Why Do Cicadas Stay Underground For 17 Years

The Bug That Vanishes for a Decade and a Half

If you were a kid in 2004, you might remember the buzz. Day to day, not the gentle hum of summer — the wall-to-wall*, tree-shaking, sky-darkening swarm of cicadas that erupted from the ground like something out of a horror movie. Then, just as suddenly, they were gone. And they didn’t come back. Not for seventeen years.

Now here we are again, and the same thing is happening. That sound isn’t just loud; it’s everywhere*. Brood X — one of the largest and most widespread periodical cicada broods in the United States — is emerging across parts of the Midwest and South. If you live in those areas, you’ve probably already heard them. It’s the kind of noise that makes you stop mid-conversation and look up, wondering where all those insects came from.

But here’s the thing that keeps scientists scratching their heads: why seventeen years? Even so, why not ten? Why not five? Why does this insect essentially sign itself up for a nap that lasts longer than most houseplants survive?

What Is a Periodical Cicada, Really?

Let’s get one thing straight — cicadas aren’t all the same. Practically speaking, there are dog-day cicadas that show up every summer, buzzing lazily in the heat. On top of that, those are annuals, more or less. But periodical cicadas are a different beast entirely. And when we talk about the seventeen-year mystery, we’re talking about Magicicada* — a genus found only in the eastern half of the United States.

These aren’t just late bloomers. And millions upon millions of them. They climb trees, mate, lay eggs, and die — all within a few weeks. Every thirteen or seventeen years (yes, some broods do thirteen), entire populations of these insects emerge from the soil at once. Think about it: they’re synchronized timekeepers. Then the cycle resets.

The nymphs that hatch from those eggs drop to the ground and disappear underground, where they’ll spend nearly their entire lives feeding on root sap. They don’t eat anything else. On the flip side, they don’t surface. They don’t molt into adults. They just… wait.

And here’s where it gets weird: they’re not just waiting for the right season. They’re counting years.

Why Does It Matter That They Disappear for So Long?

This isn’t just some quirky evolutionary footnote. Day to day, the seventeen-year cycle is one of the most extreme examples of synchronized development in the animal kingdom. And it matters because it’s tied to survival — specifically, surviving the jaws of predators.

Most insects that hang around above ground deal with constant pressure from birds, squirrels, raccoons, and countless other creatures that would happily snack on them. Periodical cicadas sidestep that problem by going underground for so long that their predators either forget they exist or die off entirely.

But there’s more to it than just hiding. By emerging in such massive numbers, they overwhelm their predators. A hungry bird can eat dozens of cicadas and still leave thousands untouched. It’s called predator satiation — flood the zone, and hope the math works in your favor.

Still, that doesn’t explain the length* of the wait. In real terms, ten? In practice, why not five years? What makes seventeen special?

How Do They Count to Seventeen?

Turns out, cicadas don’t actually count in the way we think of counting. That said, they don’t have calendars or abacuses tucked under fallen logs. Instead, they rely on a combination of biological clocks and environmental cues.

Here’s what scientists think happens:

The Biological Clock Theory

Cicada nymphs feed on root sap, which carries chemical signals from the trees they’re attached to. Plus, researchers believe these insects track seasonal changes — like temperature shifts and daylight patterns — through compounds in that sap. Over time, they accumulate enough “seasonal hits” to trigger their emergence.

Some studies suggest that cicadas may actually be sensitive to the xylem chemistry of their host trees, which changes slightly each year based on weather and climate. After seventeen of those cycles, something in their physiology flips, telling them it’s go-time.

The Prime Number Hypothesis

This is where things get fascinating. In real terms, seventeen is a prime number — it can only be divided evenly by one and itself. So is thirteen, which is why some broods follow a thirteen-year cycle.

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Biologist Isaac Asimov once proposed that prime numbers offer evolutionary advantages. If a cicada emerges every seventeen years, and a predator has a life cycle of, say, two or three years, their paths rarely align. The predator might miss the cicadas entirely, or vice versa. It’s like nature’s way of throwing curveballs.

Think about it: if cicadas emerged every six years, predators with two-, three-, or six-year cycles could sync up easily. But seventeen? Good luck finding a predator that lines up perfectly with that schedule.

The Role of Soil Temperature

Even though the internal clock seems to be the main driver, emergence timing still depends on soil conditions. That said, specifically, the ground has to hit a certain temperature — usually around 64°F (18°C) — several inches down. That’s why some cicadas pop up a week early during a warm spring, or a week late during a cold snap.

It’s not enough to mess with the seventeen-year rule, but it’s enough to fine-tune the timing. The clock says go, but the soil has to agree.

What Most People Get Wrong About Cicadas

Let’s clear up a few myths.

First, cicadas don’t sting or bite humans. They’re harmless. The only thing they’re really good at is making noise and, occasionally, pooping on your car. If you hear that buzzing and see them flying around, relax — they’re more interested in each other than in you.

Second, periodical cicadas aren’t locusts. Cicadas are in an entirely different family. Because of that, locusts are a type of grasshopper known for swarming crops and causing agricultural damage. Calling them locusts is like calling a panda a raccoon — sure, both are mammals, but that’s where the similarity ends.

Third, not all cicadas follow the seventeen-year pattern. Only Magicicada* species do. Now, the rest are annual cicadas, which emerge every summer like clockwork. Mixing them up is common, but it misses the point entirely. Most people skip this — try not to.

Fourth, the seventeen-year cycle isn’t perfectly rigid. Some individuals emerge a year early or late, and scientists have documented stragglers appearing in off-years. But the vast majority stick to the script — which is what makes the phenomenon so remarkable.

Finally, cicadas aren’t dying out because of their long cycle. In fact, their strategy seems to be working pretty well. Because of that, they’ve been doing this for millions of years, and climate change hasn’t derailed them yet. Sure, urban development and pesticide use pose threats, but the seventeen-year plan remains intact.

What Actually Works When Dealing with Cicadas

If you’re reading this during an emergence year, you’re probably wondering how to survive the next few weeks without losing your mind to the noise. Here’s what helps:

Soundproofing Your Space

Those cicadas are loud — sometimes louder than a lawnmower. If they’re concentrated near your house, consider closing windows and using white noise machines or fans to mask the sound. Trees act like amplifiers, so if you’ve got big ones near your bedroom window, you might want to relocate temporarily.

Protecting Young Trees and Plants

Female cicadas lay eggs in slits they cut into branches. On small or young trees, this can cause damage. Here's the thing — wrap trunks with netting or foil, or simply cover vulnerable plants with lightweight fabric. Most established trees handle it fine, but saplings need protection. Turns out it matters.

Embracing the Moment

Yes, they’re noisy and a little creepy. Enjoy the spectacle while it lasts. But they’re also temporary. Kids love them, dogs go crazy for them, and they’re a sign that your local ecosystem is healthy enough to support such an extraordinary life cycle.

Don’t Panic About Your Garden

Cicadas feed on tree sap, not garden plants. They won’t devastate flowers or vegetables. The closest thing to a problem is the occasional dropped exoskeleton, which is harmless and actually makes great fertilizer.