It's only been two years since we last heard that a large number of periodic cicadas would be emerging from the ground, congregating on tree branches and singing their distinctive mating calls. This year, there's news that more will be coming out again. But every time the story is in the news, it's reported that these insects stay underground for 13 or 17 years. So why do we hear about periodic cicadas so often? Duncan McFadyen asks N.C. State University entomologist Clyde Sorenson.
MCFADYEN: Why does it seem like we get these cicadas all the time?
SORENSON: In North Carolina, as it turns out, there are 7 or 8 different broods. So, I guess, 7 or 8 years out of 17 you can find cicadas someplace. So, if you have cicadas in your yard this year, even though you may have cicadas coming out in your general area in a few years, you won’t see them in your yard.
MCFADYEN: And so we may see 17 year cicadas again before 17 years passes, but we’re not going to see this brood?
SORENSON: Right, this brood, each one of these broods, in science we say they have parapatric distribution, which just means they don’t overlap between the broods. But within a year, within a brood, if you happen to have a house that’s located in a population of these things. For you, the experience is going to be the same---tens or sometimes hundreds of thousands or more of these things coming out of the ground per acre, and in a simultaneous fashion. It’s actually something worth going and trying to find, to experience, because it’s just really an impressive thing to experience.
MCFADYEN: What about it mostly, the sound?
SORENSON: The sound, you’ll see dozens, sometimes hundreds of them hanging off a single branch of a tree. One species in particular has some really bizarre behavior: the males sing, or chorus, together, and periodically they change places like a giant game of musical chairs up in the trees. When you’re in the middle of them, you’re not going to hear very much besides cicadas. They have these beautiful ruby red eyes, and so they’re striking when you see them.
MCFADYEN: And they also sound different…
SORENSON: Right. So, each one of the species has a distinct call. Actually, you can distinguish them pretty regularly. One of them has this distinct whining sound, kind of like a fire engine, a “waaaah, wah, waaaah, wah.” And another has a call that’s more of a series of clicks with a trill at the end of it.
MCFADYEN: So you’re saying if people in Charlotte do come across these, they shouldn’t throw insecticide at them.
SORENSON: Absolutely. These insects are completely harmless. The biggest threat they might pose to us is the females lay their eggs by sawing a small slit into the twigs on trees, and typically the twig beyond that dies. So where there are large populations, you’ll see a lot of dead twigs hanging sort of like forlorn flags on the ends of branches. But apart from that, these periodic cicadas cause no damage whatsoever. They’re completely harmless. They actually ought to be something people appreciate, not fear.
MCFADYEN: So you must be getting excited.
SORENSON: Yeah it’s fun. I had a large time with the 13-year brood that came out two years ago. We had several populations in the woods near Raleigh, and I spent a lot of time studying them, and bringing them home and watching them emerge from their nymphal skins to adult. It’s a really neat thing.
MCFADYEN: Do you kind of like that there’s still so much unknown about cicadas?
SORENSON: Absolutely, we’ve been studying insects for a few hundred years now, natural history for a couple thousand years. It’s kind of reassuring and endlessly fascinating that we still have so many things we don’t know.
The last thing I’d say is if you are fortunate enough to find yourself amongst them, you ought to relish the experience, ponder the experience and marvel at how wonderful it is.
Professor Sorenson encourages people who think they see or hear the periodic cicadas to contact him at clyde_sorenson (at) ncsu.edu. You can read more about periodic cicadas and hear their distinct mating calls here.