants fight only during the few hours when they happen to encounter another colony in a raid, and weaver ants engage in skirmishes that can go on intermittently for weeks, the Argentine ant battles ceaselessly. The ants actively police every centimeter of their territories, right up to a precise perimeter that constitutes a band of violence.
The quiet suburb we were visiting, it turned out, was just one front in a vast war between gigantic Argentine ant empires. Around Escondido and to the west lay the holdings of the Lake Hodges Colony, a kingdom spreading over almost 50 square kilometers. To the north was the dominion of the Very Large Colony, a single society whose territory, stretching almost 1,000 kilometers from the Mexican border to California’s Central Valley and on past San Francisco, boggles the mind. Given that the average Escondido backyard can sustain a million ants, the total population of the Very Large Colony could approach a trillion individuals, with a cumulative weight approximating that of the human residents of Carmel, one of the California cities it occupies.
Little wonder, then, that these Argentine ant republics are called supercolonies.
Argentine ants are unicolonial, which means individuals mix freely among nests, or rather, untold millions of nests. Ants from other colonies or other species, though, are attacked. In disputed areas, combat rages. As the losses build on either side, the boundaries shift. This movement of borders can be as slow as the creep of a glacier.1 The most extreme change recorded was 70 meters over the course of a month, when troops from the Very Large Colony overran the Lake Hodges Colony and seized a portion of scrubby Escondido land. In time, the Lake Hodges army took it back again.
Stepping away from the front lines, Melissa kicked a chunk of wood to reveal a mass of ants and brood. Three tea-colored queens ran to hide below a leaf. As a fourth queen rushed into view, David explained that the supercolonies owe their enormous populations in part to a myrmecological twist: they produce multiple queens that grow wings but travel only on foot, staying in their colony to give birth to still more ants. The California supercolonies contain queens by the millions. Nothing can stop a colony from thriving, growing, and expanding—except clashes with other supercolonies.
COLLIDING KINGDOMS
Until 1997, fighting among Argentine ants was unknown, and many thought the species was in effect one big happy family. That summer, UC San Diego undergraduate Jill Shanahand accidentally incited an Argentine ant skirmish—and so drew back the curtain on their internecine warfare.
Jill was assisting with a project on Argentine ants spearheaded by UCSD graduate student Andrew Suarez. The lab maintained a stock of the ants collected on campus. One day, Jill decided to augment their supply by gathering ants in her parents’ yard in Escondido, where Argentine ants were plentiful. When she put the new ants into a plastic tub containing the ants from campus, she was shocked to see the two groups rip into each other. Were the fights a function of the ants’ captivity, or were the workers clashing for some other reason, most likely because they originated from independent, hostile colonies?
The answer to this question came only in 2004. David had recently become a professor at UCSD, and another Melissa, Melissa Thomas, was his first postdoctoral student. For three months, day after day, she drove around San Diego County looking for a colony perimeter. She had batches of live ants stashed in the back seat, which she planned to mix with others from different sites like a chemist looking for a reaction. If her captive ants from UCSD were attacked by the local ants, it signified they were from a different colony; if they were not molested, it indicated they were from the same colony. Her goal: to use this test for aggression to find where the two colonies came into contact.
Melissa’s task proved more laborious than she had anticipated, given how huge their territories turned out to be. Starting in Jill Shanahand’s parents’ neighborhood, she expanded her search street by street until she eventually found two different supercolonies, each occupying one side of a road—an asphalt no-ant’s land that must greatly reduce the death toll, at least from combat. Then, early one day in April, eureka! She located a site where the nests were clashing, thus becoming the first biologist to witness a raging turf war that was hidden from view among suburban blades of grass.
Over the months it became clear to her how enormous and unique the colonies are. Even today, only four colonies are known to live in California: Escondido’s Lake Hodges Colony, two other supercolonies in the southern part of the state, and the Very Large Colony, which controls not only the UCSD campus but also much of the rest of California.
