Iowa Study: Cicadas May Take Centuries to Return to Recovered Forests

Johnson County's forests have grown back over the last century, but the periodical cicadas that once filled them have not. New research from the University of Iowa finds that natural recolonization could take centuries, if it happens at all.

Periodical cicadas spend 13 or 17 years underground feeding on tree root fluids before emerging by the trillions for a few loud weeks. When forests are cleared, the nymphs die with the trees, even years later. A forest can regrow within a human lifetime, but a cicada population that loses its trees may take centuries to return.

What the Johnson County data shows

Surveyors mapping Johnson County between 1836 and 1859 recorded trees across nearly 180 square miles, about 29% of the land. By 1930, farms, pastures, and the growing city of Iowa City had cut that to about 70 square miles, just 11.2%.

Old newspapers from 1871, 1888, and 1905 describe cicada swarms around what is now downtown Iowa City. After that, mentions near town stop, a sign the local population had already retreated.

During the 2024 emergence, researchers walked out to 301 wooded sites and listened for the insects' chorus. They heard it at 116 of them. When they compared that map against tree cover from three eras, the best match wasn't the newest map. It was 1930.

Why regrown forests still lack cicadas

Tree cover in the county has grown since 1930, reaching about 80 square miles by 2023. But only about 25 square miles have stayed forested the entire time. Much of today's recovered forest grew back after being cleared decades earlier.

The researchers set a benchmark: a site with at least 17.4% continuous forest cover nearby had a 90% chance of holding cicadas in 2024. The model matched 110 of the 116 sites that actually had cicadas, but 137 other sites met the same bar and still had no cicadas at all.

Growing a forest back isn't the same as not having lost it in the first place, and cicadas can tell the difference.

Relocation efforts failed in Iowa City

Andrew Forbes, a biology professor at the University of Iowa, wanted to know whether cicadas could be helped along. During the 2024 emergence, his team collected 20,126 adult cicadas from around the county and released them over 10 days at Pappy Dickens Preserve, a forest inside Iowa City with enough tree cover to qualify as good habitat.

The nearest wild population was about 2.4 miles away, too far to close naturally since cicadas manage only about 55 yards a generation. It didn't work. Birds and small mammals converged on the release site and picked off the cicadas as fast as they arrived.

By the last release day, no cicadas remained there. None sang, mated, or laid eggs.

“The buffet was open, and everyone knew about it,” Forbes said.

More than 80 bird species eat periodical cicadas. The insects normally survive by overwhelming predators with sheer numbers in one place at one time. Move a smaller crowd somewhere new, and that safety in numbers disappears.

Recovery could take centuries

Forbes's team modeled the forest's turnover: roughly 12% lost and 15% gained every generation. Cicada habitat shrinks under nearly every scenario tested. Even at the longest dispersal on record, about 163 yards a generation, the range barely holds.

Growing the range back would take even more. Over 95% of the forest cicadas occupy would need to stay untouched indefinitely. Even in that best case, expanding by a mile or two would take centuries. Along a fully forested corridor, natural dispersal wouldn't reach Pappy Dickens Preserve until somewhere between 2466 and 3316, the model estimates.

That outlasts most conservation programs.

What this means for Iowa's cicadas

Two cicada broods have already disappeared, one from New England and one from the Florida Panhandle. The study compares the situation to three other animals once wildly abundant and now extinct: the passenger pigeon, the Carolina parakeet, and the Rocky Mountain locust.

Whether the remaining broods follow them is still an open question. No one has run this model outside Johnson County to check. For now, the best hope is protecting the forest that's already there.

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

The full study was published in the journal Ecology.