To study swarming cicadas, it takes a crowd

Brood X periodical cicadas have captured the attention of amateur scientists across the eastern United States.

B. Douthitt/Science

The billions of periodical cicadas now crawling, fluttering, and singing from trees in the eastern United States have roused a throng of humans as well, who are mapping the insects and timing their emergence in what may be the country’s longest public science tradition. Using a free app called Cicada Safari, more than 150,000 people so far have uploaded geotagged photos of cicadas, helping scientists track their emergence after 13 to 17 years underground. 

The insects are an ideal target for science-inclined amateurs—unmissable and mysterious at the same time. “We just don’t know what’s going on in their life” underground, says Douglas Pfeiffer, an entomologist at Virginia Polytechnic Institute and State University. With just a handful of scientists trying to understand a natural event both massive and rare, aid from amateur scientists is invaluable. In recent years, community reports have caught the formation of new populations, helped study the link between emergence and air temperature, and traced how cicada populations respond to stressors.

Periodical cicadas are grouped into 15 broods based on where and in which years they emerge. Eggs laid in tree branches hatch, and the nymphs fall and burrow into the ground. There, they feed on xylem from tree roots, molting four times over the next 13 or 17 years, before emerging when the ground temperature reaches roughly 17.7°C. Then the frenzy of calling and mating begins.

The public can use the Cicada Safari app, released in 2019 by Gene Kritsky, an entomologist at Mount St. Joseph University, to send photos to researchers, who review the genus, location, and brood identity of paparazzied cicadas. Last year, the abundance of data points revealed something unexpected: The first reports of cicadas in 2020 weren’t from the expected Brood IX, which mostly lives in Virginia, West Virginia, and North Carolina. Instead, cicadas were emerging “from Georgia, then South Carolina, then Alabama and Missouri,” Kritsky says—areas where an emergence wasn’t expected for another 4 years. “We were seeing what turned out to be a 4-year acceleration of a 13-year cicada emerging after 9 years.”

Only once, Kritsky says, have scientists observed a large group of stragglers successfully splinter off from their brood. A group of Brood X cicadas (the same enormous brood currently serenading much of the East Coast), known to dominate the area around Cincinnati, emerged there in 2000, 4 years early. The cicadas came out in such force that their numbers overwhelmed predators, leaving them to survive and mate. Then, 17 years later the group emerged for a second time, having spread in geographic range, and likely formed a new brood.

Last year, Cicada Safari data showed that not one, but four broods of periodical cicadas emerged off-cycle. Stragglers, which can emerge years before or after their brood, aren’t uncommon. But the scale of their 2020 emergence was. Kritsky says last year’s stragglers may have given rise to an entirely new periodical population.

Without the public, the full scope of last year’s straggler event would likely have been missed. It was far from the beginning of cicada citizen science. In 1843, Gideon Smith, a medical doctor and silkworm cultivator in Baltimore, convinced newspapers to publish his appeals to readers to report emergences of cicadas, which were called locusts at the time. Smith claimed to have ascertained “eighteen districts or families of locusts. If each post master in places where locusts appear will drop me a line … I shall be able to make out a map of each district.” The ensuing mail campaign, arguably the nation’s first large-scale commnuity science, “was quite successful,” says Kritsky, who wrote about Smith in December 2020 in American Entomologist.

“By time [Smith] died, in 1867, he had documented all the known broods of periodical cicadas,” Kritsky says.

Another entomologist, U.S. Department of Agriculture (USDA) researcher Charles Marlatt, used a mail-campaign initiative to map periodical cicada distribution in greater detail. In 1902, Marlatt spearheaded a USDA campaign to send out more than 15,000 postcards soliciting reports of cicadas. The 1000 responses helped him delineate cicada broods by emergence year.

Later, mailed and phoned-in cicada reports went to universities across the country. But the data varied widely in quality and focus, says Chris Simon, an ecologist and biologist at the University of Connecticut, Storrs. She once received a marriage proposal in the mail instead of data.

Kritsky’s app took cicada reporting online. And more people than ever are taking part during the pandemic: Between 2019 and 2020, Cicada Safari’s user base grew by nearly 50%, to 10,000 contributors. This year, with the emergence of the massive Brood X, the app has leapt to 156,000 users.

In 2020, the team added the ability to upload 11-second videos. Although Kritsky and his colleagues have seen many videos of children chasing cicadas, clips of chorusing cicadas have also allowed the researchers to verify the presence of mating populations rather than one-off sightings.

Last year also marked the first scientific paper based on Cicada Safari data. Published by Michael Raupp, an entomologist at the University of Maryland, College Park, Kritsky, and colleagues in The Maryland Entomologist in September 2020, it cross-referenced air temperature with nymph emergences reported through the app. The aim: to improve predictions of cicada emergences using air temperatures, which are far more available than soil temperatures. With a small sample size, the results were inconclusive. Nevertheless, the paper marked another first: using cicada community science to answer nuanced questions, rather than simply reporting distribution and emergence dates.

Raupp thinks data from the public could help address enigmas that still surround cicadas, such as why stragglers break off from broods. One idea: Warm winters may cause cicadas to emerge in the wrong year. Detailed data on weather and emergence patterns might support that idea—or a different hypothesis about why some cicadas miscount the passing years. (In a further mystery, stragglers tend to miscount by specific amounts, either 1 or 4 years.)

Without understanding the factors influencing emergence, it will be hard to protect cicadas from threats that include pesticides, development, and climate change. Raupp says development may have done in Brood XI, a 17-year cicada population in Connecticut last observed in 1954. And this year’s group, Brood X, has been in decline on Long Island in New York for nearly 200 years. Cicada Safari reports should help, Raupp says, because researchers can cross-reference them with local records of land use and climate.

Michelle Watson, a retired paralegal who has submitted more photos to Cicada Safari than anyone else, says she has grown addicted to contributing to the community of cicada enthusiasts. Watson, who recently moved to Blue Ridge, Georgia, had never seen periodical cicadas before this year. While looking online for information about whether they were safe for her dogs to eat, she found Cicada Safari.

Even after this year’s cicadas disappear once more, Watson says the passion they’ve sparked for science projects will likely stick around—for her, and tens of thousands of others as well.

source: sciencemag.org