Ranked No. 1 Pest in the U.S., Surpassing Cockroaches! The Alarming Spread of the Argentine Ant Across Japan

Once they build a nest they are extremely difficult to exterminate

They swarm in large numbers, destroying household electrical appliances, and turning offerings at family altars pitch black.

While the fire ant, designated an “Invasive Alien Species Requiring Urgent Action” two years ago, is well-known, the “Argentine ant,” which becomes extremely difficult to eradicate once it establishes a nest, is expanding its habitat across Japan.

Regarding the Argentine ant, which continues to breed on a global scale, we spoke with Entomologist and Photographer Eiriki Sunamura, who received the University of Tokyo President’s Award for his research on its ecology and control. He is currently engaged in pest control research at the Forestry and Forest Products Research Institute.

It took 170 years to conquer the five continents!

The Argentine ant (scientific name: Linepithema humile) does not have any particularly notable physical characteristics. It is small, about 2.5 millimeters long, and lacks venomous stingers or thorns. Its scientific name, when loosely translated into Japanese, means “insignificant ant.” It’s a rather pitiful name for such an unremarkable creature.

“In its native region of South America, there are many prominent ants, such as the Paraponera, which is large and equipped with a powerful venomous stinger; the Atta ants, which cut leaves to cultivate fungus and have distinct thorny bodies; and the fire ant, with its red body, venomous sting, and large ant mounds.

Compared to those, the Argentine ant, with its light brown color and lack of any special weaponry, was likely viewed as just an ordinary, unremarkable ant—a typical ‘mob character,’ so to speak,” says Eiriki Sunamura.

However, this mob character has been officially designated as an “Invasive Alien Species.” Eiriki Sunamura, who first learned about its existence during his university days and became more and more fascinated by this insignificant ant through his research, now calls the Argentine ant the “uncrowned king,” holding it in deep respect. But why is that?

“As the name suggests, its native habitat is in South America. However, research has revealed that descendants of a single colony (family) from its homeland have spread across Europe, North America, Asia, Australia, and other parts of the world, forming a global colony.

The expansion of this colony’s distribution began around 170 years ago. Since a new queen is born once a year, they have passed down their lineage through roughly 170 generations.”

Moreover, these ants have the ability to remember the origin of their colony. Each colony has a distinctive scent, which they use to identify members. When they encounter ants from a different colony, fierce fights break out, but when they meet ants from the same colony, they quickly become friendly.

“Although the descendants of this colony, found in Europe, America, Japan, and elsewhere, never meet in the wild, if they are brought together through importation, they can still recognize each other as part of the same colony.”

In general, ants have a reproductive season when new queens fly out from the nest to mate with winged ants from other colonies. The new queen then establishes a new nest, cutting ties with her original mother nest.

However, in the case of the Argentine ant, after mating, the new queen takes worker ants with her and quietly moves to a relatively nearby location, on foot. Since the blood relationship remains intact, there is no hostility with the mother nest, and they freely travel back and forth, cooperating with one another.

This splitting of nests is repeated over and over, eventually expanding into a massive extended family spread over several hundred meters to several kilometers. This marks the birth of a “supercolony.” Even if transported far away in a vehicle, they all remain part of the same supercolony, and in many continents, a single supercolony can stretch over several hundred to thousands of kilometers.

“People often imagine ants building well-structured nests with many separate chambers, and the queen’s room deep in the back, but Argentine ants are more casual. They just dig a quick tunnel-like structure and hang out there,” says Sunamura.

Because nest building is incredibly simple, they can quickly start foraging for food in their new location. Additionally, since they spend far less energy on territorial disputes, they can redirect that energy into reproduction.

 

“It seems that in their native habitat, they originally lived along riverbanks. Since floods often occurred there, if a single queen tried to establish a new nest far away, there was a high chance she would be swept away and die before she could succeed. So, they evolved to think, ‘Instead of that, it’s better to quickly spread the nests nearby and be well-prepared. If we spread out to a certain extent, we can avoid total annihilation, and in the case of a disaster, everyone can evacuate to the nest that sustained the least damage.'”

I see. Hearing the story, it sounds almost like a cheerful, cooperative community. But the situation is far more serious than to allow any fondness or attachment toward them.

Argentine ant infestation causes real estate values to drop

The first Argentine ant in Japan was discovered in 1993. By 2004, when Sunamura first learned about this ant in a university lecture, it had significantly increased in numbers in Iwakuni City, Yamaguchi Prefecture, causing problems for the residents.

“Compared to other insects, methods to exterminate Argentine ants had not yet been developed, and the issue was quite significant. I thought I could conduct some useful research on it, so I chose it as my study subject,” he explained.

So, what kinds of problems arise when this ant proliferates? Let’s summarize briefly:

Disruption of the Ecosystem: Native Ants Are Driven Out

In invaded areas, native ants are driven out, leaving only Argentine ants. In a survey conducted by Sunamura in Kobe City, there were 13 species of ants observed in areas where Argentine ants hadn’t yet invaded, but only two other species could occasionally be seen in invaded areas. The loss of native ants affects various organisms, including plants.

