Are Firefly Populations ‘Blinking Out’?

17:27 minutes

a forest at night with beautiful glowing trails of fireflies
The forest princess firefly, Hime botaru (Luciola parvula) in Japan. Credit Tsuneaki Hiramatsu

If you take a walk at night during the summertime, you might catch a glimpse of fireflies lighting up the sky. But scientists are learning that these bioluminescent insect populations are vulnerable to habitat loss, pesticides, and light pollution.

Biologist Sara Lewis talks about conservation efforts including Firefly Watch, a citizen science project that maps out firefly populations around the country. She joins geneticist Sarah Lower to discuss how individual species of fireflies create different blink patterns, as well as the difference between fireflies, lightning bugs, and glow worms. Explore photos of these glowing critters below.

a cat looks curiously at a glowing firefly
Who’s curious about fireflies (Photinus pyralis)? Credit: Terry Priest
a close up of a firefly with a glowing body -- the light is slightly blurred as the bug is in motion
A male Big Dipper firefly (Photinus pyralis) in Indiana. Credit: Alex Wild
a firefly in mid-flight, with its appendages spread out
Photinus pyralis, a common eastern U.S. firefly. Credit: Terry Priest/flickr/CC BY-SA 2.0
a lake at night with glowing yellow lights from fireflies
Synchronous fireflies (Photinus carolinus) in Tennessee’s Smoky Mountains. Credit: Radim Schreiber

Further Reading

Segment Guests

Sarah Lower

Sarah Lower is an assistant professor of biology at Bucknell University in Lewisburg, Pennsylvania. 

Sara Lewis

Sara Lewis is a professor of biology at Tufts University in Boston, Massachusetts and author of Silent Sparks: The Wondrous World of Fireflies (Princeton University Press, 2016).

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. One of the key signs that summer is here is the flickering of fireflies. Haven’t we all tried to catch one? Right? Catch them when you were a kid, watch them light up, put them in a jar. This bioluminescent beetle captures our imaginations.

But how much do we really know about fireflies? Do you know the difference between a firefly, a lightning bug, and a glowworm? Do you know that? And are firefly populations in trouble? How healthy are they? My next guests are here to walk us through all these burning firefly questions. Let me introduce them to you.

Sarah Lower is an assistant professor of biology at Bucknell University in Lewisburg, Pennsylvania. Sara Lewis is the author of the book, Silent Sparks, The Wondrous World of Fireflies. She’s also a professor of biology at Tufts University in Boston. Welcome, both of you, to Science Friday.


SARAH LOWER: Thank you so much, Ira.

IRA FLATOW: Oh, it’s nice to have you. It’s one of my favorite topics Dr. Lower, everyone, not just scientists, are captivated by fireflies. But what got you interested in them?

SARAH LOWER: I got interested in fireflies in part from reading some of the work of our other guest, Sara Lewis. So we go out and catch them during summer, put them in our jar, as you said. And we think we know them. They’re so familiar. They’re very cute. But if you look just beyond the surface, if you start to read more about them, you learn that they have incredibly interesting biology. They have toxins. Some of them eat each other. They give each other gifts. So there’s all this really interesting biology that not that many people know about.

IRA FLATOW: Well, I’m glad– yeah, let’s get into that. But first, I want to clear up something with you, Dr. Lewis. What is the difference between a firefly, a lightning bug, and a glow worm? Or are they just different names for a firefly? Different– you know.

SARA LEWIS: Yeah, so here in the US, sometimes people call them fireflies. Sometimes people call them lightning bugs. And sometimes people call them glow worms. And it’s kind of puzzling because they’re not flies, they’re not bugs, and they’re not worms, right? Fireflies are actually beetles. But we don’t call them lightning beetles. I don’t understand English at all.

So lightning bug usually is a term that’s used to refer to the kinds of fireflies that we’re used to here in North America, the ones that fly around, exchanging flash signals. Males fly around flashing, females respond. We call those lightning bugs, especially in the South.

