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Billions of Brood X cicadas emerging for first time in 17 years: What to know

The insects of Brood X are tunneling out of their underground homes in 15 US states, plus Washington, DC. Prepare yourself for a lot of buzzing.

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Periodical cicadas emerge from underground in the spring of their 13th or 17th year. Fifteen US states, and Washington, DC, are about to see a whole lot of them.  

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Parts of the eastern and southern United States are about to witness a remarkable natural spectacle: billions of cicadas tunneling out from underground for the first time in 17 years to fill outdoor spaces and share their loud collective mating calls. 

Periodical cicadas, as they're known, are different than annual cicadas. They spend almost their whole lives a foot or two underground, living on sap from tree roots. Then, in the spring of their 13th or 17th year, mature cicada nymphs burrow out from the ground for a short adult stage, synchronously and in huge numbers. Really huge numbers. 

A newly emerged periodical cicada sheds its exoskeleton and unfurls its wings. Soon it will be mating time. 

GIF by Leslie Katz/CNET

The insects climb up the nearest vertical surface, often the tree whose roots sustained them. They shed their exoskeletons and inflate their wings. Then, after a few days resting, recovering and waiting for their shells to harden, the mating begins. The frenzy is impossible to miss once the males start emitting their high-pitched mating song. That happens via sound-producing structures called tymbals on either side of their abdomen. 

"They may amass ... in parks, woods, neighborhoods and can seemingly be everywhere," Michigan State University entomologist Gary Parsons explained in an MSU question and answer session on the phenomenon. "When they are this abundant, they fly, land and crawl everywhere, including occasionally landing on humans." 

Here are answers to some of the main questions about this spring's big cicada emergence.

What is Brood X? 

The Eastern United States is home to six species of periodical cicadas that emerge in different years. Groups of cicadas that share the same emergence years are known as broods. 

This spring, it'll be time for members of one of the largest broods of 17-year cicadas, called Brood X or the Great Eastern Brood, to crawl out from their subterranean hideouts and show off their black bodies and bold red eyes. Expect to see all three 17-year species: Magicicada septendecim, Magicicada cassini and Magicicada septendecula.  

Which states will be impacted? 

Parts of 15 states, as well as Washington, DC, will hear the romantic serenades of males in trees, trying to attract females. The states are Delaware, Georgia, Illinois, Indiana, Kentucky, Maryland, Michigan, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia.  

When will the cicadas emerge? 

cicadasafari

Through the app Cicada Safari, citizen scientists can help map the insects. 

Cicada Safari

The return of the cicadas typically starts around early- to mid-May (though it could come earlier) and runs through late June. It is, needless to say, a quite sight. Some people view the mass of insects as a pesky annoyance, but others welcome it as an awe-inspiring wonder of nature. Some in the latter category even regularly travel around the US to cicada emergence areas to experience the sights and sounds and help scientists map the creatures. 

A free app created at Cincinnati's Mount St. Joseph University, called Cicada Safari and available for iOS and Android, lets the cicada-curious record periodical-cicada sightings. They can also record sightings at the websites Cicada Mania and iNaturalist, a joint initiative of the California Academy of Sciences and the National Geographic Society. Cicada mapping helps scientists verify the periodical insects' life cycles, as well as broods' relationships to one another, to gain a better understanding of biodiversity, biogeography and behavior and ecology

Because Brood X occurs four years after Brood VI and four years before Brood XIV and because the three broods are adjacent to one another in parts of their geographic ranges, cicada trackers may spot "stragglers" from other broods this year.

"From a biological perspective, four-year stragglers from either of these broods are of interest because they can cause gene flow among these broods," the University of Connecticut explains. "From a practical perspective, four-year stragglers from any of these broods complicate mapping efforts, because populations may be difficult to assign to a brood."  

Stragglers may confuse mapping efforts, but the university stresses that a "misleading map is worse than no map at all."

Why do so many come out at once? 

It's thought that by emerging in such huge numbers, enough of them can live on to mate -- basically, strength in numbers. 

The cicadas typically begin to come out when soil temperatures 8 inches (20 centimeters) underground reach 64 degrees Fahrenheit (18 degrees Celsius). "That seems to be the trigger that causes them all to emerge over a few days or weeks in one area," Parson says. A warm rain often triggers their emergence. 

Because periodical cicadas are sensitive to climate, patterns of different broods and species reflect climatic shifts, note John Cooley and Chris Simon, professors of ecology and evolutionary biology at the University of Connecticut. 

"For example, genetic and other data from our work indicate that the 13-year species Magicicada neotredecim, which is found in the upper Mississippi Valley, formed shortly after the last glaciation," they write in a piece for The Conversation. "As the environment warmed, 17-year cicadas in the area emerged successively, generation after generation, after 13 years underground until they were permanently shifted to a 13-year cycle."

What does the male mating call sound like? 

It varies by species, but it can sound like a high-pitched electric buzz, a chirp or a rattle. The calls of a group of males can reach 90 decibels or more, about the same level as a motorcycle at 25 feet (about 8 meters) away. Fast-forward to 2:14 in Sir David Attenborough's video above to get a good listen. The females respond to the males' come-hither calls by clicking their wings, and all the back and forth makes for a distinctive symphony.

Can cicadas hurt me? 

Nope, the insects are harmless. They don't sting, bite or carry diseases, and they typically don't come indoors, though they do gather on outside walls. 

"The only way they could get inside is accidentally flying in through an open door or window, or because they had landed on a person who then carried them inside unnoticed," Parsons says.  

During dense emergences, females can lay enough eggs in branches to damage trees, however. Here's more on cicadas and your yard. And here's everything you need to know about cicadas and pet safety

How long will Brood X stick around? 

The mass mating lasts at least three to four weeks. Soon after, the newly hatched nymphs will crawl to the edge of the tree branches where the females laid their eggs, drop to the ground and burrow in. And so the cycle begins again. Godspeed, Brood X.