Periodical Cicadas Move About 50 Yards Every 17 Years. That Arithmetic Is Why They May Not Come Back.

Periodical cicadas spend thirteen or seventeen years underground drinking sap from tree roots, then emerge in numbers that can reach hundreds of thousands per acre, mate, and die. The strategy works by overwhelming every predator in the neighborhood at once. A study published in the journal Ecology finds that the same strategy has left the insects almost incapable of recovering from the loss of the forests they depend on.

The researchers, led by Andrew Forbes, a professor of biology at the University of Iowa, with rising University of California, Berkeley senior Ian Chan as first author, measured how far a population spreads with each emergence. The answer is roughly 50 yards, about 46 meters. Every seventeen years. Feed that into a model of forest regrowth and the conclusion is arithmetic rather than speculation: it will take many centuries for the insects to recolonize restored forest even under the very best of conditions.

"Their interesting biology has also doomed them," Forbes said. The synchronization that protects a mature brood also means there is no dispersal between cycles, no wandering individuals to seed a new patch of woods. When the trees go, the whole local population goes with them. "Forests get cut down, these animals die out, and it might take them hundreds of years to get back," Forbes said.

The team tested whether that gap could be closed by hand. They collected 20,000 cicadas from Brood XIII and released them into suitable habitat, reasoning that a large enough founding group would swamp local predators the way a natural emergence does. Every one of them was gone inside a week. Birds, raccoons and everything else in the area simply ate them.

"The buffet was open, and everyone knew about it," Forbes said. "I thought if we got big enough numbers, it would work. I was wrong." Twenty thousand insects sounds enormous until it is set against a natural emergence, where a single acre can produce ten times that.

There are seven periodical cicada species, four on thirteen-year cycles and three on seventeen-year cycles, all descended from a common ancestor roughly four million years ago, and all of them live only in North America. The old-growth forest that supported them across the eastern United States was cut down over the nineteenth and twentieth centuries. Second-growth woodland has returned across much of that range, but the cicadas have not, and at 50 yards per generation they will not arrive on any schedule a conservation program can plan around.

The practical implication is that the maps of surviving broods are close to a permanent inventory. What is there can be protected. What is gone is not coming back through replanting.