A rare burst of billions of cicadas will rewire our ecosystems for years to come

This spring is a very good time to be a bird.

In forests across the Midwest and Southeast, the ground is about to erupt with billions of loud, protein-packed cicadas. They’ll buzz about for a few weeks as they search for mates, providing snacks for pretty much every living creature in the forest, from songbirds and swans to frogs and even fish.

This is an especially big year for these red-eyed bugs: Brood XIX and Brood XIII — which pop up every 13 years and 17 years, respectively — are emerging at once. The last time such an event happened was the spring of 1803, when Thomas Jefferson was president. It will be hundreds of years before it happens again.

While the insect explosion will be brief, it will shape forests for years to come. The binge-fest that birds enjoy during these periods supersize their families and, in turn, shift the eating and hunting patterns of many other species. These effects send ripples throughout the ecosystem. As one recent study put it, pulses of periodical cicadas can “rewire” entire forest food webs. Call it the butterfly cicada effect.

Why billions of cicadas erupt all at once

For most of their lives — either 13 or 17 years, depending on the brood — periodical cicadas live several inches underground, slurping up sap from plant roots with their straw-like mouths. Then, when the soil temperature hits about 64 degrees Fahrenheit, they emerge, typically after sunset. Cicadas in more southern states, like Alabama, usually emerge in April or early May, whereas those in colder states like Illinois tend to appear later in the spring.

The teenage insects then march up plants, trees, and fences, where they metamorphose into winged adults. That’s when giant groups of males start singing loudly to attract females (you know, lady bugs). During these events, a single acre of land can have more than 1 million cicadas on it. That’s roughly 2,700 pounds of bugs.

This mass eruption, scientists believe, is strategic. “They effectively satiate their predators,” Louie Yang, an entomologist at the University of California Davis, told me a few years ago, when the famous Brood X emerged.

The cicada defense strategy is to flood the forests so that predators become so full they literally can’t stomach another bite. That leaves plenty of insects left to mate and lay eggs that will become the next generation of cicadas.

This approach seems to work for cicadas, and it’s an absolute delight for birds.

Birds lose their minds during cicada outbreaks

Birds can be fussy about their food. Some prefer plants, like the trumpeter swan, while others specialize in seeds or small insects, like chickadees.

Those preferences get tossed out during cicada explosions. The birds stop what they’re doing and go to town on the bug buffet. During the Brood X emergence in 2021, researchers documented more than 80 different avian species feeding on cicadas, including small birds that couldn’t fit them in their mouths.

“We saw chickadees — tiny, tiny little birds — grab the cicada and drag it to the ground with their body weight and then peck it apart,” said Zoe Getman-Pickering, an ecologist at University of Massachusetts Amherst, who led the research.

She also saw purple martins, which typically catch small insects like winged ants and flies from the air, go after loads of cicadas. “There was one family of purple martins that got 23 cicadas into their nest in three hours or so,” Getman-Pickering said.

This feeding frenzy can seriously benefit some birds. Simply put, more food can lead to more babies.

“Following emergences, you do tend to get an increase in a lot of the apparent avian predator populations,” Walt Koenig, an ornithologist at Cornell University and research zoologist emeritus at UC Berkeley, told me in 2021.

One analysis he co-authored, based on 37 years of data, linked cicada eruptions to a population bump in a number of species including red-headed woodpeckers and common grackles.

Remarkably, many of these knock-on effects lasted for years, Koenig said. The number of blue jays, for example, was significantly higher even three years after the cicada eruptions.

“These results indicate that, at least in some species, the effects of cicada emergences are detectable years after the event itself,” Koenig and his co-authors wrote.

Fat caterpillars, rejoice

It’s not just birds that are benefitting. During big emergences, avian predators are eating so many cicadas that they eat much less of everything else — including caterpillars. That means caterpillars get a rare reprieve from the constant threat of attack, at least from birds.

Researchers have actually measured this. In the years surrounding Brood X, Getman-Pickering and her collaborators filled forests in Maryland with fake caterpillars made of clay. They then measured how many of them had signs of bird strikes — beak marks indicating that birds tried to eat them.

In May, when Brood X was emerging, the portion of caterpillars with strike marks fell dramatically, from about 30 percent in a typical year to below 10 percent during the emergence, according to her study, published in 2023.

She also looked at real caterpillars. Remarkably, the number of them roughly doubled in the forests she studied during the emergence, relative to the two following years. “It was pretty staggering how many caterpillars that we saw,” Getman-Pickering said.

A lot of them were extra plump, too, like the spiny larvae of the dagger moth. When there are few cicadas, the juiciest caterpillars are often picked off first; they’re much easier for birds to spot. But during cicada eruptions, caterpillars are free to eat and grow at their leisure.

“The biggest, most visible caterpillars benefited immensely from the release from predation,” she said.

Trees might prefer life without cicadas

A surge in caterpillars, meanwhile, has effects of its own. These animals famously eat leaves. So when birds eat fewer of them, the caterpillars chew their way through more of the forest canopy.

Getman-Pickering’s recent study measured this too: In the summer of 2021, after Brood X debuted, oak trees experienced “a spike in cumulative leaf damage,” the paper states. A doubling of the number of caterpillars meant a doubling of the damage, she said.

It’s not clear what that ultimately means for forest health. Previous studies have shown that cicadas themselves, however, can harm trees. After breeding, females carve slits into branches and lay eggs, which often damages the wood.

Research by Koenig, of Cornell, found that oak trees produced fewer acorns in a year with a cicada emergence, and in the following year. Older studies have also shown that emergences can slow the rate of tree growth.

The long-term picture is hazier. Unpublished data from Karin Berghardt and Kelsey McGurrin, researchers at the University of Maryland, shows that trees seem to bounce back from the harm caused by egg-laying. There’s also some research suggesting that cicada carcasses could actually fertilize the forest floor.

Ultimately, what all of these studies show is that cicadas can transform entire ecosystems in just a few short weeks. Think about that the next time you walk through the woods: The birds, the butterflies, the trees themselves are all shaped, in some way, by one very weird bug.