SCOTT SIMON, Host:

Summer is a time for picnics, barbecues and ants. They invade our countertops and kitchens, searching for that spilled soda or crumb of cake. And it seems that over the past decades one especially tiny species has been invading our cities.

From member station WAMU in Washington, D.C., Sabri Ben-Achour reports.

SABRI BEN: This is a story about a party girl. Her name is Sessile, and her story is an old one, really. She moved from the country to the city and totally reinvented herself.

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BEN: She got caught up in the glitz and glam of urban life, and now she's dead in my friend Aaron's apartment.

AARON: It was really scary.

BEN: Sessile - scientists call her Tapinoma sessile - is also known as the odorous house ant. You know the small brown ants you find in your kitchen? Yeah, that's probably her. Ant colonies are almost entirely female. She started crashing Aaron's parties last winter.

AARON: Just everywhere. And, you know, on the floors. Everywhere you look, swarms of ants. I mean...

BEN: Over the past few decades, this ant has adapted to city life - in a major way.

WAYNE WHITE: They're our number one pest.

BEN: Wayne White is 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.

WHITE: You know, 20 years ago, I don't believe we had nearly the calls about odorous house ants. Now, we would've had still a large number of calls about ants, but they were different kinds of ants.

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

WHITE: I'm going to let a couple of these guys out here.

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

It smells kind of like blue cheese and coconut.

WHITE: Okay.

BEN: Not a bad smell, just not what I expected ants to smell like. But that's how you know it's sessile. Smell is also how sessile knows it's sessile. Ants, it's believed, use smell to recognize whether or not they belong to the same colony. To demonstrate this, White pulls out two vials of ants, each from different sides of the building, probably different nests.

WHITE: Let's see what happens here if we just dump a few in there.

BEN: 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 100 feet apart. That means their colony could be really big - that is a far cry from what sessile does in nature.

WHITE: In nature, they'll be just a small colony, with a queen and maybe 50 workers, and they might live in an acorn in the forest.

BEN: But in the city, they form massive super-colonies with thousands of queens and millions of workers. And not just in Washington. It's happening in Tennessee, Colorado, Utah, Indiana. One study at Purdue University found one supercolony that spanned an entire city block. Sean Menke is a research fellow at North Carolina State University. He recently wrote a paper on sessile.

SEAN MENKE: Urban populations that we've seen are thousands - possibly even tens of thousands - times larger than the largest thing I've ever found in a natural environment. So it is on a completely different scale.

BEN: He also noticed sessile is behaving more aggressively in the city.

MENKE: It's expressing something, at least in some areas, very differently than anything anybody's ever found in a more natural type of habitat.

BEN: So basically, buried in the DNA of this little ant, is a time bomb. Given the right conditions - which, it turns out, don't seem to occur very commonly in nature - it can change its behavior and form these massive supercolonies.

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 Menke says that raises the question: What in the city is driving sessile to change?

MENKE: If you think of in urban areas, we basically knocked down the forest or the grassland and built a new environment. So you start out with zero species in there. The first thing that gets in has no predators, no parasites and no competitors.

BEN: That's one possibility. The other is that...

MENKE: We've just made a buffet table for them.

BEN: In nature, they don't come across high fructose corn syrup too often, or shelter basements with year-round heating.

Scientists don't yet know which it is for sessile - 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 sessile raises, they'll learn more about what makes a species invasive.

For now, they just know sessile has a new urban attitude and loves a good party.

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