And glow worms is a term that is used more generally around the world to refer to, ha, a bunch of different kinds of insects. But mostly in the firefly world, it refers to the females of some firefly species that can’t fly, right? They’ve got no wings. They can’t fly. They just glow, and they sit on vegetation and they glow for hours, trying to attract flying males. Those are called glow worms.

IRA FLATOW: That’s really– I know the song. Glow, little glow worm.



SARA LEWIS: Great song, Mills Brothers.

IRA FLATOW: We played it last time, and my staff is balking at playing it again.

SARA LEWIS: Aw, man. I love that song.

IRA FLATOW: That’s what I say. Dr. Lower, another common question is where do the fireflies go? In the wintertime when the summer is over.

SARAH LOWER: So generally, the ones that we see in the summer are the adults. A lot of species are only living for about two weeks as an adult. So they have their two weeks, they find a mate, they lay eggs.

And over the winter– well, over the rest of the summer, the fall, those eggs are hatching out into larvae, and those larvae are growing. And they are eating. They’re searching for worms, and slugs, and snails on top of the soil. And they will overwinter as larvae. They’ll pupate in the spring time and emerge as adults in the summer.

Now there’s one group of fireflies that actually has the opposite characteristic, where they actually come out in the fall and they actually exist– they persist on trees. Over the winter, these are the winter fireflies. They’re all in a group of fireflies called Ellychnia. And so we found the first ones out around us this past spring, February 28, after a pretty good snowfall.

IRA FLATOW: All right. So they’re pretty hardy then out there. Our number, 8–

SARAH LOWER: Oh yeah, those ones live a long time.

IRA FLATOW: 844-724-8255. Can also send us a tweet at the SciFri. The fireflies glowing at night seem like a pretty big target, Dr. Lower. I mean, why would you advertise your presence to other predators at night time like that, and do they have natural predators that would like to eat them?

SARAH LOWER: So they do have natural predators. They have– actually, there is a particular group of fireflies called Photuris that will prey upon the males of other species. So females of the group of fireflies Photuris are called femme fetales, and they’ll actually mimic the flash signals of a prey species to lure males in and eat them. And we think they do this to acquire toxins from the prey that they can then put into their own eggs to protect those larvae from predation.

Other organisms that really like fireflies are spiders, so wolf spiders. All sorts of spiders that you go out in the field, and you see fireflies stuck in webs a lot.

IRA FLATOW: I didn’t know that. That’s quite interesting. Dr. Lewis, the common traits of fireflies is not that the adults glow, but that the larva glow in every species. Why would a larva need to glow?

SARAH LOWER: Yeah, so it’s really interesting. People wonder why we call the fireflies that don’t light up at all during their adult lives, why we still call them fireflies. And you’ve just provided the answer, which is that, well, they’re genetically related. And all juvenile fireflies can light up. So even as juveniles, those fireflies light up. They lose that ability once they become adults. Other fireflies keep that ability when they’re adults.

And so this is actually a really cool thing that firefly scientists have discovered just in the past 20 years, is how fireflies actually make their light, and how they control their light, and where it came from in the beginning. And so we’ve discovered that fireflies– first, fireflies’ light producing talent first evolved in that juvenile stage that Sara was talking about, the ones that live underground, they spend a long time in that juvenile stage.

And imagine being a soft-bodied little kind of grub-like thing that lives down in the soil. How are you going to advertise that you’re toxic? So we know that these juveniles contain nasty tasting chemicals. How do you advertise those chemicals if it’s dark, and you can’t have, like, the classic warning colors of orange, and black, and red that we see in the daylight world?

So we think that firefly larvae evolved their ability to make light bioluminescence as a warning signal. It’s a highly visible sign that they originally used to warn off predators, saying, hey, I’m toxic. You don’t want to eat me.

IRA FLATOW: That is kind interesting, but it also answers a question I’ve had from many years ago. I know a scientist who, many years ago, was attempting to make shark repellent out of firefly lanterns.