“They gang up, with five or six Argentine ants surrounding a single native ant, pulling at its legs and antennae like a scene from Gulliver’s Travels. Then they poison it or tear it apart.”

This impact on native species has been observed worldwide. In the U.S., lizards that primarily fed on native ants have drastically decreased. In Hawaii and South Africa, notable ripple effects and concerns for other animals and plants, such as the Cyanea plant and the Fynbos ecosystem, have also been reported.

Agricultural Damage: Ant Parts Found in Homemade Jam

Argentine ants love the “honeydew” secreted by aphids and scale insects. In their pursuit of honeydew, they protect these pests from their natural predators, causing their numbers to increase and spread diseases in crops, leading to damage. When nests are built in fields, root crops like carrots can be gnawed on, leading to deformities such as split roots.

“I’ve also seen cases where they end up inside the fruit of figs. I was once treated to some homemade fig jam in an invaded area, and upon closer inspection, I could see fragments of Argentine ant heads and legs in the jam.”

There have also been reports of the ants attacking beehives and hindering the collection of nectar, negatively impacting beekeeping.

Damage to Living Environments: Overtaking Cockroaches as the No. 1 Pest

There are numerous cases of Argentine ants invading homes. Due to their sheer numbers—five to ten times more than native species, according to Sunamura’s estimate—they can’t be contained just in underground nests and end up invading houses. In addition to the nuisance of swarming food and offerings, disturbing sleep, and biting pets, they also cause damage to electrical equipment.

“It seems they crawl into areas where they shouldn’t be, get electrocuted, and their dead bodies cause short circuits. The damage is extensive, leading to air conditioner or intercom malfunctions, flickering lights, and other breakdowns.”

In the U.S., Argentine ant invasions have led to decreased property values in some areas, even forcing people to move out.

“In California, they’re considered the No. 1 pest, surpassing cockroaches. The best-selling insecticide spray there features ants prominently in its illustrations. The same is true in South Africa.”

Japan’s climate is a strike zone.

The natural expansion range of Argentine ants is only about 50 to 150 meters per year, but most of their spread to distant areas is caused by human activities, such as being transported by cars. Unfortunately, Japan’s climate falls right within their ideal range. Their expansion shows no signs of slowing down, and areas like Hiroshima and Yamaguchi, which previously had spotty distributions, have now become a continuous belt of infestation.

Currently, the pesticide Mr. Sunamura is involved in developing is being sold to local governments. Since the Argentine ants haven’t spread everywhere in Japan yet, regional eradication is essential. Although it’s not available in stores yet, what should you do if Argentine ants invade your home?

“In the U.S., research has shown that a component found in aromatic cedar (pencil cedar) can repel Argentine ants. It’s used in chip form to cover gardens, but the effect seems to wear off over time, lasting perhaps one season.

Also, since they tend to build nests under flowerpots or stones, it’s a good idea to avoid placing such items around. Sealing cracks in exterior concrete walls can help prevent them from entering the house.”

To differentiate them from common ants, if you notice a sudden increase in small, fast-moving ants, they may be Argentine ants. Unlike regular ants, they don’t march in single file but spread out in a wide formation, attacking like a carpet.

Mr. Sunamura is so fascinated by Argentine ants that he travels across five continents researching their extermination methods, taking countless photos of trash bins and the walls of people’s homes, sometimes even getting questioned by authorities (though he always gets permission to photograph buildings).

Despite his research on extermination, you can sense a certain fondness for the ants in his words. What kind of future does he envision for them?

“While it’s necessary to control them, I don’t want them to go extinct. Ideally, they’d thrive in their native habitat.

In places like California or the Mediterranean coast of Europe, for example, it seems almost impossible to eradicate them. In such areas, I hope they don’t disrupt the ecosystem, but instead find a way to coexist. Argentine ants were unintentionally transported and are now trying to survive in their new environments, so it might be our responsibility as humans to find a way to coexist with them.”

▼Eiriki Sunamura (Eiriki Sunamura)
Entomologist and photographic artist. He earned his PhD from the University of Tokyo, researching the ecology and control of the Argentine ant (recipient of the University of Tokyo President’s Award). After working in insecticide research and development at Sumitomo Chemical, he is currently engaged in pest control research at the Forestry and Forest Products Research Institute (on secondment from the Forestry Agency). His work focuses on recording the ecology and creating artistic works using invasive species such as ants and longhorn beetles. He is the recipient of the Asahi Camera Award at the Yoshiaki Tabuchi Photography Competition. Sunamura is a member of the Japan Society of Scientific Photography and a part-time lecturer at the University of Tokyo (insect phylogeny and taxonomy). He is the co-author of books such as The Argentine Ant: The Most Invasive Species in History (University of Tokyo Press) and The Society of Ants: The Big Wisdom of Small Insects (Tokai University Press).

Click here to purchase Sunamura’s book, “Survival Strategies of Ants that Rule the World.