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Beneath the feet of millions of New Yorkers, an ant species found nowhere else in North America has been thriving in the concrete jungle for more than a decade, surprising scientists with its unique ability to thrive in Manhattan and other boroughs. But where the insect came from and why it has adapted so well to this urban environment has remained a mystery.
Now, scientists have discovered the ant’s identity and origins, and it’s several thousand miles from its home. According to a new study, the globe-trotting insect is a European species known as Lasius emarginatus and is commonly observed in more natural habitats in Central Europe.
Researchers first noticed the insect, dubbed the “ManhattAnt,” while conducting a survey of ants in New York City in 2011. Surprisingly, the black-headed and red-chested ant did not match any of the approximately 800 species found in North America, and the researchers speculated that it might be a European species; no other studies were conducted at the time to confirm the species.
But now the insect has become so numerous that it has become the second most common ant species in the region, drawing the attention of scientists and New Yorkers who live with the pest, says Clint Penick, an assistant professor of entomology and plant pathology at Auburn University in Alabama.
While entomologists have several theories to explain how the ant came to North America and why it’s doing so well in the big city, they’re keeping a close eye on this alien species to observe its impact on the environment, according to a study published May 28 in the journal Biological Invasions.
The beginning of an invasion
A 2009 ant survey did not find the Manhattan Ant, suggesting it was a recent arrival in the city. Despite only calling the Big Apple its North American home for a little over 10 years, the species has been busy, spreading at a rate of about 2 kilometers (1.2 miles) per year.
The insect has already been spotted in several cities on Long Island and New Jersey, according to the study. Based on the European climates where the insect can thrive, the authors suspect it could travel as far north as Maine and as far south as Georgia in the future.
“The species has more or less taken over Manhattan for a decade,” said Penick, the study’s senior author. “It’s the second most common species after the pavement ant (Tetramorium immigrans), which has been around (in New York) for at least a century.”
Penick said the invasive ants have so far spread naturally through mating flights (winged ants fly away from their nests during the summer months to establish new colonies), but the authors predict that the species could spread even farther via vehicular transportation, such as ships and cars, if it becomes dominant in a large city like New York.
The ants’ initial arrival in North America could be the result of such a move, said Corrie Moreau, a professor of biology and entomology at Cornell University who was not involved in the new study.
“A lot of non-native or invasive species are accidentally introduced by humans, usually through things like soil from potted plants or sometimes ship cargo,” Moreau said, but it’s less common for such insects to take hold and live in their new environments, he said, pointing to a 2005 study that found only 12% of previously introduced ant species established themselves in their new environments.
Ants in the Big Apple
To identify the Manhattan Ant species, the international team that conducted the May study first extracted DNA from ants and compared it with DNA from other species using a global database.
Genetic tests confirmed that the ant was from the genus Lasius, but to determine exactly which species it was, the researchers sent samples to co-author Bernhard Seifert, a leading ant researcher in Germany who studies native European species. Seifert was able to confirm the ant’s identity by comparing the New York ants to European ants.
Because non-native ants have spread so rapidly in New York City, researchers initially suspected that the Manhattan ant shared a trait common to other successful invasive ant species: a tendency to form what are known as supercolonies. Supercolonies are extraordinarily large ant colonies made up of nests that have been known to spread for hundreds of miles in some cases, Penick said.
“(Ants in supercolonies) have really low aggression and just take over the world and are peaceful within their colonies,” he said. Because the nests work together rather than against each other, they can spread more quickly and can easily dominate an area by competing with native species, he added.
But when the authors tested this hypothesis on Manhattan ants, collecting samples from different nests around the city and pooling them, they found that the ants displayed aggression and fought with each other, a sign that these insects were defending the typical territories seen in normal colonies and were not forming supercolonies.
Once that theory was disproved, researchers looked for other ways in which Manhattan ants differed from other common ants in the region. They discovered that Manhattan ants, one of the only species regularly found on New York City sidewalks, uniquely exploit a particular food source in trees planted along city streets—a sticky substance known as honeydew, secreted by tree insects such as aphids and spotted lanternflies.
“Their ability to make this shift and use urban structures and habitats (along with food resources) that other ants can’t access is one of the big reasons why they’re so successful in the city,” said Samantha Kennett, a doctoral student in the department of forestry and environmental conservation at Clemson University in South Carolina and lead author of the study.
The ants have been observed eating human food in some cases, while European ants feed primarily on other insects and their feces, Kennett said.
Moreau said that generally, non-native insects that manage to thrive are generalists, meaning they are not tied to a particular diet or particular environments. “Because (ManhattAnts) are more generalists, they have a better chance of landing in a new place and being successful,” he said.
Impact on ecosystem
Kennett said Manhattan ants are not harmful to humans, but like any insect they can be a nuisance when they enter apartment buildings. The ants have been observed up to the sixth floor of apartment buildings, according to the study.
But the real concern is the potential negative impact the ants could have on the environment they live in, such as the trees they inhabit or the native species they may compete with, he added.
“One of the things we know about invasive ant species is that they displace native species, and the native species they displace often have very specific roles in the ecosystem, so when those species are introduced you’re kind of disrupting the ecosystem,” Moreau said.
“Maybe if (ManhattAnts) became the birds’ favorite food, we would be less upset about it. But we haven’t seen a way in which they’re beneficial yet, so we don’t know,” he added.
The authors of the May study said they hope further research will identify other traits the ants may have evolved to be so successful in their new home and current range. Through the iNaturalist project, citizens can contribute their observations of the insects by taking notes when they see them and taking photos to upload to the site.
Time will tell whether the ants are no longer just a nuisance but are now disrupting the ecosystem, Penick said.
“Having a few ants in your kitchen isn’t the worst thing in the world,” he said. “I would guess it’s probably the worst thing we see. That’s really my hope.”
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