A parasitoid wasp on a monarch chrysalis.
Carl Stenoien | University of Minnesota
After a caterpillar goes into its chrysalis, you would expect a beautiful butterfly to emerge. But when this parasitoid attacks, the results are very different.
Monarchs eat milkweed — and that makes them toxic to most predators. But biologist Carl Stenoien discovered one parasitoid that is immune to milkweed’s effects. Listen to hear his story.
Wasps emerging from a monarch chrysalis.Carl Stenoien
• Listen to our episode on monarch butterflies here.
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INTERVIEWER: You are listening to Brains On, where we get curious about the science of everything.
MOLLY BLOOM: You're listening to Brains On. I'm your host, Molly Bloom, and I'm here with one of our producers, Marc Sanchez. Hi, Marc.
MARC SANCHEZ: Hey, Molly. Hey, I want to play a word association game with you, OK?
MOLLY BLOOM: OK.
MARC SANCHEZ: So when I say a word, you just tell me the first thing that pops into your mind.
MOLLY BLOOM: OK.
MARC SANCHEZ: Butterfly.
MOLLY BLOOM: Caterpillar.
MARC SANCHEZ: Wasp.
MOLLY BLOOM: Sting.
MARC SANCHEZ: Parasitoid.
MOLLY BLOOM: What is a parasitoid?
MARC SANCHEZ: You don't know what a parasitoid is?
MOLLY BLOOM: I do not.
MARC SANCHEZ: You know what a parasite is, right?
MOLLY BLOOM: Yes. A parasite--
ROBOT: Parasite.
MOLLY BLOOM: --is an organism that lives in or on another organism. That's the host.
ROBOT: Host.
MOLLY BLOOM: And they use the host as a source of food, shelter. They're super fascinating.
MARC SANCHEZ: Totally. And the difference between a parasitoid and a parasite is that a parasitoid will always kill its host. Parasites don't always do that.
CARL STENOIEN: Hi, I'm Carl. I'm a graduate student at the University of Minnesota, and I study tiny little wasps that lay their eggs in other insects. They're called parasitoid wasps.
MARC SANCHEZ: That's our biologist pal, Carl Stenoien.
MOLLY BLOOM: Oh, right, Carl. I met him at the monarch butterfly lab at the University of Minnesota when we were working on our monarch episode. And if you haven't heard that one yet, check it out.
MARC SANCHEZ: For sure. Check that out. It is quite a fascinating journey to Mexico and back. And those tiny little parasitoid wasps that Carl studies use monarchs as their hosts. They feed on the chrysalis of a butterfly.
MOLLY BLOOM: Chrysalis? I think I need a little refresher.
MARC SANCHEZ: The chrysalis.
ROBOT: Chrysalis.
MARC SANCHEZ: Or pupa--
ROBOT: Pupa.
MARC SANCHEZ: --is that stage in a caterpillar's life that comes after crawling on leaves and before flying off on new butterfly wings.
ROBOT: Pupa is a funny word.
MOLLY BLOOM: This robot is easily distracted.
ROBOT: Also, the plural of pupa is pupae. Pupae is a funny word. Ha ha ha ha ha.
MOLLY BLOOM: Oh, robot.
ROBOT: Apologies.
MARC SANCHEZ: During chrysalis, a caterpillar forms a protective shell around itself, so it can go through its transformation to a butterfly.
[MUSIC PLAYING]
Now, monarch butterflies eat milkweed, which provides them with a natural defense. Milkweed is toxic to parasitoids like wasps.
MOLLY BLOOM: So the monarch would be poisonous if the wasp ate it?
MARC SANCHEZ: True. But there's one type of wasp that Carl studies that seems to be immune to these milkweed toxins.
CARL STENOIEN: All right, so this wasp, pteromalus cassotis, P-T-E-R, like a pterodactyl, it means wing. Pteromalus cassotis is its name, and we really don't know much about it at all, except that we know it will lay its eggs in the chrysalis of certain butterfly species, and it seems like it might be specialized on monarch butterfly chrysalis.
MARC SANCHEZ: So when these wasps lay their eggs, what you get at the end of the chrysalis isn't the beautiful butterfly you would expect.
CARL STENOIEN: Instead of, two weeks later, a butterfly emerging, you'll have 100 wasps emerging instead.
[MUSIC PLAYING]
One female will somehow find a chrysalis in the wild, and she'll spend about maybe a day or two laying eggs into it. There's not much a pupa can do to defend itself because it doesn't have any limbs or any teeth and it can barely move. And once it hardens up, it's kind of just sitting duck. Sometimes, we find more than 200 offspring from one chrysalis.
