As billions of cicadas emerge after 13 years underground, St. Louisans are noticing something mysterious: It seems this brood is avoiding big parts of the city.
Over the weekend in Tower Grove Park, just a few cicadas flitted through farmers market crowds. In Grand Center at the St. Louis Public Radio studio Tuesday, a lone cicada made a solitary clicking sound from a tree. Residents on social media have been noting similar experiences around the city.
Meanwhile, in St. Louis County last weekend, hordes of cicadas swarmed in trees outside Chesterfield restaurants and filled Wildwood’s suburban streets with chorusing — the high-decibel undulating sound the cicadas make as they screech for mates.
Experts who study cicadas and insect ecosystems have some ideas about why this city-county divide may have formed. The most popular theory is simple.
“If there are no trees around, you're not going to have cicadas there,” said Kasey Fowler-Finn, an associate professor of biology at St. Louis University who studies insect communication.
Cicadas depend on trees at multiple points in their life cycles; first, they find mates by screaming from high perches, then the females lay their eggs in tree branches. Later, the larvae burrow into the soil to feed on tree sap from roots as they mature underground.
“Trees are crucial habitat for cicadas,” said Allison Roth, who studies ecology and evolution with a focus on animal behavior at the University of Missouri.
With fewer trees, fewer cicadas emerge, meaning birds and other predators can more easily eat a large chunk of the population. It doesn’t help that cicadas are slow crawlers and clumsy fliers. “They're really excellent fodder for predators,” Fowler-Finn said.
This all results in fewer cicadas reproducing for the next cycle. But trees aren’t necessarily the whole story. Some local parks with plenty of canopy still aren’t seeing robust cicada broods. So local scientists are setting out to answer some questions about where and why cicadas emerge.
Mapping the brood
Though their areas of research typically focus on other insects — namely wasps, flies and tarantulas — biologists Aimee Dunlap and Sara Miller have caught the cicada bug this summer.
“When you have an animal that only emerges every thirteen years, that's a long timescale,” said Dunlap. “And it's a long timescale if we think about patterns of development in a city or in an urban area.”
That’s why the two professors at the University of Missouri-St. Louis’ Whitney R. Harris World Ecology Center are working to map cicada density in the St. Louis region.
“You can get quite different numbers of cicadas block to block,” said Miller.
While they’re working to personally get out to as many cicada hot spots as possible, the researchers are also asking for community participation.
“The more observations we have, the better our dataset is going to be,” said Miller.
A three-minute sound recording on the Arduino Science Journal phone app can measure the average decibels of cicada noises, and then be uploaded along with location and time information to the Miller Lab’s research page.
“We wanted to do a sampling method that would be kind of easy and fast for people to do,” said Miller. “And then at the same time, use their eyes and ears and look around and see, you know, do you actually see cicadas? Do you hear cicadas as well?”
Dunlap says moments of relative silence are also useful for their research.
“There’s some blocks in the city that have cicadas and some that don't,” she said. “Knowing where they aren't is just as valuable to us as knowing where they are.”
Has development disturbed cicada slumber?
Experts say other factors besides tree cover could be disrupting cicada numbers in St. Louis and even across the country.
One St. Louis subreddit commenter theorized the lack of cicadas on the Gateway Arch grounds could have to do with the recent $380 million renovation that involved extensive landscaping. Roth agrees that in general, if something disturbs the ground where the cicadas are burrowed, that could lead to a dip in numbers.
“These cicadas are living underground, and if there's been too much underground disturbance that's going to kind of disrupt their larval cycles,” Roth said.
That is also true for neighborhoods that have undergone extensive development, said Gene Kristky, a national cicada expert and professor emeritus of biology at Mount St. Joseph University. He studied cicadas in a redeveloped Cincinnati suburb.
“The houses were built in the mid-to-late '90s,” Kritsky said. “And that was a beautiful little suburban development with trees everywhere. Not a single cicada emerged under any of those trees in 2004.”
The UMSL researchers are also focused on this. They’re referencing historical city maps in their work to determine how the changing landscape above ground may have impacted the broods living inches below the surface.
“You could think about 200 years of development in St. Louis. And for any given plot of land, if that soil was super disturbed at any point in those 200 years, you're not going to see the cicadas,” Dunlap said. “We are expecting to see an overlay of decades of development, population density, roads, impervious surfaces, green spaces, historic green spaces. We should see cicadas in Lafayette Square Park, we should see cicadas in Forest Park — we may not see cicadas in Grand Center.”
There’s a bright side to this potential cicada hurdle — “They can recover,” Kritsky said. The bugs will travel up to a mile to find new tree habitats so they can repopulate disturbed landscapes over time.
The climate factor
But there are also larger threats to insect populations around the world, ones that could affect cicadas in the St. Louis region.
“We are experiencing a massive die-off of insects,” Fowler-Finn said. “And this is having a cascading effect for the ecosystem, where bird populations are also declining because many birds rely on insects for food.”
Cicadas and their insect relatives are vulnerable to factors such as habitat loss, saltwater runoff from winter road treatments and chemical treatments for lawns. In fact, Roth said there is evidence to suggest lawn pesticides and herbicides might be responsible for some cicadas emerging with crumpled wings.
At first, some speculated that the patchiness of cicada populations in the St. Louis region could be attributed to differences in soil temperature — cicadas emerge when the soil reaches 64 degrees. But the experts pointed out that in general, cities are warmer than their surrounding suburbs, so that wouldn’t explain this difference.
As with many ecological challenges, climate change is also an underlying threat.
“It seems that their cycle of 13 years is actually being thrown off,” Roth said. “A lot of scientists think that this might be because of global warming, climate change.”
Cicadas tell time underground based on sap flow in the roots of trees, which is triggered by the formation of buds, Kritsky said. But climate change is throwing off that cycle. Recently, there have been more years in which a random warm period in winter prompts trees to bud early, but a subsequent frost damages the buds. When spring finally comes around, the same trees bud a second time, which throws off the cicadas’ calendar.
“It's tricking the cicadas to think that two years have passed in one,” Kritsky said. “And if that happens at a particular time in the cicada’s underground development, it can trigger a four-year acceleration of their growth. They’ll come up four years early.”
Kritsky said a cicada “straggler event,” as this is called, happened four years ago in the St. Louis suburbs with reports of Brood 19 emerging four years early, the same brood that is currently out. On the bright side, he said most of these events don’t affect the main population of the brood. He also found that some of the early emerging cicadas seemed to have a lower rate of a fungal infection that affects cicadas.
The experts say there are things people can do to help cicadas and the birds that eat them. To support insect populations, people can cut back on chemical lawn treatments and go longer between mowing. They can also keep fallen leaves in their yard for longer because leaves are important habitats for many insect species.
But Fowler-Finn wants to take pressure off individuals and focus more on improving the environmental practices of large corporations, from the use of plastics to greenhouse gas emissions.
“We are making a difference,” Fowler-Finn said. “Overall, if we want to keep seeing these wonderful phenomena that we see far into the future, we have to make larger changes at the societal level.”
The cicada songs will fade out in the coming weeks, as Brood 19 finishes its breeding cycle and dies off. But until then, Dunlap encourages those dealing with particularly noisy neighborhoods to embrace what it means for the surrounding nature.
“If you've got cicadas, I would be very happy for you,” Dunlap said.
Hear Abby Llorico’s conversation with University of Missouri-St. Louis biologists Sara Miller and Aimee Dunlap by listening to St. Louis on the Air on Apple Podcasts, Spotify, YouTube or by clicking the play button below.
