When huge broods of cicadas emerge after years underground, they provide an all-you-can-eat buffet for birds. And Brood X—the group that swarmed the Eastern United States in 2021—brought as many as 1.5 million of the protein-rich insects per acre.
These mass cicada emergences have cascading effects on the broader ecosystem, according to a new paper published last week in the journal Science. Researchers found that birds, faced with a sudden abundance of food when Brood X emerged, ate fewer caterpillars. In turn, the insects flourished, munching their way through oak forests.
“Our findings really show how… plants, animals and all sorts of organisms are all deeply connected,” lead author Zoe Getman-Pickering, a former researcher at George Washington University, says in a statement. “When you shift the behavior or the population of one of those organisms, the effects ripple through the ecosystem in surprising ways.”
Some types of cicadas appear every year. But others, called periodical cicadas, spend either 13 or 17 years underground, feeding on tree root sap, before surfacing in huge groups called broods in the spring or early summer. Once a brood has emerged, adult cicadas mate and lay their eggs in tree branches before dying. About six weeks later, the eggs hatch, and cicada nymphs plummet to the ground. They tunnel into the soil, and the cycle starts over again.
Three broods of 13-year cicadas and 12 broods of 17-year cicadas exist in the eastern United States. For the new study, researchers focused on Brood X, the largest of the 17-year broods. The team collected data before, during and after Brood X’s 2021 appearance to understand the bugs’ effect on the world around them.
Prior to the cicadas’ arrival, scientists counted the number of caterpillars on oak trees near Washington, D.C., and recorded the damage they did to the leaves. They also created clay models of caterpillars, placed them on branches and counted how many of them got pecked by birds.
In years without the Brood X cicadas, the share of clay caterpillars with peck marks was around 25 percent. During the bugs’ emergence, however, when birds shifted their attention toward the cicada smorgasbord, only 10 percent of the clay caterpillars had peck marks on them. That number returned to 25 percent after the cicadas went back underground.
When broods surface, birds have easy access to billions of new, nutritious snacks. It makes sense, then, that they would stuff themselves with cicadas and neglect the caterpillars.
“What would you do if you walked outside, and you found the world swarming with flying Hershey’s Kisses?” says Gene Kritsky, an entomologist at Mount St. Joseph University who was not involved in the study, to Science’s Erik Stokstad.
While birds were busy feasting on cicadas, caterpillar populations boomed, and individual caterpillars grew larger. In 2021, researchers counted twice as many caterpillars as they did in non-emergence years, and those caterpillars caused twice as much damage to oak leaves.
“In a normal year, birds regulate insect herbivore damage, but that gets disrupted in cicada years,” says study co-author John Lill, a biologist at George Washington University, to New Scientist’s Brian Owens.
Researchers did not determine whether this leaf damage caused any lasting harm to the trees. However, earlier research found that trees grow more slowly in the years during and immediately following cicada brood emergence.
The scientists also asked local birders to record any birds they saw eating cicadas. All told, people spotted more than 80 different avian species munching on the insects, from large trumpeter swans to teeny-tiny blue-gray gnatcatchers.
While cicada broods cause temporary changes to ecosystems, other issues—like habitat loss or human-caused climate change—may have longer-lasting consequences. More than half of bird populations in the United States are shrinking, and without as many birds to keep insect numbers in check, “there will be more damage to forests and food crops,” Lill tells New Scientist.