IRA FLATOW: Is it because of that nasty tasting chemical that you talk about that might be suspected of as being a good shark repellent?

SARAH LOWER: It could be. I really don’t know. The nasty tasting chemical is– yeah, it’s called lucibufagan. And as Sara said, it’s one of the things that wards off most predators. Like birds will not eat fireflies. Toads don’t eat fireflies. Lizards don’t eat fireflies. And so it’s possible–

IRA FLATOW: It’s possible

SARAH LOWER: –it could ward off sharks. I doubt that it evolved for that reason, though.

IRA FLATOW: Yeah. Let’s go to the phones. Let’s go to Joe in Phoenix. Hi, Joe.

JOE: Hi. How’s it going?

IRA FLATOW: Hi, there. Go ahead.

JOE: OK, so a few years ago, I was in Hollywood behind the Hollywood Bowl up in the Hollywood Hills. And I was very attentively listening to a Radiohead concert. And I looked down on the ground, and what I thought was like a glow bracelet or the end of a glow bracelet was a glow worm. It was green. The very tip of it was glowing green. And I know we don’t have glow bugs or these fireflies in California around Hollywood, but there was a glow worm. So what was it?

IRA FLATOW: OK, let me– but well, thanks for the question. Let me bring a tweet in that asked the same question. Maybe we can do a two-for-one. Erin tweets, my family and I recently saw desert fireflies, glow worms at the North Rim of the Grand Canyon. Quite a surprise as we had always understood that fireflies lived east of the Continental Divide. Sarah, what do you say about that?

SARAH LOWER: So which Sara wants to answer this question?

IRA FLATOW: Oh, that’s right. And you both have L’s in your name. Yes.

SARAH LOWER: I’d be glad to give it a go.

IRA FLATOW: Go ahead.

SARAH LOWER: So yeah, so this is one of the myths about fireflies, is that there aren’t any on the West Coast. There are actually fireflies on the West Coast, and there’s a lot of them. Some of them belong to this group that Sara just talked about, which are the day time dark fireflies, the Ellychnia fireflies. And some of them are glow worm fireflies where the females are flightless.

So what you saw at the Radiohead concert was probably a California pink glow worm. And these are the flightless females of species of firefly that lives actually all up and down the coast of California. And it’s really quite common. If you check out iNaturalist, which is a citizen science website, you can see the observations for the California pink glow worm, just like a whole bunch of them up and down the coast. So I’m really glad that you had a chance to see that.

There also are– so just in the last few years, we’ve discovered– scientists had discovered that yeah, there are actually lightning bug fireflies that show up in isolated pockets in the desert southwest in Utah, and Arizona, and other places. And they’re restricted to moist places. So fireflies need moisture in all of their life stages. The adults need moisture. The females need a moist place to lay their eggs. The larvae need moisture.

So if there’s a desert seep or a small stream, in the west, you can actually often find fireflies around those. And the Utah Natural History Museum has a whole firefly website with sightings of fireflies.


SARAH LOWER: The Southwest.

IRA FLATOW: That’s terrific. Here’s a question on Twitter from @photosite, who asks, how is firefly luciferin biosynthesized? I thought about this for about five years, and I still don’t know. The answer–

SARAH LOWER: Sara, I’m going to let you answer that.

SARA LEWIS: I can take that one. So the answer is we don’t know. So fireflies light up in a chemical reaction. And two of the really important ingredients are called luciferase and luciferin. Luciferase is a protein that’s made by the firefly. We’ve actually found it in the genome of the firefly. And it works on its substrate luciferin. And so you need these two ingredients together to make light.

We still don’t know how luciferin is made, so that is one of the burning questions in the firefly world, is where does it come from? Is it made by the fireflies? Is it made by an organism that lives inside the firefly? Does it come from their diet? We do not know.

IRA FLATOW: Wow. I’m Ira Flatow. This is Science Friday from WNYC Studios, answering your burning questions about fireflies. We had a couple people on Twitter comment that they had seen fewer fireflies over the years, Dr. Lower. And I know when we were kids catching them in jars, we thought we were going to get– that we’d get rid of all them that way. Is the firefly population declining over the years?

