Something incredible is happening right now that takes place just once every 17 years: Brood X cicada nymphs are wriggling out of the soil to climb the nearest trees, where they will transform into red-eyed, black-bodied adult cicadas by the billions.
If you live in the eastern United States, you're in luck. Brood X (the "X" stands for the number 10, as this brood is one of 12 cicada populations that emerge in 17-year cycles) is also one of the most geographically widespread broods. The 2021 emergence, which begins mid-May and lasts into mid-June, spans the District of Columbia and 15 states: Delaware, Georgia, Illinois, Indiana, Kentucky, Maryland, Michigan, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia, according to the website Cicada Mania.
Brood X cicadas, which are also known as the Great Eastern Brood, can emerge in densities up to 1.5 million cicadas per acre, which means that the total number of insects in this brood may climb into the trillions, the Cincinnati Enquirer reported.
Related: What are Brood X cicadas?
Cicada emergence is happening a little later than usual this year because of the unusually cool spring weather, said Chris Simon, a periodical-cicada researcher and a professor of ecology and evolutionary biology at the University of Connecticut in Storrs.
In the second week of May, Simon recorded sightings of Brood X in North Carolina and northern Georgia, where cicadas "had been out for a week and starting to chorus in the treetops at lower elevations, but [are] not yet out in the higher mountains," she told Live Science in an email. "Because of cool rainy weather, the cicadas are coming out over a longer period and are not noticed in many places," she added.
The first signs of Brood X cicadas are holes in the ground near the bases of deciduous trees (those that shed their leaves annually) where the long-buried nymphs are preparing to emerge. They're waiting for the soil temperature to reach 64 degrees Fahrenheit (18 degrees Celsius) to a depth of about 8 inches (20 centimeters) below the surface, according to Cicada Mania. Once the nymphs start to come out and transform into adults, you may also find trees that are littered with piles of discarded, hollow exoskeletons, which are left behind when the nymphs metamorphose into adults.
During metamorphosis, "basically the back of their skeleton splits down the middle and the adult crawls out," said George Hamilton, chair of the Department of Entomology and director of the graduate program in entomology at Rutgers University in New Brunswick, New Jersey. Adult cicadas are pale and squishy when they emerge from their nymph shells, but after a couple of hours their wings expand, their bodies harden and turn black, "and when that happens they can fly into the trees and do their thing," Hamilton told Live Science.
While Brood X will appear in counties across more than a dozen states, they will be most numerous and emerge nearly statewide in Indiana and Maryland, said Gene Kritsky, periodical-cicada expert and dean at the School of Behavioral and Natural Sciences at Mount St. Joseph University in Cincinnati, Ohio.
"Populations are likely to be densest in the D.C.-Baltimore area (but patchy)," Simon said in the email.
Western Ohio will also see dense swarms of Brood X, as will southeastern Pennsylvania, western North Carolina and eastern Tennessee. Parts of New Jersey, Delaware and Kentucky can also expect to see plenty of cicadas, Kritsky told the Cincinnati Enquirer.
Biologists predict where the biggest populations of Brood X will show up this year based on maps showing where the insects were densest when they last appeared 17 years ago — provided that the trees where the nymphs hatched and fed are still standing, Hamilton told Live Science.
Related: Why do cicadas sing?
For 17 years, periodical cicadas like those in Brood X live underground as nymphs, slurping up nutrients from tree roots. If the soil remains undisturbed and the trees aren't taken down, a brood can thrive in a cicada "hotspot" for generation after long-lived generation, Hamilton said.
"Historically — this brood and other broods — we probably had a larger distribution of them," Hamilton said. "But as we developed, we've taken their habitat away."
People who live in counties where Brood X cicadas are expected to appear should look for "the oldest forested areas, the biggest, oldest trees," said Elizabeth Barnes, an entomology educator at Purdue University in West Lafayette, Indiana. "State parks tend to also be great places to see the cicadas," and cicadas should be numerous in Indiana's Turkey Run State Park and McCormick's Creek State Park, Barnes told Live Science.
"Like a really loud lawnmower"
Cicadas in a population "hotspot" don't all emerge at once, so Brood X emergence typically lasts for a couple of weeks, and then the adults stick around for another week or two to mate and lay eggs. In locations where cicadas appear in large numbers, the din of their mating calls can be quite overpowering — "like a really loud lawnmower right next to you," Barnes said.
"The good news is, they're not singing 24/7. They mostly like to sing during the warmer part of the day, around 10 [a.m.] to 5 [p.m.] is the peak of their singing," she explained.
Other than having overwhelmingly loud sex, cicadas are not harmful to people; they don't sting or produce toxins, and they don't have chewing mouthparts, so they can't bite. However, female periodical cicadas may damage young branch tips on trees when they lay their eggs through an organ called an ovipositor, Barnes said.
"Their ovipositors are like a large sewing needle; when they stick it into a small twig it cracks the bark," Barnes said. If enough cicadas lay eggs on a single branch, it can kill the tip. This won't hurt older, bigger trees but can harm young trees, Barnes said. Fruit trees may also be affected, as the tips of the branches are where they produce flowers and then fruit, Hamilton added. Homeowners in high-emergence areas may want to avoid planting new trees this spring, and should cover young trees with burlap until cicada season is over, Hamilton said.
"You can also use netting, but it has to be very small mesh so the females can't get the ovipositor through," he said.
If you're checking for Brood X populations near you, you'll find the most up-to-date map of Brood X on the University of Connecticut's cicada website. And when you do see Brood X cicadas, smartphone apps such as Cicada Safari and iNaturalist will enable you to report the location of your sighting so that entomologists can create even more detailed maps of Brood X populations for the next emergence 17 years from now, Barnes told Live Science.
"I've had people asking me, 'You said cicadas will be in central Indiana, but what about at my house?'" Barnes said. "I can't tell them where precisely the cicadas are, but maybe in the future I'll be able to do that."
Originally published on Live Science.