Around this time of year, Marianne Alleyne hosts dozens of houseguests in her basement. Far from using camping equipment or cots, they sleep upside-down, clinging to a curtain. The entomologist at the University of Illinois Urbana-Champaign has collected cicadas, those bizarre and misunderstood cyclical insects, for four years.
‚ÄúIn Illinois, we have 20 species, and hardly anything is known about them,‚ÄĚ Alleyne says. ‚ÄúWe know very little about what they‚Äôre doing underground.‚ÄĚ
Cicadas have a longstanding reputation as loud, swarming pests that keep obnoxiously particular schedules. In the United States, they got a bad rap from the beginning, as early colonists misidentified these clouds of emerging cicadas as locusts. ‚ÄúThey were thought of as a biblical plague,‚ÄĚ says John Cooley, an assistant professor in residence at the University of Connecticut. That impression has been a lasting one: a group of cicadas is still referred to as a plague or a cloud. ‚ÄúThe question I get the most is ‚ÄėHow do I kill them?‚Äô‚ÄĚ Cooley says.
Chris Simon, an entomologist with more than 40 years of experience working with cicadas, says that feeling has changed‚ÄĒsomewhat. ‚ÄúSome people freak out,‚ÄĚ she says. ‚ÄúBut the other half…they take their kids out, they go watch [periodical cicadas] come out of their shells. They think it‚Äôs amazing.‚ÄĚ As another group of cicadas awakens in some U.S. states this spring, experts still have much to learn about them. What we do know, however, is that they are delightfully weird, and researchers across the sciences are studying these creatures to answer big human challenges.
Cicadas spend the majority of their lives underground. They spend years developing into adults before they can emerge to sing, mate and lay eggs. For a majority of the nearly 3,400 cicada species, that emergence happens every two to five years and can vary from cycle to cycle. The strange periodical cicadas, on the other hand, are very different.
Periodical cicadas like Magicicicada spend 13 or 17 years underground, and millions of them surface together. To make sense of it all, biologists classify the periodicals into one of 15 existing ‚Äúbroods‚ÄĚ based on their species, location, and‚ÄĒimportantly‚ÄĒwhich years they emerge. This year, for example, Brood IX is emerging in North Carolina, West Virginia and Virginia for the first time since 2003.
Once cicadas do emerge, the sheer volume can be overwhelming. Some people wake up to find millions of cicadas blanketing nearby cars, trees, and houses. According to Cooley, when male cicadas sing in a full chorus on a hot sunny day, they immerse you in sound from every direction. ‚ÄúIt‚Äôs the most unusual sensation,‚ÄĚ he says. Many species sound pleasant, but the periodical cicadas ‚Äúare like a jet engine or a buzz-saw.‚ÄĚ Only a handful of weeks after emerging, the chorus fades away with the cicadas. They leave behind only calories for their predators, nutrients for the soil, and eggs destined to repeat their multi-year cycle.
But why do cicadas emerge in 13- and 17-year cycles, anyway? One hypothesis with much buzz among mathematicians is that it‚Äôs because both numbers are prime; the theory goes that the cycles prevent specialized predators from springing up. Cicadas are easy prey. They‚Äôre not hard to catch, Cooley says, and ‚Äúanything that can catch ‚Äėem will eat ‚Äėem.‚ÄĚ But predators, such as foxes or owls, whose populations cycle up and down every one to ten years can‚Äôt sync up with such irregular prey.
Cooley sees the merits of the hypothesis but is skeptical. Of the thousands of cicada species, only a handful are periodical. If pressure from predators was exceptional enough to make these species periodical, then why aren‚Äôt all cicadas periodical? He says we just don‚Äôt know.
‚ÄúThis work has been characterized by a hell of a lot of surprises,‚ÄĚ Cooley says. ‚ÄúEvery time you come up with a great idea for why [cicadas] are periodical, it‚Äôs pretty easy to just blow a hole in it. And they do have specialized predators‚ÄĒfungus.‚ÄĚ
In recent years, researchers have unearthed peculiar and sometimes horrifying relationships between cicadas and fungi. Massospora fungi infect cicadas and hijack their bodies. The fungi can even synchronize to the cicada‚Äôs life cycle, staying dormant until the cicada is ready to emerge. Once active, they take over the bottom half of the cicada‚Äôs body while somehow keeping the cicada alive. The infected cicada flies away, spreading spores that infect future generations.
