Periodical cicadas, the insects whose mass emergences every 13 or 17 years turn entire forests orange-eyed and deafening, may be running out of time. A new study from biologists at the University of Iowa concludes the insects could be heading toward extinction, and the researchers trace the cause back a century, to the old-growth forests that were cleared for farmland and timber long before anyone was tracking the cicadas that depended on them. The forests have since grown back in many places, but the cicadas have not, and the research team argues they may never manage it on their own.
Seven Species, Two Life Cycles, One Shared Vulnerability
Periodical cicadas exist only in North America and are distinct from the annual cicadas that emerge every summer. There are seven periodical species in total, four that emerge on a 13-year cycle and three on a 17-year cycle, all descended from a common ancestor roughly 4 million years ago. Each brood spends nearly its entire life underground, feeding on fluid drawn from tree roots, before surfacing synchronously in densities that can reach hundreds of thousands of insects per acre. That overwhelming abundance is the species’ only real defense: individual cicadas have no sting, no camouflage and no speed, so they survive by emerging in numbers no predator population can eat through. Andrew Forbes, a professor in the University of Iowa’s Department of Biology and the study’s corresponding author, says that same biology is what makes the insects fragile. Their multi-year underground development ties their fate directly to whatever happens to the forest above them for the entire span between emergences. Mass emergences also do more than put on a show: they briefly flood forests with an abundant, easy food source for birds and small mammals, and the insects’ own decomposing bodies send a pulse of nutrients into the soil that researchers say helps shape the ecosystem around them for months afterward.
Reading a County’s Tree Cover From the 1930s
Forbes became interested in the problem after returning to the Iowa City area expecting a cicada emergence and finding almost nothing. To figure out why, his team combed through historical records, including old newspaper articles and aerial photographs, to map where periodical cicada broods had lived in Johnson County, Iowa, back in the 1800s. Those records showed that most of the county’s original forest had already been cleared by the 1930s. When the researchers compared where cicadas actually emerged during the 2024 brood event to two different maps of the county’s tree cover, the distribution matched the 1930s forest map far more closely than it matched current tree cover. In other words, decades of regrowth have not brought the cicadas back to areas they lost nearly a century ago.
A Relocation Experiment That Failed in a Week
To test whether the insects could be helped into new territory, Forbes assembled a team of students and colleagues to attempt a direct relocation. They collected roughly 20,000 Brood XIII periodical cicadas as they emerged in 2024, catalogued each one by sex and species, and released them into a new wooded area in Johnson County that appeared suitable but lacked an existing population. The transplanted cicadas did not survive to reproduce. Within about a week, birds and other predators had eaten the overwhelming majority of them, since a freshly introduced population lacks the sheer numbers that normally keep predation losses survivable. Forbes has described the outcome bluntly: the buffet was open, and every predator in the area knew about it. He had expected that 20,000 insects would be enough of a crowd to overwhelm local predators the way a natural emergence does, and the experiment convinced him that raw scale alone cannot substitute for an already-established, long-running brood.
Why Cicadas Can’t Simply Move Back
Prior research cited in the study found that periodical cicadas disperse only about 50 yards from where they emerged during each cycle, meaning a brood’s range expands at a glacial pace even under ideal conditions. Using that dispersal rate, the Iowa team modeled how long it would take a surviving population to recolonize forest that has regrown near an existing brood, and found the process would take many centuries even in the best-case scenario. Because a cicada brood only gets one shot at expanding every 13 or 17 years, and because each attempt depends on there being intact, root-rich forest immediately adjacent to where the brood already lives, the margin for error is unusually thin for an insect that otherwise seems abundant beyond counting. A forest cleared once, even if it is allowed to regrow decades later, effectively resets that clock, since the cicadas that would have recolonized it were wiped out long before the new trees matured enough to support them.
Old-Growth Preserves as the Last Refuge
The study, titled “Periodical cicadas suffer legacy effects of century-old forest removal,” was published September 4, 2026, in the journal Ecology, with undergraduate researcher Ian Chan as first author. The University of Iowa’s own account of the findings notes that Forbes sees one realistic path forward: protecting the forested preserves where periodical broods still exist today, since those populations can persist and spread slowly as long as the surrounding old growth is never cut down again. New forest alone will not solve the problem, in his view, because it takes centuries for cicadas to reclaim ground they lost, and most conservation planning does not operate on that timescale. The finding reframes an insect known mostly for showing up briefly and unmistakably every decade or so as a species whose long-term survival is now tied to forestry decisions made generations before any given emergence.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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