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Peek Inside an Unexpected Moth Slumber Party

The flying insects usually go solo—but one lepidopterist surprised hundreds snoozing together in a hollow tree.

Hundreds of moths rest in a hollow tree in Florida.
Hundreds of moths rest in a hollow tree in Florida. Andrei Sourakov/Florida Museum of Natural History

Andrei Sourakov, a lepidopterist at the Florida Museum of Natural History, is something of a bug paparazzo. Every time he passes a hollow tree with an opening he can reach, he sticks his camera inside, flips it upside-down, turns the flash on, and snaps a photo—just to see “what lurks there,” he says. He is often surprised: Once, he interrupted about 100 mating stick insects. “They sprayed my camera with nasty stuff,” he remembers. “I became quite a bit more interested.”

Other discoveries are somewhat less titillating. In 2010, Sourakov found a dozen moths, perched on the inside of a small sweetgum tree near the museum. They weren’t eating. They weren’t mating. They were just… resting. “When I zoomed in, they were all the same species”—a particular member of the genus Idia, he says. “I immediately thought, this must be some kind of specialized behavior.”

Over the next eight years, Sourakov kept returning to that tree, as well as another, a large red oak in San Felasco Hammock Preserve State Park. Often, he found moths: 60 of them, or 100, or once, over 400. Eventually, he collected his observations in a paper, which was published last month in Tropical Lepidoptera Research.

It’s not every day you get to peek inside an insect dorm. On the 400-moth day, Sourakov shot a video, which you can see above, and which is somehow both sleepy and exciting. “This is unbelievable,” he says, gazing at walls hung with slumbering critters. “So many moths.”

But this discovery is notable for other reasons, too. Adages aside, most butterflies and moths aren’t very social. There are exceptions—monarchs congregate famously and spectacularly every winter in California, milkweed butterflies travel together in Taiwan, and bogong moths hang out in groups during Australian summers. But even in these cases, “this is seasonal behavior,” generally associated with migration, says Sourakov.

With these Floridian Idia, “it looks like they do it daily, basically,” he says. “It’s like roosting in bats… They go out at night, and then they come back for the day.” He suspects they group up by sensing each others’ pheromones, and that resting in large numbers might help ensure they don’t all get gobbled down by predators.

A close-up of some sleepy Idia moths.
A close-up of some sleepy Idia moths. Andrei Sourakov/Florida Museum of Natural History

Sourakov still wants to figure out exactly why the moths are doing this, and how common it is. Since he published the paper, he’s gathered a few more data points: A correspondent started smacking on dead trunks around North Carolina and managed to scare some Idia moths out.

And last week, Sourakov was running a science camp for middle school kids and decided to check a hollow tree, just for kicks. “I wasn’t hoping for much,” he says. “But there were about a dozen Idia moths.”

The campers were impressed, and Sourakov was happy for the opportunity to pass along his philosophy. “It kind of proves my general feeling that while many young researchers want to go far to make discoveries, you don’t have to,” he says. “You should look in your neighborhood.”