17-year cicadas are coming already? Here's why

If cicadas emerge from the ground only once every 17 years, why do they seem to show up so often?

There are actually 15 separate broods of cicadas in the United States — five of which include Pennsylvania as part of their territory.

Related story: Cicadas will swarm Southwestern Pa. this spring

Most of these broods come out every 17 years, though three exist on a 13-year timer.

The last time we saw the insects was in 2016, when Brood V emerged. Brood V cicadas are mostly concentrated in Ohio and West Virginia, but Southwestern Pennsylvania is on the edge of the swarm.

Brood VIII will arrive this spring, concentrated almost entirely in this region.

“I call it our brood,” said Bob Davidson, invertebrate zoology collection manager for the Carnegie Museum of Natural History. “It’s pretty much just localized in the Allegheny plateau.”

After millions of cicadas emerge, mate and die off, their offspring will burrow in the ground and not return until 2036.

Southwestern Pennsylvania will get a cicada break for a while, though we might see some activity from Brood X in 2021 and Brood XIV in 2025.

Not all cicadas belong to broods. Many cicada species have a life cycle like other insects, without the long years underground.

“We have plenty of cicadas that are basically a normal insect,” Davidson said.

That’s why you can here the distinctive chirping of cicadas every summer, not just in swarm years.

Unlike their brood-based cousins, non-periodic cicadas aren’t likely to be joined by millions of their closest friends and family.