Great question! So what gives with the 17-year cicadas this year?
“Periodical cicadas,” those who follow a 13-year or 17-year life cycle, are unique to eastern North America. They’re fascinating and mysterious animals!
In addition to the mystery of why these noise-making insects have settled on prime numbers for their life cycles, they pose the mystery of why they don’t all emerge at the same time (slthough it’d be pretty spectacular if they all emerged, continent-wide, on the same schedule).
As things stand, periodical cicadas tend to form “broods,” or groups that emerge on a certain schedule and in a particular region. Here’s a map showing where each brood emerges, color-coded by year:
It’s pretty much guaranteed that there’s always a cicada emergence somewhere in a particular year. (And, for the folks in Tennessee, it can feel like it happens every year!)
Here in Geauga County, we most recently experienced a cicada emergence in 2016. Our Geauga County cicadas mostly follow the Brood 5 life cycle, emerging in 2016 plus or minus 17 years.
The cicadas emerging in 2021 follow the Brood 10 life cycle. (Get it? It’s five years after Brood 5. Those entomologists know what they’re doing!)
The Brood 10 emergence in 2021 includes most of western Ohio; the Cincinnati region is always a cicada hotspot! However, this means we’re not expecting a mass cicada emergence in eastern Ohio in 2021.
Since Brood 10 also includes the New York City metropolitan area (the epicenter of noise-making humans), there are a lot of nationally focused reports to the effect of “The Cicadas Are Coming!” Subtleties of geography can easily be lost when a story is shared online; the fact that cicadas aren’t emerging everywhere this year is one such detail.
(To be fair, though, the Brood 10 emergence is one of the most widespread of all the cicada groups, so if you’re going to trumpet them nationwide, 2021 is the sensible time to do that.)
With that out of the way, we can ask the fun question: WHY?
Why is there a line in central Ohio that separates cicadas into two different life cycles separated by five years?
Biologists are still trying to puzzle this out, but one proposal makes a lot of sense: In every brood, there are always some stragglers who emerge in the off-years. One possible reason is because the trees they feed on experienced some odd weather conditions that made the cicadas “miscount” the number of sap runs. Whatever the reason, if enough stragglers emerge in a particular year, they can start a new brood following a new cycle; if they turn the tide, their region might become an independent brood.
Or, maybe not! There are plenty of other hypotheses about the mysterious origins of the cicada broods. The University of Connecticut has a great website summarizing the state of cicada-brood research.
Be sure to look and listen for any stragglers this year.
Thanks for your question!
-Naturalist Chris Mentrek