The sound of cicadas, for many, is the sound of summer. Some, the aptly named annual cicadas, emerge every year.
Periodical cicadas, though, only emerge from the ground every 13 or 17 years, depending on the species. And there’s a growing risk they won’t come up at all.
Researchers at the University of Iowa said in a study published in the scientific journal Ecology this month that periodical cicadas have a chance of going extinct.
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A factor researchers cited was the loss of forests in the Eastern United States.
In the nineteenth and early twentieth centuries, people were “cutting down trees like crazy,” Andrew Forbes, a professor in the University of Iowa’s Biology Department, said in an interview with Straight Arrow.
And once a tree is cut down, the periodical cicadas living under it die because they no longer have food. So, researchers wrote, “clearcutting a forest patch, a riparian strip, or even a suburban landscape would remove all cicadas present at that site.”
“That’s almost certainly why we lost a lot of cicadas — is deforestation,” Forbes said.
Since the 1950s, though, there have been more trees planted, and “forests have been rebounded,” Forbes noted.
Still, “by and large, the cicada ranges have not followed suit,” he said. “In fact, in a lot of places, people have documented them shrinking.”
That’s because, being underground for years, periodical cicadas don’t move from the location of their original tree hosts. And when they’re out of the ground, they only have “intermittent opportunities for dispersal (just 5–7 times per century)” and short dispersal distances, generally less than 50 meters, researchers said.
“On average they’re just losing a little bit of ground every 17 years, and then they can only move a tiny little bit out from that range,” Forbes said.
During that time period, if you’re a periodical cicada whose tree is cut down, “you’re toast,” Forbes said. That’s not true for annual ones.
There is a brood of periodical cicadas four miles north of Iowa City, where the researchers are based.
“We calculated it would take something like 2,000 years for them to move back down here, if they’re just going 50 meters every time, which is crazy,” Forbes said. “There’s trees all throughout, but it just is going to take them a really long time to get here.”
Researchers said the presence of periodical cicada breeding populations is best predicted by tree cover in the 1930s, not present-day.
“Cicadas have largely not recolonized restored habitat even after nearly 100 years,” researchers wrote.
A model researchers made based on tree cover turnover and cicada dispersal suggested their ranges will continue declining under “nearly all conditions,” they said.
What happens if cicadas die?
Ecology research shows that periodical cicadas, when underground, amass nitrogen, phosphorus and other important nutrients, Forbes said.
After emerging, he said, they die a month later after laying their eggs, so there’s a “huge influx of nutrients that goes back into the soil.”
“All the plants and the trees benefit from that,” Forbes said. “We don’t know how important that is overall, but it’s certainly like a really interesting ecological process that’s happening every 13 or 17 years in these places.”
Birds and mice also love chowing down on cicadas, which allows other insects in the forest to survive, as they aren’t being eaten anymore.
“There’s these important ecological connections that they’re involved in,” Forbes said.
What happened when researchers tried to repopulate
Forbes usually studies very different insects in his line of work. But then, in 2024, there was supposed to be a periodical cicada emergence.
Unfortunately for Forbes, he found out they weren’t going to come out anywhere in the forests of Iowa City.
“I was sad about that, but I also wanted to know why, because I’m a scientist, and so we started thinking about how we could study their historical ranges and why they are not back here in places where the forests have been replanted,” Forbes said.
After doing this historical analysis, Forbes and a team of student and faculty researchers went to places where there were adult cicadas, collected more than 20,000 of them, moved them to a restored forest in Iowa City, and released them over the course of a few days — with permits to do so, of course.
It didn’t turn out so well for the cicadas, though.
“As far as we can tell, every single one of them got eaten by birds or mice,” Forbes said.
The problem is, if cicadas emerge all at once and cover many acres of land, birds are going to eat as many as they can until they get full.
“I think you would have to release 10 million cicadas in order to make a successful reintroduction,” Forbes said. “That’s a bummer because it means we can’t help them along unless we were able to take literally truckloads of them from one place to another.”
To stop the decline in the periodical cicada population, Forbes said, preserving areas of forests where cicadas currently are is important.
It’s so important, researchers said in the study, that “preservation of habitat across centuries may be the only bollard standing between periodical cicadas and their extinction.”
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