How do cicadas know what season it is?

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1:05Hello and welcome to CrowdScience from the BBC World Service. I'm Anand Jagatia and to get us started this week, I want to take you back to the middle of summer 2025. It is a lovely warm day, just a little bit of moisture in the air, nice and humid, very pleasant day. Where I am in the UK, people are enjoying the sunshine. Planned a big family barbecue, loads of us round at my house, and we had all the food prepared, ready to go out and start cooking. But unbeknownst to many of them, a swarm is about to emerge. What I didn't know that day is that the weather was going to be all about the ants.

1:48Nice flying ant situation happening in our garden right now. That disappears to be the epicentre. It was Flying Ant Day, so that was a nice surprise. Yep, it's Flying Ant Day, which is when thousands upon thousands of winged ants emerge from their nests as part of a huge mating ritual. Sorry, they're flying all over me. Oh God. At first you think it's just one fly, and then you realise that actually you are being completely attacked from all directions. Once all the flying ants have taken off all the other ground ants, we need to find a hole so they can go back in it.

2:36Despite the name, it's not actually a single day. But for a couple of weeks in June or July, you can see ants taking to the skies, ruining picnics wherever they go. Oh, this isn't going to work. Lots of kids, lots of family members trying to eat whilst these little bugs flying around us. But it's not all bad. For many people, these winged beasties mark the start of summer. It is a happy day, after all, because it signifies things getting really hot and nice. The ants tend to time their mass flight to coincide with warm, fine weather. But this seasonal emergence isn't unique to ants. It happens with many different species all over the world.

3:18And that's what this week's question is all about. It was sent in by listener Ryusuke from Japan, who currently lives in the UK. So we thought we'd go and visit him in person. Lovely to meet you, Rio. Thank you for inviting us down to Bournemouth. Yeah, thank you for coming to Bournemouth. The sun is out. Weirdly, the moon is also out. Yeah, we've come down to the beach. It's beautiful here. Yeah, it is beautiful. I like the sound of the waves and I like the sound of seagulls. So, Ryo, what is your question for CrowdTimes? So my question for CrowdScience is how do cicadas know what season it is now?

3:58So cicadas are insects that you can easily find in Japan in the summer, and they always start singing at the beginning of July, always. When they sing, I always find it hotter than usual. Right now, you know, we have the problem of global warming and the temperature just keeps going up every year so I just wondered how they can know what season or weather or temperature it is. It's a fascinating question. So are cicadas a big deal in Japan? Yes, it's kind of a part of our culture as well. So you know when they film some movies or tv3s set in summer they usually use the sound of cicada in the background.

4:43Oh really? It's true. Japanese TV shows often have a background track of singing cicadas.

4:55For Ryusuke, the association between cicadas and summer is deeply ingrained. I don't know if children today are doing this, but I personally went out to the park and just caught them. I think it's a kind of activity, summer activity, the Japanese people do. Catching cicadas. How cool. While we're researching, is there anything else that you'd like us to find out? Yeah, I also want to know, how do they spend time in underground, like I said. How do they eat food, or how do they drink water, or how do they communicate with each other? Awesome questions. Okay, so we'll try and find out a bit about the different species, what they do, how they behave, and I guess, yeah, how they sense their environment.

5:44Thanks, Raiske. To answer your questions, we're going to take a deep dive into the lives of cicadas, large winged insects found all over the globe that are famous for the noise they make.

5:59Raiske mentioned that they spend part of their lives underground. What are they doing during that time? How do they survive? and ultimately how do they time their emergence with the seasons? Before we get into all that, let's cover the basics. What is a cicada?

6:19There's more than 3 ,500 species of cicadas in the world and they're found on all continents except Antarctica. This is Professor Chris Simon from the University of Connecticut in the US. She's a world expert on cicadas. Some live in grasslands, some live in forests, some live in rainforests, some live in temperate forests. They're relatively large for an insect, although some are quite tiny, about the size of your fingernail, but others can be several inches long. Something all cicadas have in common is their singing. Only the males sing. They have these membranes on the sides of the abdomen that are ribbed.