[MUSIC PLAYING]
MARC SANCHEZ: Molly, do you want to hear something crazy about these parasitoid wasps?
MOLLY BLOOM: You mean more crazy than hundreds of wasps feeding off a chrysalis and flying out when I expected to see a colorful butterfly?
MARC SANCHEZ: Right. OK. Another crazy thing about these parasitoid wasps, there are more species of these wasps than all of vertebrates combined.
MOLLY BLOOM: So more than birds plus fish plus amphibians plus mammals?
MARC SANCHEZ: Yep, combined.
MOLLY BLOOM: That's a lot of wasp species.
MARC SANCHEZ: Don't worry. I'm not trying to scare you.
MOLLY BLOOM: Too late.
[LAUGHS]
MARC SANCHEZ: These things have been around since before we even kept records, and they haven't overtaken us yet.
ROBOT: Attack of the parasitoids.
MOLLY BLOOM: Robot, let's not get ahead of ourselves.
MARC SANCHEZ: OK, my point is that there are so many different species of parasitoid wasps, they can be hard to identify. Tiny differences in size, and color, and shape can be tough to detect, but they're important.
MOLLY BLOOM: So how did Carl do it?
MARC SANCHEZ: This is where his detective work started. The lab that he works in was trying to figure out what might be killing monarchs during the chrysalis stage. So they found them in the wild and brought them back to the lab.
MOLLY BLOOM: And then?
MARC SANCHEZ: And then they waited.
[MUSIC PLAYING]
CARL STENOIEN: Almost luckily, we had one instance of parasitism that first year of the study, and we said, let's go to the books, look it up. It looks like this really common wasp that's used in biological control, pteromalus puparum.
MARC SANCHEZ: But they had to be sure, so they took the next step.
CARL STENOIEN: We sent it to the Smithsonian. They have a large collection of insects to compare it to, and they responded yet probably is pteromalus puparum. We're not 100% sure, but let's go with that.
MARC SANCHEZ: For some people, being close to 100% sure, that'll do. But for scientists, not quite.
CARL STENOIEN: Then we dug into some really old literature and found a paper from 1888. There was this one record of monarch hosts being parasitized by this one wasp. Then we looked a little closer at the catalogs and realized there were a few differences, like the color of the legs were slightly different, and things like that. And so then we realized that not pteromalus puparum. We actually have this other species pteromalus cassotis that no one's paid attention to in over 100 years.
MOLLY BLOOM: So nobody had seen this wasp since 1888. That's pretty amazing.
MARC SANCHEZ: I know. It had been so long that people had actually forgotten all about it. Can you imagine looking at this little teeny wasp that's about as big as the grain of rice, and you're the only person who knows it exists. Think about that for a second.
ROBOT: That is bananas.
MOLLY BLOOM: Bananas, indeed, Robot.
MARC SANCHEZ: Every time I learn something new that nobody else has ever learned, it's a huge rush. I love it. It's a super exciting feeling, and it-- I can't really describe it.
[MUSIC PLAYING]
MOLLY BLOOM: That was Carl Stenoien from the University of Minnesota monarch lab. Thanks for bringing that story to us, Marc.
MARC SANCHEZ: You're welcome.
MOLLY BLOOM: And Thanks, Robot.
ROBOT: My pleasure.
[MUSIC PLAYING]
MOLLY BLOOM: Do you have any questions for us about parasites, robots, anything else? Send them to us at brainson at M, as in Minnesota, pr.org.
[MUSIC PLAYING]
Right now, we're working to find an answer to this question from six-year-old Rupert Kong from Toronto.
SUBJECT 1: How do volcanoes erupt?
MOLLY BLOOM: Did you catch that? He said, how do volcanoes erupt? We'll have an answer for you very soon. Until then, send us your questions. You can also follow us on Twitter and Instagram at brains_on. And there's one more thing we need to take care of before we sign off. It's time for the mystery sound.
SUBJECT 2: Mystery sound.
MOLLY BLOOM: We've been asking you to send us your mystery sounds, and it's been fun hearing what you've come up with and trying to guess the answers. They're pretty tricky. We're going to see if you have an easier time guessing this one than we did. Ready? Here it is.
[MUFFLED NOISES]
SUBJECT 3: My name is Bert Turtle. And I'm 6 years old, and the sound of that was going down the hill at like 25 miles per hour, basically sledding.
[MUSIC PLAYING]
Well, it also sounds like to me is a friction piano. It's like a bunch of notes at the same time, and it sounds like a full melody, like they're just all playing at the same time.
