In 2021, more than 125,000 people downloaded Cicada Safari to photograph Brood X; by May 17, they had submitted over 50,000 photos and videos for mapping

In spring 2021, billions of cicadas broke through the soil all at once in 15 states across the US. By May 17 that year, 125,000 people had downloaded Cicada Safari, a free smartphone app designed to track the event, and had already submitted more than 50,000 photos and videos. According to Gene Kritsky's own account of the emergence, published in American Entomologist, this was the 19th recorded occurrence of this particular brood since its first documented sighting in 1715, though the "Brood X" name itself is more recent, dating to a Roman-numeral classification system entomologists introduced in the late 1800s. That long-running tradition continued in 2021, but the tool people used to document it changed.

What is Brood X and why did everyone care?

Brood X cicadas spend 17 years underground before emerging together, mating, singing, and dying within a few weeks. The first written report of this brood is from 1715, when a Philadelphia pastor named Andreas Sandel noted the emergence in his journal, Kritsky’s paper says. The study also points out that people have been crowdsourcing the tracking of these cicadas for longer than most people realize; it credits Gideon B. Smith with collecting public sightings as far back as 1851. The 2021 emergence was a rare chance to witness Brood X, which will not emerge again until 2038.

Meet Cicada Safari, the app that makes phones into research tools

Cicada Safari encouraged people to get out, locate a cicada and take a picture or short video. The date, time and location were auto-tagged on each submission. Kritsky’s team then went through the photos one by one to make sure each was a periodical cicada and not just some random bug. Kritsky said the photographs and videos submitted to the map were like voucher specimens, allowing researchers to verify the observations and turn public sightings into usable data.

The numbers behind the hype

The app had more than 125,000 downloads and more than 50,000 submitted photos and videos by May 17, 2021, according to Mount St. Joseph University’s own tracking of the rollout. That’s a lot of data for one insect event, and much of it was gathered by people on walks or in their backyards. Kritsky’s team first tried the app on smaller broods in 2019 and 2020, giving them a chance to practice before Brood X, the largest of all the periodical broods, made its 2021 appearance.

How photographic evidence changed the game

Public sighting reports aren't new in cicada research, but they used to have a catch: the 2012 report on the rise of South Carolina's Brood XIX, a different, 13-year periodical brood found mainly in the American South, unlike the 17-year Brood X of the Mid-Atlantic and Midwest, published in the Florida Entomologist, utilized written reports submitted by citizens through university websites, not photos. The project results were still useful. It discovered periodical cicadas spreading out more widely from their emergence sites than previous research had indicated, along with a mixture of excitement and frustration among residents. But written reports are harder to verify than a photo with a location stamp, which is precisely the gap that Cicada Safari's photo-first design was developed to close.

Not all states told the same story

Brood X did not look the same everywhere. Research led by Evan Lampert and colleagues, published in Environmental Entomology, combined field searches with public reports and citizen science data to document the 2021 emergence across nine counties in northern Georgia, the brood's southernmost edge. The study found that cicada populations were patchy, with densities varying greatly from spot to spot. They also found that remains of female cicadas occurred less frequently than those of males and were more often dismembered, a pattern that the researchers associated with differential predation on females. Extra eyes on the ground are hard to come by for a small research team to gather local detail like this.

What Cicada Safari adds to cicada research

Brood X won’t return until 2038, but the design that Cicada Safari popularized, a photo, a GPS tag, a human check, keeps getting reused for other broods and other species. It’s a low-effort way to add something real to a research record for anyone already documenting daily life on a phone. No lab coat required. Just a working camera, some curiosity and a few minutes looking closely at something most people usually ignore. The next time you hear cicadas start their chorus, or spot a weird bug in the garden, it’s worth remembering what happened in 2021. One image from a person’s evening walk once assisted scientists in tracking billions of insects across half a continent.