Every 13 or 17 years, swarms of cicadas emerge from the ground, their black bodies, orange-tinged wings, and ruby-red eyes enveloping trees and forests. It is a biological marvel unique in the insect world, and when it happens, it is impossible not to notice.

But those mass emergences may be coming to an end.

In a new study, biologists at the University of Iowa have concluded that periodical cicadas — species that emerge every 13 or 17 years — could be heading toward extinction. The main reason: the historical loss of old-growth forests, whose roots are the insects’ sole, essential food source during their prolonged time underground. Even when those old, intact forests are replaced by new trees, the insects are unable to adapt because they move very slowly into new habitats, the biologists found.

It’s a bit of a sad tale, admits Andrew Forbes, professor in the Department of Biology at Iowa and the study’s corresponding author.

“I think that cicadas are really cool because they have this amazing biology. But that interesting biology also has doomed them,” Forbes says. “Their viability is connected with human land use. Forests get cut down, and they regrow, but these animals are living underground for 17 years, and when you cut down the old trees, the cicadas lose their food source, they die out, and it might take them hundreds of years to get back.”

Periodical cicadas are found only in North America and are different from annual cicadas, which appear each year. There are seven periodical cicada species, four with 13-year life cycles and three with 17-year cycles, all of which originated from a common ancestor nearly 4 million years ago. Each brood spends most of its life cycle underground, tracking seasons and years, before emerging synchronously and en masse in densities that can reach up to hundreds of thousands of insects per acre.

They persist through sheer numbers. Periodical cicadas have no known defenses against predators, making them easy pickings for birds and small mammals. Their overwhelming abundance is their evolutionary trick to survive.

Forbes got interested in periodical cicadas as a graduate student. Years later, he went looking for an expected emergence in the Iowa City area but found little.

“I was really sad. I wanted to find out why that is,” he says.

The researchers scoured through historical records — newspaper articles, aerial photographs, and other sources — to identify areas where periodical cicada broods existed in the 1800s in Johnson County, home to Iowa City. The researchers learned through those historical records and accounts that much of the forests in the county had been removed by the 1930s. Previous research has shown that periodical cicadas spread about 50 yards, on average, during each emergence, meaning the species is not inclined to disperse widely.

“A new finding is that we used that distance to model cicada movement and found that it will take many centuries for them to recolonize restored forest even under the very best of conditions,” Forbes says.

Forbes also decided to try to relocate the insects, to find out whether, and how, they might adapt to a new habitat. He assembled a team, from undergraduate students to peer faculty, to collect 20,000 Brood XIII periodical cicadas when they emerged, catalog each one by sex and species, and reintroduce them over several days into a new area.

Those 20,000 cicadas didn’t last one week.

With no major defense against predators other than numbers, the vast majority were gobbled up.

“The buffet was open, and everyone knew about it,” Forbes says. “I thought if we got big enough numbers, it would work. I was wrong.”

The relocation attempt, and the fact that periodical cicadas spread minimally each time they emerge, means these species are mostly confined to areas where they already exist, with little chance to adapt or expand their range. That’s why Forbes is pessimistic about their long-term viability.

There is one ray of hope, though. If forested preserves with periodical cicada broods are left undisturbed, the species could continue.

“Those can be places where they can survive and disperse slowly,” Forbes says. “In order to expand their range, you also would need new forests to remain intact and not be cut down.”

The paper’s first author is Ian Chan, a rising senior at the University of California, Berkeley. Chan learned about Forbes’ research after his first year of college when he applied for the Summer Research Experiences for Undergraduates program at Iowa.

“It was the first program I got accepted to, and I found Professor Forbes' proposal really interesting when we chatted,” says Chan, who was especially intrigued by the conservation goals behind the cicadas’ relocation idea.

Chan contributed to many facets of the project, including collecting, storing, and relocating cicadas; using GIS and other models; analyzing the historical forest records; and helping write the manuscript.

“It was the first more serious research experience I had,” says Chan, who plans to pursue a doctorate in conservation ecology. “I learned how important it is to adapt when things don’t go as planned. Of course, I also learned a lot about cicadas and entomology as a whole from the rest of the lab.”

The study, ”Periodical cicadas suffer legacy effects of century-old forest removal,“ was published Sept. 4 in Ecology.

Contributing authors, all from Iowa, include Guerin Brown, Fabiola Castaneda-Santiago, Jacky Chitty, Tamra Elliott, Mattie Gose, Stephen Hendrix, Carter Leerhoff, MaKella Steffensen, and Christian Weinrich, from the Department of Biology; and Charlotte Hanfland, Susan Meerdink, and Adam Skibbe, from the School of Earth, Environment, and Sustainability.

The research was self-funded. Iowa provided material and institutional support.