With his stories of supercolony mayhem, described from the curb in Escondido, David had our full attention. Melissa asked how the Argentine ant had become invincible. “Why don’t you visit Argentina with me?” David suggested. “That’s where the Argentine ant comes from. What we’ve learned about Linepithema humile in its home range tells us a lot about how the species has conquered California. It’s stunning.”2
Melissa glanced around at the California ranch houses as we got back into David’s car. Now that, she agreed, sounded more like a proper adventure. “Perhaps,” said David. “But before you leave California, I must show you something.”
BEWARE, LOCAL ECOSYSTEMS
David drove us to the edge of the housing development, where a potpourri of native chaparral plants spread off into the distance. As we hiked past buckwheat, sagebrush, black sage, and laurel sumac, David explained how Californians are accustomed to finding Argentine ants trespassing in their pantries, on their kitchen counters, and in their gardens, where they’re regarded as a nuisance. However, the damage they do is far more insidious. The ants tend to immense numbers of Homoptera—aphids and scale insects—helping them to flourish on backyard roses and in California’s fruit orchards, with severe economic consequences. But it is on the scale of natural ecosystems that their impact is most serious, for they are harbingers of death for many indigenous species. This is especially true for local ants and everything that depends on them.
Argentine ants are as tenacious in the wars they wage with other ant species as they are in battles with their own, annihilating even California ants with far bigger and meaner workers. Though the Argentines can’t sting and are too small to bite humans, they use the energy-rich honeydew from their homopteran herds as fuel to quickly find and dominate every food resource they can reach, thereby leaving the competition hungry. But their depredations go further than that, for even when native species don’t vie for the same resources and offer no physical threat, the Argentine ants plunder their brood for an easy meal.
Some native ant species mount a weak defense. David pointed out a large circular nest mound of seed-harvester ants. They had plugged their entrance in much the same way that Formica block slave raids by Amazon ants. To no avail: the passing columns of Argentine ants would whittle the nest away over time through starvation or repeated assault, giving this native species no chance to proliferate. Even when mature nests manage to hang on, they are, as David put it, “essentially the living dead.”3 The few indigenous ants that survive are able to do so only because they forage underground or come to the surface when it’s too frigid for Argentine ants. All other species that live in the habitats taken over by the Argentine ants have disappeared, their defensive tactics no match for the persistence of the South American conquerors.
The cleansing of indigenous ants has occurred everywhere Argentine ants have spread, namely, to every continent except Antarctica—especially in regions with a Mediterranean or subtropical climate, which include some of the most coveted human real estate on earth, southern California included.4 In each case, the ants form supercolonies, the largest of which extends from Italy to Spain’s Atlantic coast, a distance approaching 2,000 kilometers.
The effects of the Argentine ants’ conquest have only begun to be studied. On the mountain slopes of Hawaii, where the species invaded in the 1940s, the workers ravage populations of other ants, as well as predatory species like wolf spiders, herbivores such as caterpillars, detrivores such as snails, and pollinators like moths and bees.5 These organisms may be far larger than the Argentine ants, but the workers wear them down over long periods of time until they succumb. When it has eliminated these prey, a supercolony continues to thrive by increasing its dependence on honeydew.6
Even some vertebrates have been disappearing from coastal southern California, thanks to the Argentines. Horned lizards, shaped like thorny pancakes, require a balanced diet of diverse native ants, whereas the Argentine ants are themselves too small and quick for the horned lizards to catch. Adding insult to injury, the Argentine workers are experts at harassment, literally driving the reptiles into the sand.7
Argentine ants feeding on a protea seed in the fynbos habitat of the Western Cape of South Africa.
The best-documented effects of Argentine ants on plants are in the fynbos, a Mediterranean-like ecosystem in South Africa. Here, the original ant fauna is pivotal in seed dispersal, as I saw in 1996 on a visit to these vibrantly colored heathlands with Cape Town myrmecologist Hamish Robertson. Beneath the blood red flower stalks of one of the resident proteas, Hamish showed me native Anoplolepis custodiens ants that were hauling the plants’ seeds to their nest, where the workers would eat outgrowths on the seeds called elaiosomes. Afterward, the discarded seeds, like those of hundreds of other local plants, would have a chance to sprout in the ants’ nutrient-rich trash heap. A short drive away, an encroaching Argentine ant supercolony had taken over a different area, wiping out the Anoplolepis. There, we found the ants eating the elaiosomes in the open but leaving the heavy seeds behind to dry out and die.8
The native plants of southern California face a similar threat. A small tree called the California bush poppy, which David pointed out on our visit to the besieged harvester ant colony, depends on the vanishing harvesters to carry off its hefty seeds.9 This is only one of many native California species in decline in the areas encroached on by the Argentine ant. The result is that the flora and fauna of California and other affected regions are becoming increasingly uniform, in a tragic process known as biotic homogenization. In this battle, we are all the losers.
