A Co-Emergence of Periodical Cicadas
Matt McGowan: Welcome to Short Talks from the Hill, a podcast of the University of Arkansas. My name is Matt McGowan. I’m a science writer here at the University. Today we get to talk about one of my favorite things in the natural world, cicadas. With us is Austin Jones, an entomologist and instructor in the Dale Bumpers College of Agricultural Food and Life Sciences. This spring and summer, Jones and other scientists will observe an extremely rare phenomenon, the co-emergence of two specific broods of periodical cicadas. This coincidental event happens only every 220 years. Today, Jones will help us understand the rarity and importance of this event. Welcome, Austin. Thanks for being here.
Austin Jones: Absolutely, a pleasure.
MM: I want to back up a little bit and not assume that every listener knows what cicadas are. I think probably most do. But let’s assume that maybe one or two people are listening from outside the South or Midwest. Can you tell us what cicadas are and maybe distinguish them from locusts or crickets, which we also have around here and other insects that we hear in the summertime?
AJ: Well, I love that question because, you know, I’m somebody that grew up calling cicadas locusts, as many people do. And I’m not 100% sure why that misidentification has happened. I would imagine it’s because both locusts, which are more like grasshoppers, and cicadas, which are actually part of a group of true bugs, which includes things like bedbugs and stink bugs and some other well known ones. They both come out in large groups. So I think that maybe that had something to do with it. There’s definitely some differences in the fact that when locusts, those grasshoppers, all come out in mass, they can be destructive. Going all the way back to biblical times, it’s been reported how they can come out in these large mating groups and then they destroy agriculture and therefore food chains and food supplies, etc.. Whereas cicadas, when they emerge in mass, they’re really not agriculturally relevant. They’re not really doing much damage. They’ve been accruing their biomass over 13 or 17 years underground before their emergence. We used to have locust swarms in the U.S. up until about the early 1900s, and then they actually miraculously kind of went extinct. And we really don’t know why. They still erupt in other parts of the world, in Asia, on the African continent. As far as a difference, we hear things in the summertime like katydids, which are kind of like grasshoppers. A lot of them look like leaves. You do hear certain grasshoppers. And then, as you mentioned, crickets. Grasshoppers, crickets and katydids are part of a group of insects called Orthoptura. And they are males, just like with cicadas, that are making noise trying to find a mate. But they do it by rubbing body parts together, whereas cicadas have a special organ embedded into the sides of their exoskeleton that they flex and it creates a click. So I don’t know if you or any of the listeners have ever had an iced tea or a Snapple that has a metal lid on it with the freshness pop, that’s very similar to how the organ is structured on the sides of a cicada. They have muscles in their thorax that make that freshness lid click, and then they’ve hollowed out their entire body to be a Helmholtz resonator to try to make that sound broadcast out as much as they can. Some cicadas can make up to 120 decibels of sound, which is enough to cause permanent hearing damage if it’s going off right next to your ear. Most of the ones in the US don’t get above about 100 or 105 decibels. But one of the real main ways to tell what you’re listening to in the summertime around here is cicadas are primarily daytime callers, whereas when you hear stuff calling in the evening into the night, those are going to be the katydids and the crickets.
MM: It was confusing to me when I started hearing about this co-emergence coming up in the summer of 2024, because as most people who live in the South know, we have cicadas every year. We hear them every year. So I had to do a little bit of reading and educate myself. So what we are hearing every year are so-called annual cicadas. What will happen this summer is slightly different. Species or brood or maybe both?”
AJ: Both.
MM: The periodical cicadas. Can you talk about the difference between these?
AJ: There are about 3,000 species of cicadas worldwide. There’s about 175 or so in North America. There are said to be about two dozen species in Arkansas. Most of those are the annual or dog day cicadas that you speak of. Some of them actually have dog day in their common name. And those are species that live underground for about 2 to 5 years before they come out and emerge. And they’re not all synchronized to come out in the same year or as the same brood. A few of them are going to be coming out each and every year, as they reach maturity. An interesting thing about insects is that their development isn’t just measured in time, like it is for human development. Insects also rely on heat. I’m 40 years old and I measure my time in years. But if I was an insect, if I was raised at a ten degree warmer temperature than I have in my life, I might look like I’m 60. Whereas if I was raised in a cooler climate, I might still look like I’m 20, or be at that maturity level. So most insects mature based on accruing heat. But periodical cicadas have a different mechanism that allows them to somehow count seasons and synchronize between individuals that may be undernourished and not quite ready to go yet. And others that have had really good luck finding food over that 17 or 13 years and they’re ready to go. It’s kind of interesting. It’s almost like I think about the Olympics because they happen in four year cycles. These cicadas, if they’re ready to go, they’re going to all emerge at once. But there’s a few stragglers that are going to come out off sync. So some will come out four years later, some will come out four years earlier sometimes. Also some that will come out one year off sync on occasion. Back to your question, though, about the difference between periodical and annual. Of the species that we see coming out every year in Arkansas, the vast majority are going to be around 2 to 3 inches long, and they’re mostly going to be camo in coloration. They’re going to have some sort of olive drab. They’re going to have mostly clear wings. But there are a few species of cicadas in the state that only get to be a quarter or a half inch long. And you find these more in remnants of prairies and glades and grassland habitats. The periodic cicadas that we’re going to see emerging in the state this year look drastically different from either of those annual cicada groups. They’re kind of mid-sized. They’re about an inch and a half long. They’re going to be mostly black bodied. They’re going to have orange veins in their wings and they’re going to have red eyes. And so they’re going to be real distinct from others in the area.
