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Country Ants, City Ants

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Tapinoma sessile, or the odorous house ant, is becoming a major nuisance to urban dwellers.
Sabri Ben-Achour
Tapinoma sessile, or the odorous house ant, is becoming a major nuisance to urban dwellers.

By Sabri Ben-Achour

Invasive species are foreign pests that are introduced into a new environment and then go wild, dominating that landscape and overpowering native species. Examples abound, from Kudzu in the South to Snakeheads in the Potomac. Some new research is showing that a species doesn’t have to move very far to become invasive.

Cecily’s story is an old one really. She moved from the country to the city, totally reinvented herself. She got caught up in the glitz and glam of urban life, and now, she’s dead in my friend Aaron’s apartment.

“Everywhere. Even on the floors. Everywhere you looked, just, scary,” says Aaron.

Cecily, her scientific name is Tapinoma Sessile, is also known as the odorous house ant. You know the small brown ant you find in your kitchen? Yeah that’s probably her, ant colonies are almost entirely female. She started crashing Aaron’s parties last winter.

“Swarms of ants. There were few surfaces that were NOT covered in ants. The worst part was of course over here where the liquor is. But they’re very small, and easy to kill,” he says.

Over the past few decades, this ant has adapted to city life. In a major way.

“They’re our number one pest," says Wayne White, an entomologist with American Pest, a pest control company in Takoma Park Maryland. He, and many other scientists, say Sessile is becoming a dominant urban species.

“You know, 20 years ago, I don’t believe we had nearly the calls about odorous house ants. We would’ve still had a large number of calls about ants, but they were different kinds of ants,” he says.

White dumps a jar of them out onto the table at his office, to show me how to identify this type of ant.

"I'm gonna let some of these guys out here. If you just take one like that between your fingers, roll it between your fingers and crush it good. Do you smell that odor?," he asks.

White holds up his finger, smeared with ant paste, and we take a whiff.

"It smells like bleu cheese and coconut."

Not a bad smell just, not what I expected ants to smell like. But that’s how you know it’s Cecily. Smell is also how Cecily knows it’s Cecily. Ants, it’s believed, use smell to recognize whether or not they belong to the same colony. Workers that serve unrelated queens from different colonies will know by smell, and kill each other.

To demonstrate this, White pulls out two vials of ants, each from different sides of the building, probably different nests.

“Lets see what happens if we dump a few in there,” says White.

See, if these ants aren’t related, they’ll know from the smell, and they’ll fight.

“You watch those guys and I’ll worry about these running all over the table," he says.

“Huh…oh. Well I see one that’s kind of dead, do you think it was bitten?"

"Well it’s hard to tell what’s going on in there," laughs White.

These ants should be at war if they’re from different colonies. But they’re not, they’re actually pretty calm, like, they know each other. And remember these ants were collected at least a hundred feet apart. That means their colony could be, really big. That is a far cry from what Cecily does in nature.

“In nature they’ll be just a small colony, with a queen, and maybe 50 workers and they might live in an acorn in a forest,” says White.

But in the city, they form massive supercolonies with thousands of queens and millions of workers. One study at Purdue University found one supercolony that spanned an entire city block. It had millions of workers and thousands of queens all acting as a unit.

“Urban populations are thousands, possibly tens of thousands times larger than anything I’ve ever seen in nature. So it is on a completely different scale," says Sean Menke, a research fellow at North Carolina State University, he recently wrote a paper on Cecily.

“It’s expressing something very different from anything anybody’s ever found in a more natural habitat,” he says.

Menke says she’s also behaving more aggressively. In nature she’s meek and timid in the face of competitors. In Urban environments, but she is meaner, tougher, and will defend her food from other ants. All of this was enough to lead Menke to wonder, is this a new species that’s developing right under our noses? And floorboards? He looked into the genetics and it turns out, no.

“In any part of its range in North America, it appears they’ve moved into urban environments and are behaving very similarly in each urban environment but they’re from very different lineages in the species, so very different populations are all doing the same thing. Which suggests the ability to change your colony size is inherent in the species,” he says.

So, basically, buried in the DNA of this ant is a little time bomb. Given the right conditions, which, it turns out don’t occur very commonly in nature, it can change it’s behavior and form these massive super colonies. There’s a word for this in science, it’s called Phenotypic Plasticity, the ability of an individual to change its traits in order to adapt to its environment. But that begs the question, what in the city is driving Cecily to change?

“If you think in urban areas we’ve knocked down the forest and the grass land and a whole new environment. So you start out with zero species in there. The first thing that gets in has no parasites, no predators, no competitors,” says Menke.

That’s one possibility. The other is that “We’ve just made a buffet table for them,” comments Menke.

In nature, they don’t come across high fructose corn syrup too often. Or sheltered basements with year round heating.

Scientists don’t know, yet, which it is for Cecily, whether it’s the abundance of food or the new environment. Nor do scientists know why now: Is it the expansion of cities? Climate change? Undetected genetic changes? They don’t know. But as they try to answer all the questions that she raises, they’ll learn more about what makes a species invasive. For now they just know Cecily has a new urban attitude and loves a good party in the city.


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