Animals Endangered Species What Are Brood X Cicadas and What to Expect When They Emerge By Olivia Young Freelance Writer Olivia Young covers a wide range of environmental topics, from low-impact travel to conservation. She is passionate about tiny living, climate advocacy, and all things nature-related. our editorial process Twitter Twitter Olivia Young Updated April 21, 2021 Jeremy_Hogan / Getty Images Share Twitter Pinterest Email Animals Wildlife Pets Animal Rights Endangered Species Brood X cicadas emerge every 17 years in 15 states throughout the Eastern and Midwestern United States. Of the 12 active periodical cicada broods, Brood X (pronounced "Brood 10") is one of the largest and most concentrated, comprising all three known 17-year cicada species, Magicicada septendecim, Magicicada cassini, and Magicicada septendecula. On select years, in late April to early May, billions of these large-winged arthropods emerge from the ground simultaneously, promptly blanketing the trees in discarded exoskeletons and filling the air with their vociferous, rhythmic mating calls. Despite their unsettling appearance — with veiny mega-wings and bulging red eyes — cicadas are harmless. They don't bite humans or harm the mature trees that temporarily house them. They may, however, hinder the growth of a juvenile tree (but for that, there's netting). Learn more about the world's potentially densest, most widely distributed cicada brood and what to expect when it arrives. Brood X Cicadas cmannphoto / Getty Images The first mention of Brood X was in a 1715 journal entry by Philadelphia pastor Rev. Andreas Sandel. Another mention of them decades later — in a letter by botanist John Bartram describing their 1732 emergence — verified their 17-year periodicity. While their distribution is believed to have once been wider, they now occur most heavily in Maryland, Indiana, Ohio, Tennessee, and parts of Pennsylvania, Delaware, New Jersey, Illinois, North Carolina, and Georgia. "Brood X historically included a large swath of the mid-Atlantic, west to Ohio and Indiana, and south to Kentucky and Tennessee," says Dr. John Lill, chair of George Washington University's Department of Biological Sciences. "But intensive deforestation, followed by farming and/or grazing, likely reduced and fragmented Brood X into its current distribution (the exact causes of the current distribution remain a bit of a mystery)." It's no mystery, however, why U.S. periodical broods tend not to travel outside their longtime territory. Dr. Zoe Getman-Pickering, a postdoctoral scientist at GWU, says they're "relatively helpless" because their only individual survival mechanism is singing (at 120 brain-rattling decibels, no less), so when they stray from the swarm, they're more likely to become meals for birds, rodents, snakes, and mammals. Thus, they rely on their strength in numbers — and Lill says there could be up to a trillion of them. Besides, they feed on tree roots as nymphs, and the grassy plains of the Midwest simply can't support them, says Getman-Pickering. What Are 17-Year Cicadas? There are two types of cicadas: annual and periodical. There are seven species of periodical cicadas in North America — four that emerge every 13 years and three that emerge every 17 years. Experts believe the longer periodicity of 17-year species is caused by colder temperatures, as 13-year broods are more common in the warmer Southern states. Life Cycle Cicada molting on tree trunk. Joe McDonald / Getty Images The lifespan of a cicada is concurrent with its periodicity — so, 17-year cicadas live 17 years, and so forth. They drop from cracks and holes in trees to the ground immediately upon hatching six to 10 weeks after their eggs are laid, then they burrow underground and find a patch of tree root to live on for the next 17 years. Cicadas are not exactly dormant during their juvenile phase (in which they're called nymphs). Rather, they spend nearly two decades just feeding on xylem and waiting for their next emergence, which they may be able to determine by monitoring the trees' blossoming cycles underground. When the ground 8 inches below the surface reaches 64 degrees Fahrenheit — usually late April or early May — the cicada nymphs will tunnel to the surface and climb up a nearby tree to evade ground predators. They shed their exoskeletons (exuviae) during the last growth cycle, and once their wings inflate with fluid, they take flight — singing their piercing calls to attract mates all the while. Brood X cicadas emerge every 17 years (1987, 2004, 2021) and live only up to six weeks above ground. Females lay about 500 eggs each before they retire, and then the cycle repeats. Cicada exoskeleton. OSCAR MAGANA PASCUAL / Getty Images Are Cicadas Dangerous? Cicadas are not dangerous to anyone or anything except, perhaps, juvenile trees. Cicadas continue to feed on trees once they emerge from the ground, and if you have trees whose main branches are less than a half inch in diameter, their feeding and egg-laying can cause damage. You can easily protect juvenile trees by covering them with mesh. It's best to not use insecticides as the chemicals can compromise future cicada generations. Cicadas do not bite or sting. They're so harmless that animals — including cats, dogs, and even humans — eat them. Report Cicada Sightings Richard Ellis / Staff / Getty Images All three Brood X species are listed as Near Threatened on the IUCN Red List. "The populations are definitely declining," Getman-Pickering says, and the main culprits are climate change and deforestation. Whereas large quantities of Brood X cicadas once emerged on Long Island, the New York City-adjacent refuge now sees very few. To monitor populations across the region, cicada expert Dr. Gene Kritsky of Mount St. Joseph University developed a free citizen science app called Cicada Safari where people can upload photos of Brood X and future broods. The goal is to map cicada sightings for future research. Additionally, Lill says people in cicada-prone regions can help conserve the prehistoric critters by planting trees, supporting conservation organizations (particularly local ones that promote reforestation efforts like Chesapeake Bay Program, Casey Trees, the National Forest Foundation, American Forests, and The Nature Conservancy), and voting for local development projects that protect forest lands. View Article Sources "Periodical Cicadas are Poised to Emerge." Ohio State University. 1 April 2021. "Evolution of periodicity in periodical cicadas." Sci. Rep.5, 14094. 14 Sept 2015. "How 17-year cicadas keep track of time." Richard Karban Department of Entomology, University of California, Davis. 5 April 2002. "Magicicada septendecim." Animal Diversity Web. "Magicicada." International Union for Conservation of Nature Red List of Threatened Species.