Chicago cicada invasion 2024: What to know about these loud, unique insects

CHICAGO (WLS) -- The cicadas are coming! The 2024 double emergence of Brood XII and XIX will bring billions of cicadas to northern Illinois. According to the Field Museum, the emergence of the Great Southern brood and the Northern brood simultaneously only happens every 221 years.

But what do you actually know about these cyclical critters?

Cicadas are one of the longest lived insects on the planet

Most insect life cycles are pretty short; some last for days, and the overall average is less than a year, according to scientists. But the periodical cicadas emerging this year are among the longest lived insects on the planet. Their life cycle spans 17 years, with 99% of that life spent underground.

When they emerge as adults, both females and males only live about a month above ground before dying.

Cicadas are loud - but only the males

Cicadas are notoriously loud insects. The buzzing song they make can reach up to 90 decibels, about the sound of a loud lawn mower or a motorcycle.

That may not sound too bad, but, here in northern Illinois, when broods emerge they can pack anywhere from 100,000 to 1 million insects into a single acre, depending on tree and soil coverage. A million cicadas buzzing away at 90 decibels is enough to give anyone a headache.

But not all cicadas sing a song; only the males buzz away. They do so to attract female cicadas and find a mate. And each species of cicada has a unique song.

Cicadas begin and end in trees, but spend most of their lives underground

Female cicadas lay their eggs in tree branches that are usually 1 inch in diameter or less. They use their egg-laying organ, called an ovipositor, to cut the branch and then lay their eggs in the cut. They typically lay 10 to 20 eggs per cut, and in total lay up to 600 eggs each in their reproduction cycle.

While this reproductive cycle won't damage mature trees, new trees can be damaged by the females' cuts and egg-laying process.

The eggs then stay in the trees for six to 10 weeks before cicada nymphs emerge. These nymphs then drop from their branches and start burrowing into the ground. They dig 8 to 12 inches into the ground and then feed on the tree roots for the next 17 years, growing and maturing, waiting their turn to emerge as adults and start the life cycle over again.

Cicadas stick close to home, which is why broods stay concentrated in certain areas

Looking at a periodical cicada, you can see how large their wings are, but they don't actually stray too far from home. Once they emerge, cicadas travel at most about half a mile by air.

Because they stay so close to where they emerge, the North American broods are geographically set, and can be tracked by scientists, making the periodical emergences easier to predict.

There are perhaps 3,000 to 5,000 species of cicadas around the world, but the 13- and 17-year periodical cicadas of the eastern U.S. appear to be unique in combining long juvenile development times underground with synchronized, mass adult emergences. There are two other known periodical cicadas in the world, one in northeast India and one in Fiji, but these have only four-year and eight-year life cycles, respectively.

Cicadas are a very old species, and well documented in early American history

As species, periodical cicadas are older than the forests that they inhabit. Molecular analysis has shown that about 4 million years ago, the ancestor of the current Magicicada species split into two lineages. Some 1.5 million years later, one of those lineages split again. The resulting three lineages are the basis of the modern periodical cicada species groups, Decim, Cassini and Decula.

Early American colonists first encountered periodical cicadas in Massachusetts. The sudden appearance of so many insects reminded them of biblical plagues of locusts, which are a type of grasshopper. That's how the name "locust" became incorrectly associated with cicadas in North America.

During the 19th century, notable entomologists such as Benjamin Walsh, C.V. Riley and Charles Marlatt worked out the astonishing biology of periodical cicadas. They established that unlike locusts or other grasshoppers, cicadas don't chew leaves, decimate crops or fly in swarms.

Cicadas are susceptible to climate change, too

From the date of emergence to the length of their life cycles, cicadas are as susceptible to climate change as every other species on earth.

In northern Illinois for 2024, scientists had pegged May 15 as the date that the brood was most likely to emerge, as they require soil temperatures of 64 degrees Fahrenheit to do so. But with a warmer-than-average winter and spring, residents started reporting some cicadas emerging in their area by mid-April.

Because periodical cicadas are sensitive to climate, the patterns of their broods and species reflect climatic shifts. For example, genetic and other data indicate that the 13-year species Magicicada neotredecim, which is found in the upper Mississippi Valley, formed during a previous interglacial period about 200,000 years ago.

As the environment warmed, 17-year cicadas in the area emerged successively, generation after generation, after 13 years underground. Eventually, they permanently shifted to a 13-year cycle.

But it's not clear whether cicadas can continue to evolve as quickly as humans are altering their environment. Although periodical cicadas prefer forest edges and thrive in suburban areas, they cannot survive deforestation or reproduce successfully in areas without trees.

The Associated Press contributed to this report.