Millions of cicadas will soon emerge in the U.S. Here’s why.

Each year, warm weather in North America brings the familiar buzzing and clicking of cicadas that have surfaced from their underground burrows in search of mates. Once every decade or so, though, that cacophony turns deafening as millions or more of the winged insects emerge at once in dense throngs. They stick around for about a month, and then they die.

This spring, a group of cicadas known as Brood X is expected to emerge throughout the mid-Atlantic for the first time since 2004. It is among the largest of the cicada broods with a 17-year life cycle, numbering in the hundreds of billions.

Of the 3,000 species of cicadas around the world, only seven species share synchronized life cycles that allow them to come out simultaneously every 13 or 17 years. These periodical cicadas, as they’re known, are only found in the central and eastern U.S.

This mass emergence inflicts some damage on trees, but cicadas don’t devour crops like locusts do. They instead contribute to their ecosystem by pruning weak branches, releasing nutrients back into the soil when they die, and serving as an abundant food source for birds and other animals.

“Periodical cicadas are one of these insects where everything about them is amazing and unusual,” says Andrew Liebhold, a research entomologist with the U.S. Forest Service.

But why have these cicadas developed such a unique life cycle, and why does it happen only in the central and eastern U.S.? Here’s what we know—and what scientists are still trying to sort out.

Cicadas in sync

All cicadas start their lives in the same way: Females lay eggs in the branches of trees and, after six to 10 weeks, those eggs hatch and the nymphs fall to the ground, digging burrows. They spend most of their lives in these burrows, sucking the liquids of plant roots and molting their shells five times before surfacing as adults. Males call out for mates, which respond by flicking their wings. Then, within four to six weeks, they die.

How long it takes for the nymphs to mature, though, varies among cicada species, which are split into roughly two categories: annual and periodical.

Most cicada species are considered annual cicadas—though the term is a bit of a misnomer, as these insects live longer than just a year. Their life span, which is around two to five years, depends on how long it takes for them to reach a mature size and weight. These cicadas do not have synchronized life cycles, so they emerge at different times each summer.

Periodical cicadas are the insects that make headlines by appearing en masse. Members of the genus Magicicada, these cicadas include four species that live for 13 years and three that live for 17 years.

John Cooley, an entomologist at the University of Connecticut, says their relatively long life cycles aren’t what distinguish periodical cicadas from annuals. Rather, it’s the fact that they reach adulthood at roughly the same speed and emerge in synchrony at regular intervals.

Periodical cicadas do so by counting the seasonal pulses of fluid flowing in roots from which they feed. “They don’t keep track of time, they just count [plant] cycles,” says Cooley, whose periodical cicada mapping project was initially funded by the National Geographic Society.

Once plants have completed 13 or 17 cycles, the nymphs wait until the soil gets to the right temperature—around 65°F (18°C)—to take to the skies.

Safety in numbers

Given their synchronized and lengthy life spans, you might expect periodical cicada sightings to be relatively rare. But these insects come out in droves almost every year—just in different parts of the country. That’s because they live in 15 geographically defined broods.

Most of these broods are composed of multiple species of periodical cicadas, but only those with a common life span: Twelve broods follow 17-year life cycles, while the remaining three broods have 13-year life cycles. Cicadas in a brood are synchronized with one another—but not with the cicadas in the next brood over.

Each brood has its own range. Some are confined to small regions, such as brood 7, which is only found in upstate New York; while others are scattered across swaths of the country, such as brood 19, which is found in the Midwest and along the East Coast from Maryland to Georgia. Some broods overlap, meaning two broods might even live in the same city.

Periodical cicadas' synchronized schedules boost their survival. Unlike annual cicadas, which know to quickly flee from predators like birds and moles, periodical species haven’t evolved to develop evasive maneuvers. “When they come out in low densities, they get mowed down,” Cooley says.

For them, safety is in numbers. Their broods are so dense—as many as 1.5 million may crowd a single acre—that the risk to any individual cicada approaches zero.

Unanswered questions

But why have these particular U.S. species evolved to have such long, prime-numbered life spans?

“That’s one of the big unanswered questions,” Cooley says. “It’s got to be something pretty special because it’s rare.”

One common theory has to do with avoiding predators. Long life cycles—and particularly those that are prime numbered—are difficult for predators to match, making it impossible for them to specialize in eating cicadas.

But Cooley says this theory is flawed: Cicadas don’t have 11- or 19-year cycles, yet these are prime numbers, too. Periodical cicadas also have plenty of opportunistic predators, and there’s even a fungus that specializes in killing them.

Other studies have targeted glacial cycles. Cicadas prefer warm weather, the theory goes, so perhaps the extreme cold of the last ice age favored the evolution of a longer life cycle. But Cooley questions this theory, too, as glaciation would have affected all cicada species and not just those in the United States.

Liebhold, the research entomologist with the U.S. Forest Service, agrees that it’s hard to know why cicadas behave so differently in the eastern half of the United States. He speculates it could be related to the region's forests, which are known for a high diversity of plants and insects.

“If periodical cicadas were going to evolve anywhere, the odds were pretty good they would evolve here,” Liebhold says. But, he adds, it could be purely chance that these species slipped into an unusual life cycle.

Future of cicadas

Scientists are trying to unlock other cicada mysteries, too.

Sometimes, periodical cicadas get confused. Known as “stragglers,” these individuals can emerge either a little bit early or late—either one year or four years in either direction.

Gene Kritsky, dean of behavioral and natural sciences at Mount St. Joseph University, says the brood patterns suggest some “stragglers” may emerge in big enough numbers to reproduce which, over many life cycles, could ultimately result in the formation of a new brood.

Kritsky, who discovered one of the 13-year broods, believes cicadas have probably been exhibiting these behaviors all along, but scientists had limited means to document it in the past. Now scientists can use mapping technology—and can cast a wider net with the help of citizen scientists reporting sightings through Mount St. Joseph’s Cicada Safari app.

“What we’re seeing, I think, is how cicada broods evolve,” he says. “We have to figure out, How does that fit into this bigger picture?”