Argentine ants attack a harvester ant. / photo by Alex Wild *
David Holway, an ecologist and evolutionary biologist at UC San Diego, stands in a field where he has been studying Argentine ants that have moved into the site -- which is near a residential neighborhood. / photo by Nelvin C. Cepeda * U-T

The invaders have overrun homes and jumped continents, employing tight family networks and singular group loyalty to achieve domination.

Among a world of intricately diverse insect species, uniform populations of Argentine ants have swept the globe, wiping out native ants, threatening local plants and animals, and creating a nuisance in kitchens each summer. These creatures appeared in the United States around the turn of the last century and quickly made themselves at home.

The Argentine ants that raid pantries in San Diego County nowadays are closely related to those in Northern California, Japan, New Zealand and Europe, according to scientists who have studied international “super-colonies” of them.

They all recognize their common roots, welcoming far-flung relatives without the aggression leveled at unfamiliar ant species. Their persistence and homogeneity are unusual for the insect world, where specializing and differentiating are the norm. And that gives them a competitive edge over native ants and other creatures.

“We’ve made the argument that because they don’t fight among themselves in super-colonies, that can contribute to high rates of colony growth,” said David Holway, an ecologist and evolutionary biologist at the University of California San Diego. “They’re very flexible as a species. The fact that they can associate so closely with humans gives them an advantage as far as spreading.”

But their uniformity could eventually be their undoing: Some recent research suggests that Argentine ants are disappearing en masse from areas where they once thrived.

Argentine ants originated along the floodplains of the Rio Parana in North Central Argentina and surrounding areas. When the river swells during rainstorms, the ants migrate into trees and fight ferociously with neighboring colonies for shifting territory.

“I think that predisposes them to live in an urban environment,” Holway said. “They can deal with the disturbance of flooding, irrigation.”

The ants jumped a ship to New Orleans in the late 1800s and showed up in Los Angeles and the Bay Area around 1908, he said. They spread steadily throughout California during the next century, hitching rides on flower pots, shipping pallets and trash loads.

“Their populations can grow extremely large and can overwhelm other ants,” said Neil Tsutsui, a professor of environmental science at UC Berkeley. “Extreme population size is also what makes them such big pests for humans. They can enter houses in large numbers and cause problems in agricultural areas.”

Argentine ants infest citrus groves and vineyards, where they tend to aphids that produce liquid nutrients called honeydew. To safeguard their supply of this substance, the ants will protect aphids from predators — allowing the pests free run of the orchards.

They also chase off competing ant species and various pollinators, thereby damaging plants and animals that depend on local insects. For example, horned lizards have declined as Argentine ants have displaced a prime source of food: harvester ants. And barrel cactuses have suffered as Argentine ants have harassed the bees that pollinate them.

“In a general sense, they reduce native diversity,” Holway said.

There are a few smaller colonies of other Argentine ants that were probably imported separately, including one around Lake Hodges, he said. Those do fight with Argentine ants from the global super-colony, but the vast majority of Argentine ants are harmonious.

Their uniformity contrasts with other ants around the world, whose distinct traits have served as inspiration to an array of disciplines ranging from artificial intelligence to commercial distribution.

But how do Argentine ants keep such a united front as they spread from continent to continent? Researchers investigating that question have found that Argentine ants throughout California recognize each other as immigrant relatives.

Holway conducted an experiment in which he introduced Argentine ants from San Diego County to others he collected across the state. Placed together in a jar, the ants commingled peacefully instead of fighting to the death.

Genetic analysis revealed that the ants were closely related and produced an odor profile that identified them to each other as family, Tsutsui said. Ants imported under quarantine from Japan, New Zealand, Australia and Europe showed the same reaction.

“So when they encounter each other, they treat each other as if they’re all one, big happy family,” Tsutsui said. “There are huge numbers of them because they’re so cooperative.”

That makes them dreadfully hard to eradicate. Household sprays will kill a column, but reinforcements quickly reappear. Professional exterminators favor a chemical called Fipronil, but Holway said it can persist in the environment and may have unintended consequences for other organisms.

Mike Rust, a professor of entomology at UC Riverside, said containment may be the best strategy. Sealing cracks and avoiding overwatering in yards can help keep the ants at bay.

Scientists also are trying to develop baits and biological controls, but success has been elusive on those fronts. “We don’t have any biological control measures at this point, any parasites or predators that are effective,” Rust said.

Not yet, anyway.

Ant expert Brian Fisher is exploring signs that some populations in the city of Davis and in New Zealand are declining. And by declining, he means disappearing.

“Where they’re gone, they’re gone,” said Fisher, an associate curator of entomology for the California Academy of Sciences and a professor at UC Berkeley and San Francisco State University. “I went baiting for them and couldn’t find them.”

He and other scientists believe an unidentified pathogen could be the cause. And if that pathogen kills one ant in the super-colony, it could just as easily wipe out any of its nearly identical relatives.

So the genetic similarity that gives Argentine ants their strength could be their Achilles’ heel.

The good news, Fisher has observed, is that nature may have retained some of the diversity that Argentine ants suppressed. When those ants dwindle in number, “the native ants come right back.”

A look at Argentine ants and some native ants they are displacing

Argentine ant / photo by Alex Wild *
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Argentine ants (Linepithema humile): They originated in Argentina and have invaded parts of North America, Europe, Asia, Australia and New Zealand. Brown-colored and about one-eighth of an inch in length, they eat sweets and sometimes protein. These ants don’t bite or transmit disease. They nest outdoors in shallow mounds, but often raid homes in search of food and water.

Carpenter ant / photo by Alex Wild *
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Carpenter ants (more than 1,000 species): Native to California, these ants also live around the world. Black, or sometimes red and black, they grow from one-fourth to half an inch in length. The many varieties of carpenter ants all eat sweets. They nest in tree stumps, firewood, fence posts, hollow doors or window frames. They don’t sting but can inflict painful bites.

Velvety tree ants / photo by Alex Wild *
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Velvety tree ants (Liometopum occidentale): They’re native to California and are closely associated with oaks. With a dark head, red thorax and velvety black abdomen, they grow from one-eighth to one-quarter inch in length. They eat sweets and insects, and nest in wood such as tree limbs, stumps and logs. They emit a distinct odor when crushed.

Harvester ants / photo by Alex Wild *
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Harvester ants (Pogonomyrmex): More than 20 species of this ant are native to the U.S., including California. They grow from one-fourth to half an inch long, and collect seeds for food. Horned lizards, which feed on harvester ants, are declining partly because of the loss of these insects. Harvester ants reside in underground colonies. They can inflict painful stings.