A DESTROYER ABROAD, BUT JUST ANOTHER BRUISER AT HOME
A year later, Melissa and I found ourselves shaking hands with David Holway again, this time in the stifling early morning heat of the Corrientes bus station in northern Argentina. We had just arrived from Buenos Aires, where the steaks were immense and I had managed a tango lesson without injuring either Melissa or the tiny but stern dance instructor. Still groggy from thirteen hours on a bus, we climbed into David’s tin can rental car with his former student Ed LeBrun and drove another three hours to the field site. Relieved to finally stretch our legs, Melissa and I tromped with David and Ed through a thorny pastureland along a river, watching out for stray bulls. At our feet meandered thin columns of Argentine ants.
The discovery that California’s Argentine ants join forces to create supercolonies had piqued David’s curiosity as a postdoctoral student at UCSD. Was this ant, clearly an outrageously proficient fighter in California, as murderous in its native country? With this question in mind, in 1997 David and three colleagues had headed to the region Melissa and I were now visiting, where the species demonstrates its preference for moisture by living in river floodplains.
To the researchers’ surprise, the Argentine ant is nondescript in its indigenous land, so innocuous and with the workers so sparsely distributed that the locals know it as the “sugar ant” (after one of its favorite foods) and hardly give it a glance. Moreover, workers transplanted between nearby nests often fight one another, suggesting the colonies are many but small. I confirmed this by putting some workers into a vial and carrying them to an adjacent field. There I dropped them on top of the local sugar ants, which went on the attack.
Even more unexpected, David and his colleagues discovered that the home-grown Argentine ants belong to a rich and sustainable community of ant species, among them relatives of California’s besieged harvester ants. This isn’t to say their relationships are harmonious. Just as in California, the Argentine ants wage territorial battles with other ant species that they then raid for their favorite snack, ant brood.10 But in Argentina the resident ants put up a much better fight, so all the species persist.
Indeed, the Paraná River drainage in this part of Argentina is an exceptionally cutthroat environment for ants, forcing the indigenous species to hone their battle skills. The drainage is the original home of not only the sugar ant but also other species that are invading vast swaths of the world: Pheidole obscurithorax, Pseudomyrmex gracilis, Paratrechina fulva, Solenopsis richteri, Solenopsis invicta, and Wasmannia auropunctata.11 As one friend put it, any ant colony exported from this region is like a World Cup–winning soccer team sent to suburban Ohio to compete in the junior-varsity soccer league.12
What has turned the sugar ant and these other species into unrivaled warriors? All of them harvest a broad range of foods through mass recruitment that brings great numbers of workers to a feeding site, creating intense competition. With many ant species, however, aggression between colonies declines after the combatants tussle, establish territorial borders, and then back off. Much as humans do in similar situations, the ants adjust to the other’s presence in a kind of stalemate, even going so far as to establish a strip of land in limbo between disputed areas. Though the two sides continue occasionally to test each other, mortality drops—a behavior known as the dear enemy phenomenon, observed in ant species such as the weaver ant and in vertebrates from frogs to birds.13
Argentina’s shifting floodplains allow for no such stalemate. Frequent rising waters repeatedly force colonies to high ground—even up trees—and into whatever temporary living space is available. Each time the floods drain away, the ants descend to their former homeland, where they must struggle to reestablish their territories from scratch. This training regime of agile movement and repeated and relentless marathon combat has turned the ants into expert globetrotters and efficient, cold-blooded killers. Shifting from place to place at the slightest opportunity, the Argentine ants and their floodplain adversaries move swiftly into any available living quarters and thus are able to hitchhike on ships bound for far corners of the world. Once a ship docks and the ants disembark, their aptitude for resettlement and conquest gives them control over the new landscape.