7:04And the ribbed membranes are attached to large muscles that cause the membranes to vibrate very quickly. And the males tend to have more or less hollow abdomens, which resonates and makes the sound a lot louder. And something else they have in common is that they spend nearly all of their lives underground. You may have heard of certain species of North American cicada, which stay below the surface for a really long and a really specific amount of time. Those are the 13 and 17-year periodical cicadas. They're in the genus Magicicada, which actually was named Magicicada because the person who named them thought they were magic.

7:45So these cicadas live underground for 13 or 17 years, and then when they reach a certain size and a certain maturity, they come out of the ground because they all come out of the ground within a few nights of each other. But the cicadas in most parts of the world, including the ones Raiuske grew up with, are annual cicadas, meaning that some emerge every year. And these are the ones we're going to focus on first. I want to see and hear some of these critters for myself, although right now it's winter in the Northern Hemisphere, which means they're currently underground. But I'm travelling to Spain to meet a scientist who says he can show me the next best thing.

8:37My name is Peter Pons and I am an associate professor at the University of Girona. We've come to talk to you because one of the animals that you study are cicadas. We've come to a field site where you've been working for over a decade. I mean, the view is incredible here, mountains carpeted by pine forest. Could you just tell us where we are? Now we are in the municipality of Darnius, in the Ampor D 'Ar region in the north of Catalonia. This is a very nice area in the landscape and the natural beauties. Although the landscape is silent right now, this is a recording from the same spot just a few months earlier.

9:20So could you tell us a bit about the kinds of cicadas that you get in this region of Spain, in Catalonia? Yeah. Well, in Catalonia we have 13 different species of cicadas. There is one species that is the most evident, is cicada orni. We start hearing them in May, June, and until September, more or less. And this specific species is mostly from July and August is the peak. Perry comes here throughout the summer to collect something the cicadas leave behind as they emerge, their skin, which they shed as they change into their adult form. You've just pulled out a jar from your pocket and it looks like it's full of, well, I don't know, look a bit like peanuts, but what have you got in here?

10:06These are the skins of this species, cicada orni. these forest species. Wow. So they are a little bit like paper, no? Paper. So they're kind of like the size of maybe a big peanut. And yeah, they're just like hollow, translucent. They look like ghosts, like tiny paper ghosts of cicadas. Because it's just the shed or the skin of them. These sort of skins, these little, you know, shells that they leave behind, which look like little models of them, they're really important in your research, aren't they? Yeah. We have developed a method. So what we do is working on a pathway, collecting and counting these skins.

10:47We can compare between different areas and between different years and know how the abundances of cicada populations are changing across time, across space. So could you tell us a bit more about the life cycle of cicadas? Females go where males are singing. They mate, they are going for egg laying in a plant. And more or less two months later, the eggs hatch and the small larvae fall down to the ground where they dig a hole. And if they are lucky enough, they find the root and they start feeding on the root of a plant. Our species can be between four and ten years below ground. If there happened to be some underground now around us, and if we could have a magic camera and see under the ground, what would we see?

11:41Are they in big colonies? Is it like they're on separate routes? How does it work? Yeah, they are solitary. And when they find a route, they dig what is called a feeding chamber. And this is how cicadas live for most of their lives, enclosed within their feeding chamber. a small hole in the dirt just bigger than their body that they've dug around the root of a plant. In fact, Pere says there's a good chance we're standing on top of one right now. Probably there are some of them very close to us, but probably they are deep, half a metre or one metre even, because they only go close to the surface when they are close to emergence.

12:26OK. That's why it's very difficult to find them. So what would be the chances if we had a shovel and we decided to try and dig a meter underground, like underneath this tree here, would we find baby cicadas or would it be very rare? I think it is probably not happening. Have you ever seen any nymphs underground? Yeah, but very few times in my life as a scientist I have seen only a few by chance, but there are other scientists that are working and specialised in studying them. when they are living below ground. So, Rayuske wanted to know what cicadas are doing underground. Perre and Chris have introduced us to the lives of these strange creatures, how they feed for years on the sap from plant roots, slowly growing into adulthood in their subterranean burrows.

