Trillions of the noisy insects are emerging from the ground this spring and summer in a rare, synchronized event.
Why so many bugs?
A vast swathe of the U.S. is now experiencing a natural phenomenon that occurs once every 221 years: the double emergence of two separate broods of periodical cicadas. These are not the usual annual cicadas, the mainstay of balmy summer nights, which live between two and nine years and can be found worldwide. The trillions of red-eyed, cacophonic insects now appearing live only in eastern North America. In the Midwest, there's Brood XIII, which has spent the past 17 years underground, slurping sap from tree roots, and is now crawling above ground to molt, sing, mate, and — after about four weeks — die. And in the South, there's Brood XIX, which has spent 13 years underground. Seventeen states stretching from Wisconsin to Louisiana and from Iowa to Maryland will be blanketed by the two broods, which made their last joint appearance in 1803. While some people may find the insects' chirping annoying, and their mounds of carcasses off-putting, researchers are excited. This emergence "really is one of the seven biological wonders of the world," said University of Connecticut entomologist John Cooley. "It's something that really nobody else in the world gets the privilege of seeing."
What is a brood?
It's a massive group of periodical cicadas that have timed their life cycles in unison. There are 15 broods in the U.S., each of which can contain multiple cicada species that all sing different songs. Brood XIII contains three separate species; Brood XIX has four. Scientists believe broods formed as an evolutionary survival strategy. Cicadas are slow and lack poison, a stinger, or any other defense mechanism, so simply turning out en masse means these protein-rich bugs can't be eaten into extinction by predators. "Birds everywhere will feast," said University of Maryland entomologist Mike Raupp, "and once again the cicadas will emerge triumphant.” Some researchers speculate that by appearing every 13 or 17 years — both prime numbers — the cicadas stop predators from matching their life cycles and birthing lots of young during years of abundance. But others dismiss this idea, noting that when the bugs do emerge, the world — and cicada eaters — have long forgotten about their existence.
How do they know when to emerge?
It's all to do with their diet. Cicada nymphs attach to roots and feed off a fluid called xylem. The flow and makeup of xylem changes predictably over the year, becoming richer in amino acids as a tree buds and blossoms. "We know that's what they count,” said Georgetown University biologist Martha Weiss. But "where they're putting their little chalk marks on the wall, we don't know." When it's time to emerge, cicada nymphs wait until the temperature 8 inches underground hits 64 degrees Fahrenheit. The nymphs then burrow to the surface, latch on to trees or plants, and shed their exoskeletons. After four to six days as pastel-colored "teneral" adults, their exteriors darken and harden, and the bugs get busy seeking mates. When that happens, 1 acre of land can host more than 1 million cicadas, about 2,700 pounds worth of bugs.
Do all those bugs cause damage?
While cicadas are sometimes mistaken for locusts, they don't chew leaves or defoliate crops like their voracious insect cousins. Female cicadas drill slits in trees to lay their eggs — they use a metal-enhanced shaft in their abdomen to make the hole — but this threatens only the health of very young trees and shrubs, which can be protected with netting. Any significant agricultural damage they inflict is secondary: With predators gorging themselves on cicadas, fewer caterpillars get gobbled. One study found that, during the 2021 emergence of Brood X in parts of the Midwest, the South, and Pennsylvania, caterpillar populations doubled, and oak trees experienced a surge in leaf damage. But for the entire ecosystem, periodical cicadas are a net plus. They aerate the soil when they emerge, and their decomposing bodies infuse it with nutrients. The only real threat they pose to humans is their assault on our eardrums: Some cicadas can make a noise reaching 120 decibels at close range, about as loud as a rock concert or a passing jet.
Why are they so noisy?
That's how they attract mates. Males have a washboard-like structure on their abdomen called a tymbal, which they strum with their legs to produce anything from the harsh screeching of cassini cicadas, to the tambourine-like shick-shick-shick of the decula. Females respond to these fevered cries with a simple click of their wings. After mating, they lay clutches of up to 600 eggs in branches and die shortly afterward. When the eggs hatch, the infant insects drop to the ground and burrow down, waiting for their time to dig out. But scientists note that this multiyear cycle seems to be shifting, with periodical cicadas now emerging 10 to 14 days earlier than they did on average in 1940, and more "stragglers" surfacing without their brood.
What's behind this change?
Researchers suspect global warming is messing with the bugs' time-counting mechanism. An unusually warm winter can cause trees to leaf early, which cicadas count as a year; if a cold spell causes trees to pause this process, then resume when spring arrives for real, the insects will count a second year. Some entomologists suspect warming temperatures could shift 17-year cicadas to a 13-year cycle, and 13-year cicadas to a nine-year. But many worry that climate change is happening so fast that species won't be able to evolve adaptations to keep up with it. Cicadas are "going to be handicapped in a very real sense" by their long life cycles, said John Lill, a biologist at George Washington University. "You can only evolve as fast as you can have new generations."
Sex, drugs, and killer fungus
In addition to hungry birds and mammals, cicadas are menaced by a unique, very gruesome STD. Massospora cicadina, a cicada-specific parasitical fungus, enters their bodies and swiftly starts generating spores. Eventually, spores burst out of its rear end, causing the cicada's genitals to fall off to reveal a chalky fungal plug, which contains a form of amphetamine. Despite lacking the equipment to mate, the zombified insect will try to hook up with as many other cicadas as possible. Eventually the plug disintegrates, and the disemboweled cicada rains down spores that infect others. "We call them saltshakers of death," said West Virginia University mycologist Matt Kasson. This fungus doesn't affect other species. Still, experts warn that, while healthy cicadas are fine for humans and pets to eat, it's best to exercise caution. If you find old, sickly-looking, or dead bugs, said Kasson, "you do not want to put those in your mouth."