JUMPING AND BUDDING
A mastery of leapfrogging to far locations is called jump dispersal, and it is a highly developed skill among tramp species of ants. Before humans introduced oxcarts, boats, cars, and planes, the tramps had the potential to extend their ranges only when winged queens were blown by storms or colonies caught a ride on water-borne detritus. These stressful events rarely took the ants very far and had a low probability of success. The castaways often expired in transit.
A few days after our arrival, David took us to the murky Paraná. The river was so broad that the opposite bank was a barely visible green line, with masses of vegetation drifting on the currents and container ships passing in the middle distance. To the roar of howler monkeys in the trees, David led us on foot to the river bank along a route made treacherous by haphazard piles of massive driftwood and other dreck. I passed the toothy skull of a piranha being picked clean by Argentine ants. With the abundance of such resources at the water’s edge, it’s no wonder the ants often end up on flotsam. That must have been how they were swept away by the river for millennia, until the arrival of human forms of transportation accelerated successful jump dispersal for them a millionfold, and took them much greater distances as well. With a few workers and a queen or two, a splinter group of Argentine ants can survive in just about any carrier, whether a barrel of produce or the soil nourishing a small potted plant.
The Argentine ant’s relationship to North Americans traces back to early trade out of Buenos Aires. The Plymouth Rock for these ants was likely a dock in New Orleans, the city where they were first recorded in 1891, in all probability arriving aboard a ship loaded with coffee. The first sightings of the species in California occurred circa 1907, in the San Francisco Bay area and around Los Angeles. The origin of these populations is not known, but since the Panama Canal had yet to open, it seems likely that they came from the southeastern United States rather than from Argentina, possibly stowing away in a railroad car.14
Slowly but surely, Argentine ants spread through southern California. Though the invaders never made off with our children, as some tabloid newspapers in the 1980s implied they might, the ants have become a part of the landscape, and they continue to expand onto land as yet uninhabited by their species. Even the Very Large Colony is still growing. Where moist areas are continuous, a colony can diffuse outward as the ants migrate to previously untenanted sites, an incremental process of budding new nests that remain part of the same colony in this case. Such migrations occasionally take place across uninhabitable sites such as narrow roads, but the Lake Hodges Colony is split by two freeways, a more formidable obstacle. With other species, a queen on her mating flight could have traversed such a barrier to form a new colony on the other side, but the Argentine queen doesn’t have that option. Either the Lake Hodges Colony is older than the freeways, or the colony was accidentally brought to the other side via human commerce—jump dispersal.
Argentine ants eating a piranha on the banks of the Paraná River in northern Argentina.
Jump dispersal has been essential for the Argentine ant’s success at domination, as demonstrated within California. The spread of the species by budding nests is slow—up to 150 meters a year. If the Very Large Colony had expanded only by this method, it would have grown to just a tenth its current size. However, given the volume of cargo we haul not just across the oceans but between neighborhoods, modern-day humans must aid and abet ant stowaways in reaching thousands of new sites every day.
The Argentine workers are less crowded on their home turf, most likely because they contend with more numerous, and more dense, colonies of their own species as well as colonies of other equally aggressive ants. No doubt the competition forces colonies to divert labor and resources from foraging and colony growth, keeping the size and the density of the worker populations under control and allowing other species to settle and expand. Elsewhere in the world, less skilled fighters fail, and so the Argentine ant infests new territory in prodigious numbers.
When the ants first arrived in New Orleans and spread across the Southeast, their numbers were astronomical from the start.15 In general, tramp species are expected to have few limits on their populations, thanks to what’s known as ecological release: they are free not only from their competitors but also from the predators and parasites that bedevil any organism in its native range. Even though an Argentine supercolony’s death toll is astonishing by human standards of warfare, mortality remains low overall because the vast majority of workers live far from borderland clashes. Once an Argentine ant colony lays claim to a plot of ground, the workers there may never experience conflict with their own or any other kind of ant. And so their populations shoot sky high.16
CONTROLLING THE LANDSCAPE
The success of the Argentine ant is facilitated by its fluid lifestyle. “The species is weird because nests are decentralized; they live anywhere,” David Holway said. In this way, they resemble weaver ants with their interconnected roadways and leaf nests. But unlike weaver ants, Argentine ants invest little in infrastructure, which is one reason they easily expand their colonies without territorial limits. “They take whatever’s around, usually near the ground surface,” David explained. “If a nest under a stone becomes too hot, some workers and queens move under adjacent leaves. Some of them will move again as those spots dry out, or they’ll shift locations to be near some aphids, all in the course of a day.”
