For 17 years, they’ve remained hidden underground, biding their time for the right moment to swarm the surface by the trillions. Now, the time has come for the brief but cacophonous party of “Brood IX,” a horde of cicadas in the U.S. South that has not been seen above ground since 2003.
These insects are poised to throw such a rare and raucous rager that the buzzing songs of the males may reach 90 decibels, which is as loud as a dirtbike or a lawnmower. Over the coming weeks, this sound will become a familiar background drone in many parts of Virginia, West Virginia, and North Carolina—the home turf of this particular brood of cicadas.
Brood IX isn’t the only cache of 17-year cicadas set to burst above ground: Brood X, or “the Great Eastern Brood,” is coming to the Midwest and Eastern U.S. next year. That batch will likely number in the trillions.
The bizarre life cycle of periodical North American cicadas has fascinated and perplexed scientists for decades. The species with the most extreme schedules only emerge from their subterranean lairs once every 13 or 17 years, depending on the brood. During that long stretch underground, the cicadas remain in a juvenile “nymph” state and pass the time building mud tunnels, feeding on tree roots, and going through five “instars,” or molting phases.
“The really amazing weird thing is that if you dig them up underground, they are not all growing at the same rate,” said Chris Simon, a professor of ecology and evolutionary biology at the University of Connecticut, in a call. “They’re all counting the years, but some of them are growing much slower.”
“That’s because the environment is always patchy, so some of them have better spots than others,” she continued. “Some of them have better trees, some of them are in a patch with better nutrients, some of them are more crowded or less crowded, and some of them are just healthier genotypes.”
Despite how differently this underground phase can play out for individuals, the cicadas all share an internal clock that runs on a four-year schedule. Because their first instar is atypically short, lasting about one year, their nymph period normally adds up to a multiple of four, plus one.
Once the cicadas sense that this long chapter of their lives is coming to an end, they wait for soil to reach about 18°C (65°F) at eight inches of depth. “It’s commonly in mid-May to the last week of May, depending on how warm it is, because the trigger for the day of emergence is the ground temperature,” Simon said. “The trigger for the year of emergence is that they’ve been counting, but the exact day of emergence is the ground temperature.”
When the time comes, the cicadas at last end those dark days and make a synchronized exit to enjoy a fleeting adulthood under the Sun. Roughly 1.5 million of them can crawl out of a single acre of their habitat, all eager to find a mate and seed the next generation.
Some sections of Brood IX have already started to emerge, and legions more will rise from the depths in the coming weeks. This year, the event may be particularly noticeable, assuming that communities near the insects remain in various phases of lockdown due to the Covid-19 pandemic.
With more people spending time at home, especially in rural areas, the din may be a reminder that the natural world keeps on humming even when human life is disrupted by emergencies like infectious outbreaks.
On the upside, scientists hope that quarantines and distancing measures will leave people with more time to appreciate the rare emergence event—and perhaps even help collect crucial data about this brood.
For instance, Simon and her colleagues heavily rely on crowdsourced reports for their research; her colleague Gene Kritsky, a professor of biology at Mount St. Joseph University, has developed an app called Cicada Safari to track them. If you’re hearing the endless buzz of amorous males, you can use the app to take pictures or videos of cicadas and send it to researchers with location data.
“The crowdsourcing is super-important for us, especially because of Covid-19,” Simon noted. “Right now, our university is discouraging fieldwork, and we would be worried about doing fieldwork ourselves.”
“We really want to get out there,” she added, “so it’s important to have crowdsourcing.”
Males often congregate together to produce “choruses” that seem to be especially enticing to the opposite sex. Once they’ve copulated, the females cut characteristic slits into tree bark to lay their eggs. The adult revelry is intense, but terminal, and the males and females are usually dead before the hottest days of summer. But their genetic legacy lives on once the eggs hatch and the newborn nymphs drop down to the soil, burrowing into it for another 17 years.
Scientists think the insects evolved this unusual periodicity in part to avoid predators. While cicada emergence events offer a feeding frenzy to a wide variety of reptiles, birds, amphibians, and mammals, their sheer numbers may overwhelm these hunters. Even the nymphs are not safe underground, where they are vulnerable to predators such as moles, which is why cicadas must sustain voluminous populations with billions or trillions of individuals.
“Whenever they get to low density, they are in danger of dying because they have this predator satiation strategy, or ‘safety in numbers’,” Simon said. “So if they don’t have large numbers, they’ll get wiped out by predators.”
The brood that is about to go through this amazing ritual is the offspring of cicadas that died in the earliest years of the 21st century. The new generation that they will sow this summer will most likely reappear again in 2037, though climate change may shift this rhythm in the future.
“With global warming, it seems like much more often we see a lot [of 17-year cicadas] that are ready four years early, and they’ll come out as 13 year cicadas,” Simon said. “Even the 13-year cicadas in the South can come out as four years early as 9-year cicadas.”
“If you look at the Cicada Safari map, one thing that’s confusing about it is that a bunch of the cicadas that we’re recording this year are actually four-year-early 13-year cicadas,” she noted.
Brood IX may get some special attention this spring, given that it is coming out during a pandemic. But Brood X, due in spring 2021, will be even more voluminous because it has the greatest range of any 17-year cicada population.
It’s unclear what kind of a world we’ll be in a year from now, given all the chaos and uncertainty of the current moment in history. But if there’s one thing we can bet on, it’s that trillions of cicadas will be singing their hearts out under the Sun for the first time since 2004.