Every 17 years, billions of cicadas emerge from the soil in the eastern United States to climb trees, mate, and lay eggs. For a few weeks, the plump insects provide an all-you-can-eat buffet for birds, mammals, and other predators. “It’s a phenomenal explosion to an ecosystem,” says Kathy Williams, an insect ecologist at San Diego State University.
The consequences of this gluttony ripple through the ecosystem, as satiated birds ignore their usual prey of caterpillars, which then grow fat eating the leaves of oaks, researchers report today in Science. “I thought this was an amazing study,” says Jalene LaMontagne, a population ecologist at DePaul University who was not involved. “It really adds to the big picture of cicadas,” says Gene Kritsky, an entomologist at Mount St. Joseph University.
Many species of cicada appear every year, chirping and buzzing through summer days. Other species, called periodical cicadas, live for 13 or 17 years underground before appearing in massive numbers. They live in populations, or broods, that create a geographical patchwork across the eastern United States, emerging in different places on different schedules. Each brood produces so many periodical cicadas that predators can’t eat them all, and enough individuals survive to reproduce. Although the cicada feast can help predators increase their numbers the next year, their populations shrink back to normal before the periodical cicadas emerge again.
Ecologists John Lill of George Washington University (GW) and Martha Weiss of Georgetown University have studied insects in oak forests for many years. While Brood X, the largest of the cicada broods, was still preparing to emerge, Lill and Weiss wondered what the massive numbers of cicadas would mean for caterpillars. Too many caterpillars can wreak havoc on oak trees, but birds keep their populations in check. Lill and Weiss reasoned that if birds filled up on cicadas, they might not have much appetite left for their usual diet of caterpillars.
To find out, they counted caterpillars on oak trees in two locations near Washington, D.C., 1year before the Brood X emergence. They and colleagues also measured birds’ appetites by gluing clay models of caterpillars onto branches. Each week, they found, about 25% of these decoys had been pecked.
When Brood X finally emerged in 2021, the feast was on. Weiss and Lill gathered 983 observations from birders in Mid-Atlantic states. All told, 82 bird species were reported feeding on the cicadas. “We were really surprised at how many bird species were eating cicadas,” says entomologist Zoe Getman-Pickering, who at the time was a GW postdoctoral researcher. Trumpeter swans wolfed down the 1.2-gram cicadas. Even blue-gray gnatcatchers, which weigh less than 7 grams, managed to pick at bits of a cicada like eating pieces of BBQ chicken.
Kritsky, however, isn’t shocked. After all, Brood X cicadas are easy to catch and nutritious “What would you do if you walked outside, and you found the world swarming with flying Hershey’s Kisses?”
All this easy food meant birds didn’t eat as many caterpillars. After the cicadas emerged in May 2021, fewer than 10% of the clay caterpillar models were pecked each week. By August 2021, after the cicadas had finished mating and died or been eaten, those levels returned to about 25%. A tally of real caterpillars in the oak forests revealed their population was twice that of the following 2 years, when the cicada brood had returned underground. The researchers also found that a common caterpillar, the eclipsed oak dagger (Acronicta increta), was much more likely to grow large—defined as developing to a stage called the third larval instar or beyond—when the birds were full of cicadas. In fact, in 2021, more than half of measured caterpillars grew to this large stage compared with 13% or 1% in the subsequent 2 years, respectively.
As the very hungry caterpillars grew plump, they feasted on oak trees. The researchers observed that leaf damage was twice as extensive during the cicada glut. “The leaves looked like lace, they had so many holes of different shapes and sizes,” Getman-Pickering says. It’s not clear whether such leaf damage harmed the ability of the tree to reproduce, because they were not old enough to produce acorns, but previous studies suggest this level of herbivory could set the forests back. Research on tree rings shows that oaks grow more slowly during years when cicada broods emerge.
The cicadas themselves do some damage when females lay their eggs inside twigs. But the dead bodies of cicadas also release nutrients back into the soil, which could benefit plants. The bugs’ burrows and tunnels help air and water penetrate the soil, as well.
Getman-Pickering hopes the new research will serve as a reminder of how interconnected ecosystems are—and that shocks to them can have wide-ranging consequences. The disruption from Brood X “was short-lived, it was intense, and then it went away.” In contrast, current changes such as declines of bird populations through loss of habitat, climate change, or invasive species are chronic and widespread.
