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Year of the cicada: After 17 years, large cicada brood will emerge aboveground

This cicada hatched in 2015 as part of Brood IV. It belongs to the species Magicicada cassinii, which is similar to, but distinct from, the Magicicada septendecim species that occurs in Western North Carolina. Greg Holmes photo This cicada hatched in 2015 as part of Brood IV. It belongs to the species Magicicada cassinii, which is similar to, but distinct from, the Magicicada septendecim species that occurs in Western North Carolina. Greg Holmes photo

This spring, the eastern United States will play host to one of nature’s great marvels — periodical cicadas, mysterious insects that live underground either 13 or 17 years before emerging for a few short weeks of furious mating closely followed by mass death. 

“Being an entomologist, I especially nerd out on these things,” said Jim Costa, Ph.D., a long-time biology professor at Western Carolina University and director of the Highlands Biological Station. “It’s one of nature’s great, amazing natural phenomenon, this mass emergence and the air just alive with this constant loud droning humming sound.”

The last time Costa saw the periodical cicadas, it was 2008 and he was in Asheville. For those doing the math, that wasn’t 17 years ago — but that’s because the 17-year cicadas are divided into 12 different broods, each of which emerge on a different schedule and occupy different geographic areas. The 13-year cicadas, comprised of a separate set of species, are divided into three broods. 

Brood XIV, the one Costa saw in 2008, won’t emerge again until 2025. Its populations are concentrated along the Appalachian foothills, mainly in Tennessee and Kentucky but also including parts of North Carolina, Virginia, Ohio and West Virginia as well as a few disjointed blips in Massachusetts and Pennsylvania. 

The brood set to come out this year, Brood X, is the most widely distributed of the 17-year cicadas, with thee distinct pockets of population. The smallest of the three wraps around the southern toe of the Great Smokies region, including eastern Tennessee, a handful of counties in northern Georgia and the very western tip of North Carolina in Cherokee County. 

“They’re definitely worth people making a road trip to see, because truly these are one of nature’s magnificent spectacles,” said Costa. 

For 17 years, the insects set to appear this spring have been underground, growing little by little as they feed on tree and shrub roots. Now that it’s their year to emerge, they’re waiting for the right temperature. In the warmer parts of their range, they tunnel to the surface in late April or early May, but here in the mountains, it will be more like late May or early June. 

“Some will begin to emerge, and that probably induces others once they start calling,” said Costa. “You begin to get a few, and it rapidly ramps up.”

 

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Brood IV cicadas, also Magicicada cassinii, crowd a branch in 2015. Brood IV cicadas, also Magicicada cassinii, crowd a branch in 2015. Greg Holmes photo

 

All told, cicadas remain aboveground for four to six weeks after the first emergence. During that time, males congregate in “choruses,” usually in high, sunlit branches, where they create their infamous sound using the ridged membranes on their abdomens called tymbals. Females don’t have these sound-producing organs, so the sound serves to guide the females to the choruses of males. They visit and mate. 

Soon after mating, females split the bark of living trees and shrubs and deposit their eggs, usually between 24 and 48 at a time. However, females can mate many times during the course of an emergence and may lay up to 600 eggs before their death at the end of the emergence. The eggs remain in the trees for six to 10 weeks, at which point the juveniles hatch, drop to the ground, and burrow into the soil, where they will remain for another 17 years. 

There are about 150 species of cicadas in the United States and at least 3,000 worldwide, but the seven species in the eastern United States known as periodical cicadas are unique among them. These species — four on a 13-year cycle and three on a 17-year cycle — are the only ones that combine an extremely long nymphal stage with a synchronized mass aboveground emergence. 

Time is just as effective at separating gene pools as mountain chains or oceans.

“You can imagine there’s a strong selective pressure against getting the timing wrong,” said Costa. “A real latecomer isn’t going to have a mate, and that’s it. So if for whatever reason they didn’t emerge and there’s any genetic component to that, that’s not going to make it to the next generation.”

Stragglers do exist, however. In every brood, there are some individuals that emerge a little bit early or a little bit late. Scientists are interested in studying this phenomenon in order to better understand how common it is and how common it must become in order to eventually create a new, self-sustaining brood, or significant genetic transfer between broods.

According to the cicada lab at the University of Connecticut, stragglers are a particular issue for Brood X, the one set to emerge this year, because it occurs four years after Brood VI and four years before Brood XIV — these broods appear adjacent to each other in some parts of their ranges. 

“From a biological perspective, four-year stragglers from either of these broods are of interest because they can cause gene flow among these broods,” reads an explanation on the lab’s webpage. “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.”

For decades, the lab has sought to better understand the who, what, where and why of cicada emergence, and this time around it — as well as various other cicada researchers — is enlisting the public to help advance scientific understanding of this still-mysterious species. Because each brood emerges so rarely, it’s difficult to impossible for scientists to collect the magnitude of data they need to answer their research questions in the limited time available between the emergence and death of any given year’s adult population. 

 

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Brood X, set to emerge this spring, is shown in yellow. USFS map

 

The Cicada Safari app — created by Gene Kritsky, Ph.D., in partnership with the Center for IT Engagement at Mount St. Joseph University in Cincinnati — is now available on Google Play and the Apple Store, and anybody can use it to submit their photographs of the cicada emergence. Once the photos are verified, they’ll be posted to a live map.

“If they were being studied a century ago by a couple of people, you can imagine they can’t be everywhere at any one time, so they have these very coarse maps of the distribution of the broods, or they relay on the occasional secondhand accounts of things,” said Costa, “but the crowdsource approach is brilliant.”

Not everybody is a cicada fan. The 1.5-inch-long insects don’t bite — they don’t have teeth, after all — but they are loud during the daytime hours, though they don’t sing at night. Some of the more raucous choruses can exceed 90 decibels as perceived by somebody standing directly under the tree, similar in volume to a motorcycle from 25 feet away or a power lawnmower. Adult cicadas eat tree sap through a proboscis, and while those meals are unlikely to impart any real damage even to young trees, it may be best to cover young woody plants with bird netting or cheesecloth in order to avoid damage from females as they pierce small branches to insert their eggs. 

However, for many, the mass emergence is a short-lived but impressive spectacle worthy of awe, and of appreciation. 

“This is just amazing,” said Costa, “and we’re so fortunate to have them in our backyard. It’s well worth going to check them out.”

 

Contribute to science

To add your cicada observations to the trove of data scientists will use to better understand the periodical cicada phenomenon, download the Cicada Safari app from Apple or Google Play — and then upload your photos once the emergence begins. Learn more at cicadas.uconn.edu or www.cicadamania.com.

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