The swarm is coming.

A massive brood of cicadas is expected to emerge from the ground this spring after its 17-year infancy, and Southwestern Pennsylvania will be right in the center of it.

“In some areas, you can get as many as a million and a half individuals per acre, when they’re really dense,” said Bob Davidson, invertebrate zoology collection manager for the Carnegie Museum of Natural History.

Cicada nymphs wait underground for years, feeding on nutrients in tree roots. When they turn 17, they wait for temperatures to hit a comfy 64 degrees before crawling up and attaching themselves to the nearest tree while they wait to grow wings. Related story: Pennsylvania is home to 5 cicada broods

Global warming has made it tricky to predict exact timing, but a cicada swarm typically starts in May and is over by late June, Davidson said.

There’s nothing to fear, said Sandy Feather, horticulture educator with Penn State Extension in Allegheny County. Cicadas are noisy and numerous, but not harmful. They don’t eat crops — or anything else for that matter.

“I know they horrify people, but I would encourage people to realize they don’t bite, they don’t sting,” Feather said.

Adult cicadas exist only to mate, then die.

A male cicada acts much like a human teenager who joins a band to try to get girls. They create “chorus groups,” hanging out together on a tree branch and “singing” to attract females, Davidson said.

“These giant chorus groups of males will call, and when the female is attracted, she’ll fly into the chorus group,” he said.

A cicada “sings” by vibrating a specialized membrane in its abdomen called a tymbal.

Female cicadas drill holes in live twigs, where they can lay up to 600 eggs. This often kills the twig, but doesn’t cause any lasting damage, said Joe Stavish, education coordinator for nonprofit group Tree Pittsburgh.

“We’re going to get a ton of phone calls this year,” he said.

Holes cicadas make can cut off the water supply to the tips of branches, causing healthy leaves to die.

“Middle of the summer, a lot of nice green trees are going to look brown,” he said.

The trees will be fine, and tree lovers should be happy to see the cicada swarm, according to Stavish.

“We really should celebrate it because when we’re thinking about trees, one of the things trees do is provide habitats for insects and animals,” he said.

Cicadas can hurt very young trees, however. The insects can be warded off by wrapping a tree in garden fabric, Feather said.

When nymphs are born, they drop from tree branches, dig underground and begin the long wait for the cycle to start again.

Scientists have long been puzzled by the unique lives of periodic cicadas, which are unique to North America, Davidson said. Most insects have much shorter life cycles.

“Because it’s so odd, it’s attracted a lot of attention by evolutionary biologists who try to figure out why,” Davidson said.

Cicadas have many natural predators — birds, frogs and small mammals. And when a swarm of cicadas emerges, these creatures eat well.

The insects ensure their survival through sheer force of numbers, providing more food than their predators can stomach.

However, if they did this every year, cicadas would have a problem, Davidson said. Predator populations would skyrocket thanks to the yearly all-you-can-eat buffet.

But by waiting underground for 17 years at a time, cicadas can outlast predator population cycles.

Every time there’s a cicada swarm, bird populations spike for a year or two afterward thanks to the abundance of food, then crash back to normal levels, according to Davidson.

“By coming out once every 17 years, they basically break the cycle,” he said.

The brood that will emerge this year contains three separate cicada species. It’s difficult for the untrained eye to tell them apart — the only physical differences are slightly different markings on their abdomen. But each has a distinctive call.

Once their descendants return underground, they will not be seen again until 2036.

The swarm grosses people out, but it provides people a look at one of nature’s rarities, Feather said.

“Yeah there’s a lot of them, but they’re unique in the world, so try to have some appreciation,” she said.

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