MM: What’s the difference between species and broods of cicadas?
AJ: Okay, that’s a great question. So broods can consist of multiple periodical cicadas species. And how we got to calling them broods actually comes from one of the first examples of crowd sourcing. As people were trying to put together the first maps of these emergences, they were relying on first hand accounts from postal carriers and people in the field that were out there seeing these things, who had really no idea because many of these species look very similar…black, red eyes, orange wings. They may not have known there were different species there. And so we started naming these emergence events, broods. They started in one year and then that was Brood one. And then the next year, wherever they came out, that was Brood two. And now we’ve moved on up to the number of broods that we have now. We have more 17-year broods in the U.S. than we do 13-year broods. And those are typically more northern, which also still kind of patterns that when it’s colder, it takes longer to develop, type of a model. As opposed to the 13-year cicadas, which are what we have in Arkansas and what we’ll see emerging on the interior highlands, the Ozark and the Ouchita plateaus this year.
MM: So let’s talk a little bit more about what will happen this summer. There’s a co-emergence of the 19 and the 13. So talk about that a little bit more. The, I guess, coincidence, or the rarity of this event, and why geographically, three states are especially lucky or special in this case?
AJ: Absolutely. Those three states a little bit north of us, I think of Indiana, Illinois and Iowa, three out of four of the “I” states have it, it turns out. In those areas, you’re going to see some overlap between the co-emerging broods. So between 19, which we’ll see in Arkansas, and 13 in the more northern reaches. Brood 13 is a 17-year periodical cicada. Ours are 13-year periodical cicadas. For the math junkies out there, 13 and 17 are both prime numbers. And so the only time that you get them coming out is when you have a multiplication year of the two of them together. The last time this has happened was 1803. So this is a 221-year cycle. It sounds awe-inspiring because we already have billions, if not trillions, of these things coming out in each of these broods. And to have two coming out simultaneously, wow, there’s going to be lots, but it’s really going to increase the range of emergence, more so than the density at any one location. So some of those special areas that we mentioned in those “I” states, they may have members of both broods in their counties that are going to be coming out. And that’s really intriguing as well, because for each of the three 17-year species, there is a paired 13-year species, and there can actually be some gene flow that exists between these different species when these broods co-emerge like that. So that’s really just kind of one of the pieces of the genetic puzzle that these things are. I’ve tried to do a little research as people have had interest in this this year in the genetics, and it’s pretty mind boggling. We look back and say these were all one group, 4 million years ago, and then at the last glacial maximum, perhaps it was reduced down to three different groups of 17-year cicadas. And then each of those branched off a 13-year group. But then there are other hypotheses that contradict that, that also have support to them. What we can say about that is that they’re showing us something about how our planet has changed in the last 4 million years, how these broods have shifted based on how climates have shifted, and how moisture has been distributed around the eastern U.S., and how forests have come to be. I think that it is pretty wondrous time to see some of these gene flow events happening that can only happen on a 221-year cycle. And, you know, the fact that these periodical emergences happen is believed to be strengthened by the idea that we all have to come out at once to overcome predators, and that benefits all the different species that are emerging at once. Now, to the average person, when they go out into the Ozark woods this summer, they may just appear to be one species of cicada and how we differentiate them, yes, there are some physical differences. The easiest way is to listen to the pitch of their call. Certain ones will have different frequencies that they call out than others, and they’ve shown that the females of these different species are highly selective to those different frequencies, suggesting why these different species still coexist at the same time in place with these emergences. In 2000, actually, there were some researchers in Arkansas from the University of Michigan that ID’d a new range of call that they ID’d as a fourth 13-year cicada species. So we in Arkansas are blessed enough to maybe have as many as four species coming out with this emergence, whereas some other broods in the country may only be composed of one single species. And actually we have lost some broods over time that no longer emerge anymore. And there are a couple of broods in the northeastern U.S. that they’re looking at as being in decline right now as well.”
MM: Austin, thank you so much for being with us here today. I really appreciate your time.
AJ: I really appreciate it.
MM: Short Talks from the Hill is now available wherever you get your podcasts. For more information, and additional podcasts, visit ArkansasResearch.uark.edu, the home of science and research news at the University of Arkansas. Music for Short Talks from the Hill was written and performed by local musician Ben Harris.
