In 2024, the 13-year cicada brood XIX and the 17-year cicada Brood XIII emerged in the U.S. It was the first time in more than 200 years two broods emerged at the same time. This led to many questions about these large, buzzing bugs. Do they really swarm out of the ground? Why do they only arrive every 13 or 17 years? In this article, we’ll answer these questions and all the other questions you might have about these fascinating insects.
What Exactly Is a Cicada?
Periodical cicadas are insects in the Hemiptera order, along with aphids and leafhoppers. There are over 3,000 cicada species in the world, with about 190 that can be found in North America. Annual cicadas occur on every continent except Antarctica. Periodical broods are unique to North America, where they are concentrated in the central and eastern regions of the U.S.
Both the 13-year and 17-year cicadas look the same. They’re about 1.5 inches long with thick, squat bodies, translucent wings, and bright red eyes. Cicadas are also noted for the unique buzzing sound they make. This distinct noise sounds like a constant, high-pitched buzzing and clicking. The male cicadas make the noise as a mating call. The noise levels can reach as high as 100 decibels, which is as loud as a jackhammer or a chainsaw.

©Chris Alcock/Shutterstock.com
What Is a Cicada Brood?
A brood is a name given to a particular group of cicadas that emerge on a regular schedule. A brood can contain different cicada species if they are all on a similar schedule. This is how researchers keep track of the various cicada groups. There are three broods of 13-year cicadas and 12 broods of 17-year cicadas.
You can think of each brood as a large family. For instance, the 17-year Brood X from 2021 was descended from the same brood that arrived in 2004, which was descended from the brood that arrived in 1987. Some cicada broods have been appearing regularly for centuries. Researchers can map the areas where different broods of cicadas arrive to learn their patterns over time.
The Life Cycle of a Cicada
The cicada life cycle consists of eggs, nymphs, and adults. Each female can lay about 600 eggs. They lay the eggs on the branches and leaves of trees, and the eggs hatch about two months later. When the hatchlings come out of their eggs, they fall off the trees and land on the ground. Once there, the nymphs burrow into the ground. Inside their burrows, they feed on liquids from surrounding plant roots.
The nymphs stay underground until they develop into adults. The nymphs go through five instar stages, which is the stage between molts, in their development underground. Once they reach adulthood, the cicadas emerge from the ground and fly to the nearby trees. Then they shed their shells, which look just like the nymphal cicadas.
Adult cicadas have a short life. They live for about four weeks before dying or being eaten by other animals. They are a favorite food of sparrows, bluebirds, robins, and other birds. Animals such as bears, raccoons, snakes, lizards, mice, and rabbits will also feast on cicadas. During those four weeks, the cicadas mate, lay eggs, and start the whole cycle over again.

©iStock.com/Jonathan Steinbeck
How Do They Know When It’s Time to Emerge?
Most scientists believe it is because they have an internal clock tied to their diets. Cicadas have little mouths like suckers that they stick into the tree roots. This allows them to feed on the sap and other liquids from the trees. The changing composition of the fluids throughout the year indicates the passage of time. Their internal clocks count the years so they know when it has been 13 or 17 years.
Once 13 or 17 years have passed, they must wait to emerge until the ground soil is warm enough. Scientists believe the ground must be 64 degrees at a depth of 12-18 inches. When it reaches this temperature, the cicadas will climb to the surface to finish the transformation to adulthood. Around 70% of the brood emerges in the first 4-5 days, followed by the remaining members over the next few days.
Why Do They Wait So Long?
The most likely reason why cicadas wait so long is to avoid predators. Cicadas practice a technique called predator satiation. The cicadas need to make sure there are enough of them to feed all the predators. When cicadas come out of the ground, they do so in the trillions. By emerging all at once, these periodical cicadas supply enough food for all the predators in the area. Once the predators are too stuffed to eat more, the remaining cicadas can reproduce with fewer worries about being eaten.
Another reason cicadas spend so long underground is that insect populations are regulated by specialized predators. If the cicadas only emerge every 17 years, predators have a much harder time specializing on them because they aren’t a reliable food source. When the cicadas do come out, using predator satiation means they can sacrifice millions of brood members without harming the species’ chances of survival.
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