East Tennessee cicadas expected in 2021, but stragglers could show in 2020

East Tennessee hasn’t seen a brood of periodical cicadas in almost a decade and is still a year away from a full flood of the chirping, flying insects.

But University of Tennessee professor Dr. Jerome Grant already is giddy for the cyclical occurrence. 

“Every time an emergence starts to get close, you get excited about it,” Grant said. “You don’t know what to expect.”

Here is what to know about the brood expected in 2021, the possibility of seeing periodical cicadas a year early and how to spot them:

Why the fascination?

Grant suggested three reasons that people are fascinated by cicadas, including the sound. Male cicadas make a sound Grant described as “high and shrill” chirping for the purpose of attracting females to mate.

Another reason is the mass number of cicadas. 

“You will have either the joy of a lifetime or one of the most terrifying experiences you have ever had. There are so many — hundreds of thousands of these insects coming out within a couple of weeks.”

The third reason Grant posed is the lifespan of the cicada. The most common periodical cicadas spend almost 17 years underground before emerging.

“They have the longest life history of any insect we have in North America,” Grant said.

Adults live four to five weeks after surfacing. The swarms serve as a food source for many creatures.

“They don’t cause much of a problem unless you are a nursery grower or trying to grow landscape trees,” Grant said.

East Tennessee broods and timelines

East Tennessee has three broods of periodical cicadas — two of the 13-year variety and one of the 17-year group, according to UT's Department of Entomology and Plant Pathology.

Brood X, the lone 17-year variety, is the cicada expected to re-emerge in 2021. It was seen in 2004 and 1987. 

Brood XIX and Brood XXIII are on a 13-year cycle. Brood XIX is expected to arrive again in 2024, while Brood XXIII is not due until 2028.

Could East Tennessee see periodical cicadas early?

Periodical cicadas have been known to appear in limited numbers anywhere from one to four years early.

“They are stragglers who developed a little bit early, but that is not the mass emergence of the population,” Grant said. “That is a small few individuals.”

Grant said the reason for the early emergence likely is physiological, as broods emerge due to weather.

“It could trick them into thinking it is time to emerge,” Grant said. “Every year, you have a few random stragglers.”

An app called Cicada Safari helps track the appearance of periodical cicadas. User-submitted data shows a handful of periodical cicadas in East Tennessee.

What is the difference in cicada types?

There are the cicada broods that appear every 13 or 17 years as well as annual cicadas.

The types range in appearance, as annual cicadas are black and green, while periodical cicadas are vibrantly colored.

“Periodical cicadas, appropriately for this area, have orange-colored eyes and legs,” Grant said. “They just stick out big-time.”

Periodical cicadas are smaller than annual cicadas, which are commonly called dog-day cicadas. The former is an inch to an inch-and-a-half, while the latter can be up to two-and-a-half inches.

The different types also emerge at different times of year. Periodical cicadas come out in May, while annual cicadas come out later in the year. 

Where are periodical cicadas this year?

Brood IX emerged this year in North Carolina, Virginia, and West Virginia, according to CicadaMania.

“It is an interesting insect and an interesting concept that they can have so many broods that can emerge in different areas,” Grant said.

The website states that periodical cicadas “begin to emerge when the soil 8 inches beneath the ground reaches 64 degrees.”

Mike Wilson covers University of Tennessee athletics. Email him at michael.wilson@knoxnews.com and follow him on Twitter @ByMikeWilson. If you enjoy Mike’s coverage, consider a digital subscription that will allow you access to all of it.