13:18Until, eventually, they emerge, cast off their skins, and start the cycle again. But Raisuke also wants to know how they sense when the right time is to come out.

13:34In Japan, where he's from, there's such a signature of the start of summer that if cicadas are late, as happened in 2025, people get worried.

13:47Japanese people believe that once the rainy season ends and the hot summer arrives, they will start hearing the songs of cicadas. Therefore, even on continuously hot days, if they do not hear these songs, some people become anxious, wondering if something bad is about to happen. This is Yasumasa Saisho, a mathematician at the University of Hiroshima. But he also has a hobby, running a website all about cicadas.

14:18For example, they fear that a major earthquake might occur. or that abnormal weather might set in. Even people who don't particularly like cicadas feel that something is off if they don't hear their songs, because summer is simply the season where cicadas are supposed to sing. Saisho was interviewed by news networks in Japan when the cicadas failed to show up on time and people became rather superstitious.

14:47Many people found summer without the sound of cicadas unusual, and posted about it on social media. Some even contacted museums for information and the topic was picked up by newspapers and broadcasters. However, by the end of July, cicadas began appearing as usual in many places and the commotion finally died down. So it seems cicadas are pretty consistent in the timing of their appearance. But if they're underground, how do they keep track of the seasons? That's what we'll be looking at next. This is summer at its peak. Whole Foods Market Summer Fruit Fest is your invitation to eat the season.

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16:24You're listening to CrowdScience, the show that emerges every week like clockwork with answers to your science questions. In this episode, we're answering Ryusuke's question about cicadas. So far, we've looked at their life cycles and how they can survive as juveniles for years underground. To learn more about how they sense their environment, back in Girona in Spain, ecologist Pere explains what triggers them to dig their way to the surface. We can monitor temperatures with devices like this. OK, so should we stick it in the ground and see what the temperature is? Pere plunges a soil thermometer into the ground, a metal skewer with a kind of screen on the top.

17:08So it's 11 degrees, it's saying, or 10 degrees actually. Temperature is a pretty good marker of the time of year. Now here maybe we were at 13 or something like this. But at night we can be here at 2 degrees. Because even though it fluctuates quite a lot above ground, below ground it's much more stable, building slowly and steadily from winter towards summer. So we could expect in summer the soil temperatures to be 20 or 25 degrees, so it's the moment for them to emerge. But how to know the specific moment when all your mates are also outside, when the sun is hitting the soil, which is a certain temperature, then they know they have to emerge, to go to the surface, and then everything starts again, all the cycle starts again.

18:01So the cicadas that Pere studies in Spain are waiting for a specific temperature threshold before they emerge. But for other species of cicada who live in temperate zones or places where there's only a wet and dry season, moisture might be more important. For example, according to one study, there are some periodical cicadas in the USA that only emerge when a certain amount of rain has fallen. 1 ,181 millimetres of rain, to be precise. And if there's any less than this, you don't get any cicadas. But there's another aspect to all this. How do they know when it's the right year to emerge? As juveniles, they're completely isolated from the outside world, apart from through the plants they feed off.

18:49So could they be getting information about the passing of time from the food they eat? So come in. Thank you. You can smell the tomatoes. Yeah. I prepared some tomatoes. To find out more, I'm meeting up with Mark Valls, a plant geneticist from the University of Barcelona, who's showing us some plants in his greenhouse. Here it's heated and everything, so normally you can produce tomatoes in winter, which is very interesting. We're taking one back to his lab so that we can cut it open and see the sap that cicadas feed on. So let's try with the younger plant. Mark cuts the stem in half, and at first, nothing happens.

19:33Now here, here it comes. But then tiny drops of liquid start to appear where the stem's been cut. Oh, OK. You can really see it better. So on the tip there of that... Oh, you can really see it bubbling up. OK. This is what cicadas survive on. It's a fluid that comes up from the roots called xylem sap. That's the stuff. So what is in that fluid? Well, in this fluid normally you have xylem sap. So xylem normally, by textbooks, is supposed to contain water and inorganic salts from the soil. It also contains low amounts of low concentrations of sugar and amino acids. All crops have xylem, like all plants do have this.

