Technology & Science

Everything you wanted to know about those horrifying fire ant rafts

No doubt you've seen the horrifying images of rafts of thousands of fire ants floating through the floodwaters left by Tropical Storm Harvey. Here's everything you wanted to know about them.

How ants that can't swim build amazing flotation devices with their bodies

Imported red fire ants are native to South America, where they evolved to survive annual flooding by creating rafts out of their bodies. (David Hu/Georgia Tech)

No doubt you've seen the horrifying images by now: seething red-brown rafts of up to hundreds of thousands of fire ants matted together, topped by squirming white larvae, floating through the floodwaters and debris left behind by tropical storm Harvey.

And no doubt you have questions: Are all those millions of venomous, stinging ants really all going to survive the flood and move happily back into Texas neighbourhoods afterward? How do they do that? And what should you do if one of them floats by while you're desperately wading through the floodwaters, trying to get yourself to higher, drier ground?

Here's what you need to know.

What kind of ants are in those rafts?

They're known as the imported red fire ant, and their scientific name is Solenopsis invicta. They're native to South America, and they were first detected in the U.S. in the state of Alabama in the 1930s, according to the Pest Tracker website at Purdue University. They have now spread throughout the southeastern U.S., from Texas to North Carolina. Fortunately, reports of fire ant stings in Canada are relatively rare, and may not be from this species.

Colonies can contain up to 250,000 workers, although most colonies contain only about 80,000.

Are they dangerous?

As its name implies, the ant injects a painful venom when it stings. The imported red fire ant is considered "a human health hazard due to its aggressive behavior and painful sting," says Purdue University.

Craig Tovey, a Georgia Tech engineering researcher who has studied fire ant raft-building and been stung "more than I'd like" says the ants' poison is in the same family as black pepper and hemlock, and is "worse than black pepper, but not as bad as hemlock."

Can the ants swim?

No — that's what got Tovey and his colleague David Hu interested in fire ant rafts. "Why does a single fire ant Solenopsis invicta struggle in water, whereas a group can float effortlessly for days?" they asked and answered in a 2011 scientific paper published in Proceedings of the National Academy of Sciences.

Individual fire ants struggle in water and will often drown. (David Hu/Georgia Tech)

So how does the raft float?

"It turns out that the ants grip each other and the distance at which they hold each other and the angles at which they hold each other are such that together they form sort of a bumpy surface and this bumpy surface is more water repellent than the individual ants," Tovey told CBC News.

He thinks a similar design could be used to make water-repellent fabrics out of sustainable materials that aren't water repellent, such as cotton, instead of out of plastics like nylon.

How do the ants build the raft?

Tovey and colleagues scooped clumps of thousands of ants (they like to cling to each other) out of a bucket using a long spoon, dropped them in a basin of water, and filmed them. "You drop that clump in water and its spreads out into this raft, which is pancake-shaped and about three ants thick," Tovey said. The ants move around on the surface of the raft as it forms.

"And when they get to the edge of the raft, they sometimes end up stuck there," Tovey said, though it's not clear whether they're trapped by their nest-mates at the edge of the raft or whether they voluntarily hung on.

The ants have barbs on the backs of their legs that interlink with those on other ants so it takes very little effort for them to stay linked together.

How long does it take to build a raft?

The clump of ants is noticeably flatter after 10 seconds, but takes several minutes to reach "a stable equilibrium."

How did the ants learn to do this?

Laboratory studies show that a clump of ants dropped into water is noticeably flatter after 10 seconds, but takes several minutes to reach 'a stable equilibrium.' (David Hu/Georgia Tech)

Tovey says the ants are originally from to the Amazon River flood plain. "Usually, there's a flood that lasts a few weeks every year … so they clearly adapted to be able to survive for several weeks at a time on these rafts.

Is there any way to break up the raft?

In the lab, the researchers were able to break it up by adding soap to the water. That reduces the surface tension - the force that holds the raft up — and causes it to sink. But Tovey doesn't recommend trying to spray soap on any fire ant raft floating outside, as the soap would become diffuse too quickly to be effective.

What should I do if I see a fire ant raft?

Whatever you do, don't poke it with a stick, Tovey warns. "whenever in the lab we would give the ants a chance to leave the raft and go upwards onto a rod, they would take that opportunity." He recommends staying as far away from it as possible.

Why didn't we see ant rafts after Hurricane Katrina?

Tovey suspects it's because the flooding during Katrina happened too quickly, due to storm surges and broken levees rather than rain.

In fact, he and his colleagues collected fire ants for their research by pouring water into their nests. At first, he recalls, they poured buckets of water into the nest all at once and the ants simply drowned. "Then we realized we have to do it slowly."

I still don't get why fire ant rafts are anything but horrifying.

Well, maybe the researchers' summary from their paper will change your mind:

"Overlooking its diminutive size and shortcomings in soapy solutions, the ant raft has attractive traits with respect to man-made flotation devices. It simultaneously provides cohesion, buoyancy, and water repellency to its passengers.

"It can be constructed quickly (in approximately 100 seconds) without any additional equipment. It can accommodate thousands to millions of passengers with zero casualties. But perhaps most strikingly, the ant raft is self-assembling."

Don't you think that's amazing?


  • A previous version of the story stated that the ants have not come close to the Canadian border. In fact, some cases of fire ant stings have been reported in Canada, although they may not be from this particular species.
    Aug 31, 2017 12:11 PM ET


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