It’s a swarm 17 years in the making.

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After 17 years in waiting, billions of periodical cicadas are set to emerge from the ground en masse next month and blanket a large portion of the eastern United States.  

Members of Brood X will be in 15 states, concentrated in Maryland, Indiana, and Pennsylvania, with scientists predicting that some places will have more than a million cicadas per acre—that's more than 25 or 30 per square foot. And that's on top of the annual cicadas that come out every summer!

17 Year Cicada
Credit: WerksMedia/Getty Images

The epicenter of the cicada explosion will be Washington, DC., where huge concentrations are expected to appear simultaneously in mid- to late May. (You can find a population map here.)

Unlike the green cicadas that pop up in some parts of the country every year, these guys have black bodies, clear wings, bold red eyes, and a particularly cacophonous song.

"There are three species of 13-year cicadas and three species of 17-year cicadas," Judy Black, vice president of quality assurance and technical services at Orkin, LLC., explained to Prevention. "The 17-year cicadas are generally found in the North and the 13-year cicadas are generally found in the South, but there is a great deal of overlap. Both types of periodical cicadas could appear in an area at the same time but would only emerge together once every 221 years."

The lives of Brood X members began in 2004 when they hatched as tiny, translucent nymphs no bigger than a grain of rice. They burrowed into the ground where they have been molting, growing, and rummaging around for the better part of two decades.

At some point in May and June—when the soil temperature about a foot below ground reaches about 64 degrees—the nymphs will emerge and begin climbing whatever vertical surface they can find. They will then molt one last time and become soft, white adults.

The adult cicadas will spend the next two to four weeks courting, mating, flying, and driving people bananas.

According to The Washington Post, the chorus of a bunch of Brood X males can reach 105 decibels—louder than a lawn mower.

After weeks of noise, the surviving adult females will lay their eggs in trees, which will hatch six to 10 weeks later and start the whole cycle all over again.

While they can be menaces, cicadas are basically harmless. Experts urge homeowners to be patient and resist the urge to kill them.  

"The mass emergence of periodical cicadas is a phenomenon that is incredible to witness," Black told Prevention. "If you attempt to kill them, you are robbing future generations of witnessing these emergence events."