Scientists trace ecological effects of cicada brood to defoliation of forests
This brown headed cowbird was one of many birds that feasted on cicadas—rather than caterpillars—when Brood X emerged in 2021.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.
Ecologists John Lill of George Washington University 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.
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.
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.
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