Cicada Rythm

2021 marks the emergence of the periodical cicada in the Tennessee Valley —a phenomenon that happens only every 17 years. VIDEO: Here’s what nature lovers can look for.

MAY 10, 2021 — Burrowed up to eight feet below the ground’s surface — and only emerging every 17 years — lives the scientific phenomenon known as the periodical cicada. 2021 is the year for you to observe them in one of the planet’s largest insect emergences since 2004. 

According to TVA Zoologist Jesse Troxler, the soon-to-emerge cicadas are part of a group called Brood X (10), or the Great Eastern Brood, and are expected to appear in early- to mid-May. They will be hard to miss, as Brood X will emerge in loud choruses throughout a geographical range that stretches from Georgia all the way up to New York. In the south, Brood X is limited to the eastern valley (to see a map of the various broods and their locations, click here).

“Eastern North America is the only place in the world that has periodical cicadas,” Troxler says. “Scientists hypothesize that their life cycle is an adaptation to climate cooling during the ice ages.”

Just like their predecessors, this brood of cicadas has been subterranean for 17 years, growing, tunneling and feeding on the fluids from grass and tree roots beneath the soil since their hatching. Troxler said that they’re now waiting for the soil to reach 64 degrees Fahrenheit before they make their mass ascent.

“It’s hard to predict exact numbers from Brood X, but we’re expecting billions to emerge,” he says. “Their 17-year cycle helps them avoid interactions with the plethora of local predators, including birds, raccoons, fish and domesticated animals. Not all of them will survive, but their excessive numbers ensure that many will have the chance to reproduce and allow their species to continue thriving.”  

Though they survive for nearly two decades underground, the cicada’s lifespan lasts only four to six weeks after emergence. This is why they immediately molt their adolescent skin and crawl or fly up into tall trees to reproduce. The males seek each other out to begin a chorus of mating calls to attract the females. Once a female is mated, she can lay up to 600 eggs — which later hatch and fall from the trees to burrow into the soil again.

Managing Their Habitat

As part of Brood X’s limited habitat, the Valley’s natural resources play a major role in the survival and success of the species — especially the forests that TVA helps to protect and manage.

Not only are the trees that cicadas seek protected under TVA’s authority, but TVA also assesses and prevents invasive exotics like Kudzu from choking out plant diversity and killing large, established trees. This is what Troxler says helps the cicadas to flourish in our area.

“The cicadas rely on local plant and tree roots for food and ultimately our forest trees’ height for singing, breeding and laying their eggs,” he says. “Our work throughout the forests and along the reservoirs to control invasive exotics, such as prescribed burns and removal projects, helps to protect their habitat. TVA works hard to encourage healthy relationships between all our Valley species, and anything that’s good for our local forests is good for the cicadas.”

This relationship is also beneficial for the trees, as studies show a significant increase in tree growth after the cicadas die and fertilize the soil beneath them. Besides providing immediate nourishment for the local animal populations, this fertilization supports the health of the ecosystem.

“It’s hard to fully determine the effects that a 17-year emerging species can have on the environment,” says Troxler. “Most benefiting species, whether animal or plant, have adapted to not depend on these cicadas, but see them more as opportunistic nourishment. However, who knows what would happen if they suddenly stopped emerging.” 

What to Watch for

For those wanting an up-close experience of the cicada emergence, here are some things to watch for:

  • There are multiple species of cicadas throughout the Valley — 17-year (Brood X) cicadas are black and orange with red eyes.
  • While searching for them in your back yard or on TVA lands/trails, look for finger-sized holes in the ground near the base of trees, chimney-like tunnels poking out of the soil or cicada shells on/near the bases of the trees.
  • Listen for their loud mating calls — cicadas have been known to reach 96 decibels and drown out the sounds of passenger jets!

“Not everyone will find them in their back yard, and the locations of their emergences are hard to predict,” Troxler says. “So, I recommend that you drive or walk until you hear them — trust me, you will hear them — and then you can look for signs to observe them.”

Though the mass emergence of these red-eyed insects may seem apocalyptic, it’s nothing to be alarmed about. Troxler said that they’re safe for both humans and animals to interact with… and even eat. 

“Brood X’s last emergence was in 2004 and it won’t happen again until 2038,” he says. “This is a rare opportunity for you and your family to experience, and TVA lands are