20:17This is essential in plants. Does the composition of the fluid change over the season? So like as the plant kind of moves through winter, summer, does it change? Yeah, it can change a lot over seasons and it changes especially, for instance, when you have trees in winter, trees that lose their leaves and where this xylem often gets more concentrated in sugars in winter so that this protects from freezing. And for instance, this has been studied by the industry of maple syrup, because maple syrup is basically concentrated, highly concentrated xylem sap. So if cicadas are feeding off xylem sap, and the composition of that fluid changes with the seasons, could that be one way in which the insects track what's going on above ground?

21:12The cicadas seem to be using these seasonal changes in their host plant as a way to keep track of how much time has passed. This is Rick Carbon, professor of ecology at UC Davis in the USA. He studies periodical cicadas, the ones that only emerge every 13 or 17 years. Rick's done experiments which seem to show that cicadas are using xylem sap to track the passage of time. But it's not an easy thing to demonstrate. First, you've got to find a juvenile cicada. It's a little bit like looking for a needle in a haystack. You can find locations where they were known to be quite abundant and dig in those particular spots.

22:02You might remember earlier that Pere in Spain said he's only seen a couple of juveniles in his entire career that he found by accident. But Rick has become something of an expert in hunting them down. He'd go out with his shovel, mark out the place they emerged the year before and start digging. Kind of dig carefully, sort of follow the path of roots. You can find cicadas attached to those roots. Once you've dug one up, then you've got to get it to the lab. They're pretty happy living inside of potatoes. Yep, potatoes. Apparently apples can also work. If you manage to transport them safely, you can introduce them to a new host.

22:46Rick decided to use peach trees because of some other work that had been done on those plants at UC Davis. Somebody here had done some work with peach trees, getting them to go through their annual cycle and in fact produce peaches in the wintertime. If you can get trees to produce peaches around Christmas time, they're worth a lot more money. Researchers have figured out how to get peach trees to go through two annual cycles in a single year by manipulating the temperature and the light regime that the trees experienced. This is the really clever bit. By growing peach trees in a special room where light and temperature are artificially manipulated, you can force them to go through two annual cycles and two cycles of xylem sap changes in a single year.

23:41In the springtime, the tree is moving sugars and other resources from the roots where they have been stored over the winter up to the shoots. Rick calls these cycles nutrient pulses, and his theory was that they could be the ticking clock that cicadas use to time their emergence. To test it, he placed 15-year-old juvenile cicadas onto the roots of peach trees and waited. The cicadas were meant to emerge after 17 years, but Rick was hoping he'd be able to trick them into emerging a year early. The cicadas that were on those trees that had gone through two annual cycles had cicadas emerge from them after 16 years.

24:29Those trees that we didn't expose to this altered regime had their cicadas emerge normally after 17 years. So that provides some evidence that the cicadas are keyed into changes that are occurring in their host trees.

24:52Rick's experiment seems to show that cicadas are using xylem sap nutrient pulses to keep track of time, because if you speed up the pulses, they think time is passing more quickly. But this raises another question. The xylem sap could be providing the signal that cicadas use to count annual cycles. But how are they actually keeping track of how many years have passed? We think there could be some kind of internal marker that accumulates over time because this has been found in other organisms like bamboos. This is biologist Chris Simon again, who has a theory on how cicadas may be counting. And they're monitoring the passage of time by changes in the plant that they're feeding on.

25:38So after the passage of a year, then a marker will be attached at the end of that year, and then the next year the same thing. Exactly what this internal marker might be is still unclear, but Chris has an idea. We think that they could be doing this by modifying their DNA. So DNA has instructions for all aspects of life. But in order to keep track of the passage of time, they somehow have to add markers to the outside of the DNA. We normally think of DNA as a code, with instructions for building an organism and sustaining life. But DNA can also be modified by adding chemical tags to it. And maybe this is the counting mechanism that cicadas are using.