That‚Äôs not the only fungus to wreak havoc on cicadas. Ophiocordyceps fungi also invade the underground cicada. But rather than keep the cicada alive, this fungal parasite coaxes its host to crawl upwards towards the forest floor and die. With nothing in its way, the fungus grows to sprout a mushroom out of the soil‚ÄĒall from within the cicada‚Äôs body.
Despite these wild parasites, cicadas are far from doomed. Recent research suggests some cicadas have flipped the script and domesticated their fungal parasites. Rather than turning into a fungal flowerpot for the parasitic Ophiocordyceps, a few species live symbiotically with the parasite. The fungus gets a home and probably provides the cicada with essential nutrients in return. This has happened in species all over the world, but the origin of this arrangement is a mystery.
Simon says this fungal relationship is currently her lab‚Äôs major project. ‚ÄúMaybe it‚Äôs the fungus that decided to give up its parasitic ways and live inside a comfy cicada.‚ÄĚ
While periodical cicada broods are enormous and remarkably synchronized, once in a while some ‚Äústragglers‚ÄĚ do come out early. In 2017, for example, periodical cicadas clouded the East coast four years early. This May, Brood XIX crashed the party ahead of schedule, too, leaving scientists curious as to whether climate change has played a role. ‚ÄúWe‚Äôve predicted that the warmer it is, the more we‚Äôre going to see these four-year accelerations,‚ÄĚ Simon says. If these 17-year stragglers keep emerging early, they may permanently synchronize to a 13-year cycle.
Or perhaps they will change in more unexpected ways. Because 17-year cicadas are so abundant, their fussiness makes them living, breathing gauges for the environment. ‚ÄúThey‚Äôre sitting down there integrating 17 years‚Äô worth of data on what the forest is doing,‚ÄĚ Cooley says. ‚ÄúAnd if the forest is screwed up or broken, that‚Äôs going to show up.‚ÄĚ
Cicadas develop differently in cities, too. In 2018, a group led by DeAnna Beasley at the University of Tennessee-Chattanooga showed that urban cicadas grow larger. Urban areas use more fertilizer, and their concrete and population density turn them into ‚Äúheat islands‚ÄĚ that can be 5 degrees warmer than rural areas‚ÄĒstimulating conditions for these insects. (Cicadas develop faster with more warmth and nutrients.)
But it‚Äôs not yet possible to conclude how (or if) climate change threatens cicadas. Since historical data isn‚Äôt as reliable as current data‚ÄĒCooley says that scientists are essentially still establishing the starting point. ‚ÄúSo if we want to be able to consider these to be indicators of forest health, we‚Äôve got to do the legwork to figure out what normal is.‚ÄĚ
Learning from cicadas
Scientists have been looking to cicadas to solve human-sized problems. That‚Äôs because cicadas‚Äô late-life wings are covered in a natural engineering marvel: minuscule uniform nanopillars that repel water, kill bacteria and self-clean. The germ-killing wings inspire chemists and engineers who want to harness these properties.
Some try to design these nanopillars as glare-free, self-cleaning surfaces for solar panels. Others, like Susan Kelleher, a chemist at University College Dublin, were captivated by the antibacterial surfaces. ‚ÄúControlling cell behavior is not only so interesting but essential for biomedical science,‚ÄĚ Kelleher says. ‚ÄúThe next step is to translate what we learn from the natural world, into a scalable and manufacturable material.‚ÄĚ
For years, engineers have focused only on the dimensions of the wing patterns. Recently, though, Marianne Alleyne‚Äôs team of biologists, chemists and engineers looked deeper. They published evidence that specific chemical compounds secreted by cicadas are essential to building and maintaining those ingenious nanopillars. The work shows that for those seeking to design technology with cicada-inspired antibacterial traits, it‚Äôs not enough to mimic what the cicadas look like‚ÄĒthe secrets lay deeper. Revealing those secrets, Alleyne says, means working with biologists to actually learn how these mysterious cicadas build what they build.
‚ÄúSometimes the engineers can go like, ‚Äėwe can make this better, we can do it in a clean room‚Äô,‚ÄĚ Alleyne says. ‚ÄúBut insects can make this material out of nothing, right? Maybe we can be inspired to do it that way.‚ÄĚ
When she goes out to collect cicadas, Alleyne makes a point to bring the engineering students along. All the collected nymphs wind up in Alleyne’s basement. Overnight, they inch their way up the curtain and spread their wings. ‚ÄúNow and then, one of them mysteriously disappears, and that‚Äôs when my family is not happy with me. ‚ÄĚ Alleyne says. ‚ÄúBut it‚Äôs all for science.‚ÄĚ