When The Brood X Cicadas Descend In Masses, So Do Bug Scientists

Nell Greenfieldboyce

May 25, 2021 / 1:38 pm

Millions upon millions of Brood X cicadas are emerging, and entomologist Marianne Alleyne didn't want to miss it.

"I didn't see it 17 years ago, and I wanted to experience it," says Alleyne. "My brother-in-law actually put it perfectly: 'This is your entomology Woodstock, isn't it?'"

Brood X isn't the only group of cicadas that spends nearly two decades underground, but it is the biggest and most famous. The return of these enormous black bugs only lasts about six weeks and is a rare chance for researchers to try to understand everything from the creature's basic biological design to the impact of its mass appearance on other living creatures.

That's why Alleyne recently made a road trip to Maryland from the University of Illinois Urbana-Champaign. She studies how the intricate structure of cicada wings makes water droplets roll right off, and she wanted to collect some cicadas from this particular brood.

Other researchers are busy collecting too. Jake Socha, a professor of biomedical engineering and mechanics at Virginia Tech, and researchers in his lab set up a makeshift research station in a rented house outside Washington, D.C. On a recent evening, as brown, shrimp-like cicada nymphs began to crawl out from the soil, the scientists scoured the grass under some old oak trees around a school in Alexandria, Va.

"Be very careful. They're everywhere right over here," called Mary Salcedo, a postdoctoral fellow who is interested in how a cicada's wings expand and unfold during its final molt.

Stepping carefully, so as not to smush any nymphs, she and Ph.D. student Josh Pulliam counted out 100 and dumped them into a brown grocery bag. The nymphs scrabbled around inside, instinctively desperate to climb something.

Back at the rental house, the researchers let the nymphs creep up a sheet of cloth taped to the wall. Partway up, the nymphs froze and began to transform into adults. The back of each shell split apart, and a soft new body started to swell out. Salcedo used cameras to closely watch the shriveled-looking bits of bright yellow tissue that become elegant cicada wings.

"It's very thin tissue that looks like a beautiful origami structure when it first comes out," she says, "so just that unfurling pattern is really interesting."

Meanwhile, Pulliam has been testing nymphs on little homemade climbing walls covered with rough or smooth sandpaper, to try to understand the biomechanics of their climbing.

"What's really interesting is how these two forelimbs move. When they climb, they sort of do this up and down climbing behavior," Pulliam says. "That's excellent for digging and gets them where they need to go on trees, but is very different than what the adults do."

Cicadas can't bite or sting, and their only defense is to overwhelm predators with sheer numbers. That makes Brood X a bonanza of food for all kinds of creatures — as if someone set up a free shrimp buffet on every street corner.

To see if birds chow down on cicadas and ignore their usual prey, like caterpillars, one team of researchers has been doing an experiment in a forest in Poolesville, Md. They gathered there recently to check on small oak trees that they've been monitoring closely.

"I have a box of beautiful green fake caterpillars which are ready to be deployed on our trees with our trusty super glue," says Martha Weiss, a biologist at Georgetown University.

Since these fake caterpillars are made of soft clay, a bird or insect that tries to eat one will leave distinctive, tell-tale little lines and other marks, explains John Lill, a biologist at George Washington University. He showed off one of the fakes that was "chomped, at least several times, by a bird trying to prey on it. Their beak marks make sort of a characteristic V-shape."

Every week, these researchers come out and replace previously deployed fake caterpillars with new ones. This lets them see how often caterpillars get attacked, and if that changes when cicadas are around.

Less predation on caterpillars could mean they're able to eat more leaves, so the research team is tracking leaf damage on the trees. They're also measuring changes in the numbers of parasites on caterpillars.

"So we're trying to catch what it looked like the year before the cicadas, what it looks like in the year of the cicadas before they come out, while they're out, and then after they're gone within a season," and for a few more years, says Lill.

What they expect is that Brood X will have ripple effects that persist long after the cicadas have disappeared. For instance, more young birds may survive this year due to an abundance of food, and that would mean more of them competing for resources next year when the Brood X cicadas are gone. There's been almost no research, Lill says, looking at these kinds of subtle ecological shifts after a mass emergence.

Periodical cicadas are generally a super-difficult creature for scientists to study because the insects spend such a long time hidden in the dirt. "It makes it really challenging to do manipulative experiments on insects that have such a brief time above ground," says Lill, adding that an academic research career might only span two 17-year cicada cycles.

"There's just an awful lot of underground stuff that is a mystery," agrees Weiss. She ticks off a bunch of unknowns: After eggs hatch and tiny cicada nymphs burrow into the soil, how do they find roots to feed on? How much do they move around while they're down there? And while they seem to register changes in tree sap to monitor seasonal changes and the passage of time, how exactly do they count the years?

"So many of the questions require these extremely long-term investments in experiments," says Lill. "It's just really hard to do for any type of scientist on typical grant funding."

Still, researchers are gathering whatever information they can, especially while the cicadas are out and about. And they hope volunteers in the community will pitch in, by recording cicada sightings and making their own observations about what birds are eating.

"We're asking anyone who can ID a few birds to go out and watch and see where they're feeding and if they are feeding on cicadas or not, and then share that data with us," says Zoe Getman-Pickering, a researcher in Lill's lab.

She remembers seeing a mass cicada emergence in 1996, when she was 5 years old. "It was quite stunning for me," she says. It helped send her on a career in entomology, and she hopes some children seeing cicadas this summer will come back 17 years from now to study them as scientists.