Thousands of exoskeletons crunch underfoot as spectators hike to hear the show. They've gathered in the forest to listen to the other-worldly drone of millions of periodical cicadas, walking on the insects' nymphal shells on their way to a good spot.
Once a brood of periodical cicadas gets started "singing" though, you don't have to travel too deep into the forest to hear them.
"I’ve measured intensities at 96 decibels. To put that in perspective, that’ll drown out a jet flying overhead," said Gene Kritsky, an entomologist, naturalist, professor and author. Kritsky has been studying and photographing periodical cicada emergences for over fifty years.
Kritsky said beginning this April and May, billions of nymphal (juvenile) cicadas that have lived underground for over a decade will dig holes to the surface. The insects are part of a brood: akin to a graduating class of students, each brood contains multiple species of periodical cicada that spend either 13 or 17 years underground as nymphs. After the nymphs have dug to the surface, they'll climb up trees and plant stems, molt from their exoskeletons as winged adults, mate, and die — all within a few short weeks.
But those few short weeks will be filled with the deafening, eerie hum of the male cicadas — who make noise to attract females — and a frenzy of mating activity. The males aren't necessarily singing; they make noise by expanding and contracting a membrane in their abdomen called a tymbal. If the males aren't having success making noise from one tree, they will fly to a new tree to try again.
"It’s like a giant cicada singles bar. If a male is unsuccessful, maybe the bar across the street will be better," Kritsky said.
Brood XIII is known as the Northern Illinois Brood: it contains three species of cicadas that are on a 17-year cycle. Brood XIX is known as the Great Southern Brood: it contains four species of cicadas that are on a 13-year cycle. These two broods have not emerged at the same time since 1803. Although the emergence of the Northern Illinois Brood will be mostly concentrated across Illinois and parts of Wisconsin and Indiana, there are historical records of members of the brood in Michigan, and experts say it's possible that they could be observed in the state's southernmost counties this year.
Hannah Burrack, an ecologist and chief academic officer of Michigan State University's Entomology Department, said that those who really want the full emergence experience could take a trip across state lines to Illinois or Indiana to find a good concentration of the insects; Illinois residents have the best chance to hear all seven species at once.
"They’re the sort of insect that can get people really fired up about entomology because you feel like you’re looking at something really unusual and really striking, that doesn’t look like the things you see regularly in your backyard. Especially during these mass emergence events, this is something that can get lots of people fired up about the natural world," Burrack said.
Not everyone will feel excited about the simultaneous emergence of billions of insects: for those who fear a cicada takeover, Burrack said not to worry, but try to appreciate the chance to witness a rare event.
"Embrace the fun and unusual and exciting parts of this. These are the kind of insects we can feel excited about and not concerned about." The cicadas do not pose a threat to people or pets, Burrack said.
So, what do they do? Four main things, according to Kritsky's educational website Cicada Safari:
"Their emergence tunnels in the ground act as a natural aeration of the soil. The large number of adult cicadas provides a food bonanza to all sorts of predators, which can have a positive impact on their populations. The females’ egg-laying in trees is a natural pruning of the trees that results in the tree producing more flowers and fruit in the following year. Finally, after the cicadas die their decaying bodies contribute a massive amount of nitrogen and other nutrients to the soil."
Burrack agreed that the insects have an important place in their ecosystems. "They’re a huge pulse of resources into the environment," she said.
"The most important thing about why they’re good? They got me tenure," Kritsky said.
Jokes aside, Kritsky's enthusiasm for cicadas was clear as he described their life cycle and their contributions to the habitats he studies. He's written a book about these two broods called A Tale of Two Broods: The 2024 Emergence of Periodical Cicada Broods XIII and XIX, and developed a phone app — also called Cicada Safari — for entomology enthusiasts to submit their observations of the broods this year.
He plans to observe and photograph this 2024 event. He may even measure a decibel higher than 96 as the two broods drone together through the forests: seven species united in their purpose to make enough noise to keep their line going.