The ants bud new nests when moving not just beyond established borders but anywhere within the colony, and they do it all the time. The frequent shifting of domiciles within a territory is called seminomadism, a practice that seems to go hand in hand with living in temporary encampments. Such camps sound like an army ant bivouac, except, as David explained, instead of staying in one compact nest as army and marauder ants do, “Argentine ants are constantly nomadic, everywhere in a colony at once.”
Supercolony sprawl and suburban sprawl are strongly linked by ants’ and humans’ thirst for water. If I could make the ants glow, irrigated human properties would phosphoresce to their edges, with additional illumination spreading along waterways and in moist natural habitats. These glowing patches would expand and contract as the workers came and went from nests, or created and abandoned nests based on temperature and water availability, a pattern tied to daily and seasonal cycles. But some luminosity would extend into the dark areas, as enclaves of the colony eke out a living up to 200 meters from obvious water sources.17 It’s in such corridors that the invaders do the most harm to California’s scrub ecosystems.
The ants’ constant redistribution of nests minimizes the time spent in foraging and commuting to meals, whatever the food is and no matter if it is clumped or scattered.18 As a result, Argentine ants have no commitment to a particular resource or site but control the useful part of the landscape absolutely. Their trails appear to emerge along lines of frequent passage, much as human paths form on beaten grass, though the ants are guided by scent rather than by the wear of footprints.19 The densest columns develop between nests or from nests to productive food-harvesting sites, with the ants both reestablishing old routes and starting new ones swiftly and easily.
Fanning out from the routes, the workers behave a bit like army ants in acting through sheer force of numbers. Foragers advance over uncharted ground by laying exploratory trails, but they radiate out in a looser, more scattered way than army ants do at their raid fronts.20 Physicist turned biologist Jean-Louis Deneubourg led the team that described this process, in which the whole group generates a trail behind it that leads back to a nest. The success of the operation depends on the workers departing a nest en masse while continually laying pheromones:
The Argentine ants’ exploratory behavior is exceptional in that they mark continually and explore collectively. Whereas other recruitment trails are constructed between two points (e.g., nest and food), their exploratory trails have no known destination, progressively advancing into the unknown. They rapidly lead new explorers to the frontier between the just explored and the about to be explored zones, avoiding situations where ants will end up exploring the same zone twice, and help returning explorers reach the nest directly. A wide corridor of the chemically unmarked area is thus systematically “swept” and marked in a minimum time with maximum economy.21
Each worker can wander at least half a meter from her neighbors, which suggests that, unlike an army ant, she is relatively free to explore on her own. Where their nestmates are scarce, the foragers take straight paths, which spreads them swiftly over new ground. As their numbers increase, the workers begin to take more irregular courses, such that each one’s movements become limited to a smaller area, until the workers saturate the terrain.22
Argentine ants employ these exploratory patterns in order to muster a concentration of workers everywhere at once. They may not attain the aggregate densities and strength of raging army ants packed in a raid, or show the multipronged communication systems of the weaver ants, but the race is not to the swift, nor is the battle to the strong. The Argentine ants turn out to succeed despite a lack of many of the organizational skills we have come to expect from large societies in this book. The workers show a minimal division of labor, without polymorphism. They do not have assembly lines and teams, and they are not adept at moving food in a group (nor do they need to, since theft from competitors is so unlikely that they can eat the food where they find it). Yet they take to phenomenal extremes the rapid dominance military practices deployed by the marauder ant. Like a starfish that succeeds in prying open a clam through persistent application of pressure, these ordinary-looking imperialists wear down nasty rivals and prey many times their weight in wars of attrition staged over hours, days, weeks, and even years.23