SARAH LOWER: So just as you said, you think about your childhood and remember more fireflies then. For the longest time, we scientists were just dealing with these reports about firefly declines, these anecdotes from people that you would meet when you go out in the field or give talks.

Just recently, we’ve been starting to get enough long term monitoring data from different scientific groups, but also from citizen scientists. So as Sara mentioned, the iNaturalist platform where you can actually take a picture of an organism that you see, submit it online, and a scientist and expert will identify that picture for you and add it to a growing list of database of sightings. We can use this massive data over time now to start tracking firefly declines.

In addition, there’s another program that is specific to fireflies called Firefly Watch. Sara, would you like to talk about that?

SARA LEWIS: Sure. Yeah, I’d actually also like to talk about some of the– are fireflies really declining? And so it’s a really interesting thing. It’s sometimes yes, they’re declining, and sometimes no. And there are kind of two threads to follow in this story. So just like other insects, fireflies go through population fluctuations, right? They have good years. They have bad years.

We know now that warm, wet springs are really, really good for the juvenile fireflies that Sarah mentioned. So these conditions are responsible for producing a bumper crop of fireflies, like the one that we had this year. I know there are a lot of fireflies in New York, in New York City this year.

But it’s also really important to keep in mind that there are 2,000 different species of fireflies around the world. And some of these are really weedy. They can live anywhere, the adults and the juveniles. But some are very, very picky. They can only live in special places. We call those habitat specialists, right? They only live in particular places. And many of these habitat specialists are the fireflies that are declining because their habitat is disappearing.

IRA FLATOW: Nice, I hear you. All right, I have two questions. I want to see if I can get them in, 30 seconds each. Let’s go to Mike in Akron. Hi, Mike.

MIKE: How are you doing?

IRA FLATOW: Quickly.

MIKE: I was wondering about the synchronicity that some fireflies undergo and they all light up at the same time and what kind of advantages that that may have.

IRA FLATOW: OK, let’s see which Sarah wants to answer that. Sara Lewis? Or I’ll give Sarah Lower a chance on this one.

SARAH LOWER: So the synchronicity, there’s a couple of species that will do this. There’s synchronizing ones, so you may have heard of this species, Photinus carolinus, that’s on the east coast of the US. There’s a really big viewing area in the Great Smoky Mountains, where there are thousands of them.

IRA FLATOW: And what is the advantage of that? I’m trying to get this answer in quickly.

SARAH LOWER: So yeah. So the advantage of that is that all of them signaling together, they can have greater reproductive success. An individual male has greater reproductive success in finding a female if they all signal together. In Malaysia, where they all gather together and signal in a single tree, we think that this is having a specific area where females know to go where males are, and mating can occur.

IRA FLATOW: Quick question, quick answer. How do you attract them to your yard, in 10 seconds, Sarah Lower.

SARAH LOWER: Attract them to your yard. Don’t apply– keep your lights off so we want to minimize light pollution. You want to make sure that you’re providing spaces for their prey, so they’re eating worms, and slugs, and snails. You need some moist nutrient-rich environment with leaf litter.

IRA FLATOW: No pesticides.

SARAH LOWER: Tall grass. They seem to really like tall grass.

IRA FLATOW: All right. There you have it. We’ve run out– so many questions, so little time. You can read more about it in Sara Lewis’s book, Silent Sparks, The Wondrous World of Fireflies. And also Sarah Lower is assistant professor of biology at Bucknell University in Lewisburg. Thank you both for taking time to be with us today.

SARA LEWIS: Thanks for having us.

IRA FLATOW: You’re welcome. If you missed any part of the program, you can subscribe to our podcast. Also you can have your smart speakers asked to play Science Friday whenever you want. Oh, so much great stuff about fireflies and all the other stuff we talked about today. So stuff to talk about over a beer tonight if you’re having one. Have a great weekend. I’m Ira Flatow in New York.

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