Periodical Cicadas Could Struggle to Survive After Forest Loss

The mass appearances of periodical cicadas may be far less secure than their spectacular numbers suggest. According to a study by University of Iowa biologists, the insects could be heading toward extinction in some areas because the old forests supporting them disappeared long before the cicadas could recover. Periodical cicadas emerge only every 13 or 17 years, unlike annual cicadas. The seven North American species spend most of their lives underground, where their roots provide their sole essential food source. When they emerge, they rely on overwhelming numbers for protection because they have no known defenses against predators. The study, published in Ecology, examined historical records, newspaper accounts and aerial photographs from Johnson County, Iowa. The researchers found that much of the county’s forest had been removed by the 1930s. Although trees can grow back, the cicadas do not quickly follow: previous research indicates that they spread an average of about 50 yards, or 46 meters, during each emergence. Using that movement rate, the researchers modeled how long it might take cicadas to return to restored forest. Even under the best conditions, recolonization could require many centuries, according to the study’s authors. Their findings suggest that the insects remain largely restricted to places where established broods already survive. The researchers also tested whether relocating cicadas could help establish them in a new habitat. They collected, identified and moved 20,000 Brood XIII cicadas over several days. The relocated insects did not survive a week, with predators consuming most of them. The result indicated that moving the insects away from their existing range may be difficult because their survival strategy depends on large concentrations emerging together. The study does not establish that all periodical cicadas will become extinct. It identifies long-term vulnerability linked to forest removal and the insects’ limited dispersal. The researchers said protected forest preserves containing existing broods could provide places where populations continue to survive and gradually spread. For that expansion to occur, the study indicates, newly forested areas would also need to remain intact rather than being cleared again.