[MUSIC PLAYING]
MOLLY BLOOM: Thanks Barry for sending that really cool sound, the sound of sledding. I don't know what a friction piano is, but it sounds like a cool invention. Good idea. If you have a mystery sound you'd like to share with us, we'd love to hear it. Just send it to brainson@npr.org. We'll play some of the sounds we get on an upcoming episode.
That's it for now. You can always listen to past episodes at our website, brainson.org, or at your favorite podcast app. Thanks for listening.
(SINGING) Ba ba ba ba ba ba ba ba ba ba ba brains on.
MOLLY BLOOM: You're listening to Brains On. I'm your host, Molly Bloom, and I'm here with one of our producers, Marc Sanchez. Hi, Marc.
MARC SANCHEZ: Hey, Molly. Hey, I want to play a word association game with you, OK?
MOLLY BLOOM: OK.
MARC SANCHEZ: So when I say a word, you just tell me the first thing that pops into your mind.
MOLLY BLOOM: OK.
MARC SANCHEZ: Butterfly.
MOLLY BLOOM: Caterpillar.
MARC SANCHEZ: Wasp.
MOLLY BLOOM: Sting.
MARC SANCHEZ: Parasitoid.
MOLLY BLOOM: What is a parasitoid?
MARC SANCHEZ: You don't know what a parasitoid is?
MOLLY BLOOM: I do not.
MARC SANCHEZ: You know what a parasite is, right?
MOLLY BLOOM: Yes. A parasite--
ROBOT: Parasite.
MOLLY BLOOM: --is an organism that lives in or on another organism. That's the host.
ROBOT: Host.
MOLLY BLOOM: And they use the host as a source of food, shelter. They're super fascinating.
MARC SANCHEZ: Totally. And the difference between a parasitoid and a parasite is that a parasitoid will always kill its host. Parasites don't always do that.
CARL STENOIEN: Hi, I'm Carl. I'm a graduate student at the University of Minnesota, and I study tiny little wasps that lay their eggs in other insects. They're called parasitoid wasps.
MARC SANCHEZ: That's our biologist pal, Carl Stenoien.
MOLLY BLOOM: Oh, right, Carl. I met him at the monarch butterfly lab at the University of Minnesota when we were working on our monarch episode. And if you haven't heard that one yet, check it out.
MARC SANCHEZ: For sure. Check that out. It is quite a fascinating journey to Mexico and back. And those tiny little parasitoid wasps that Carl studies use monarchs as their hosts. They feed on the chrysalis of a butterfly.
MOLLY BLOOM: Chrysalis? I think I need a little refresher.
MARC SANCHEZ: The chrysalis.
ROBOT: Chrysalis.
MARC SANCHEZ: Or pupa--
ROBOT: Pupa.
MARC SANCHEZ: --is that stage in a caterpillar's life that comes after crawling on leaves and before flying off on new butterfly wings.
ROBOT: Pupa is a funny word.
MOLLY BLOOM: This robot is easily distracted.
ROBOT: Also, the plural of pupa is pupae. Pupae is a funny word. Ha ha ha ha ha.
MOLLY BLOOM: Oh, robot.
ROBOT: Apologies.
MARC SANCHEZ: During chrysalis, a caterpillar forms a protective shell around itself, so it can go through its transformation to a butterfly.
[MUSIC PLAYING]
Now, monarch butterflies eat milkweed, which provides them with a natural defense. Milkweed is toxic to parasitoids like wasps.
MOLLY BLOOM: So the monarch would be poisonous if the wasp ate it?
MARC SANCHEZ: True. But there's one type of wasp that Carl studies that seems to be immune to these milkweed toxins.
CARL STENOIEN: All right, so this wasp, pteromalus cassotis, P-T-E-R, like a pterodactyl, it means wing. Pteromalus cassotis is its name, and we really don't know much about it at all, except that we know it will lay its eggs in the chrysalis of certain butterfly species, and it seems like it might be specialized on monarch butterfly chrysalis.
MARC SANCHEZ: So when these wasps lay their eggs, what you get at the end of the chrysalis isn't the beautiful butterfly you would expect.
CARL STENOIEN: Instead of, two weeks later, a butterfly emerging, you'll have 100 wasps emerging instead.
[MUSIC PLAYING]
One female will somehow find a chrysalis in the wild, and she'll spend about maybe a day or two laying eggs into it. There's not much a pupa can do to defend itself because it doesn't have any limbs or any teeth and it can barely move. And once it hardens up, it's kind of just sitting duck. Sometimes, we find more than 200 offspring from one chrysalis.
[MUSIC PLAYING]
MARC SANCHEZ: Molly, do you want to hear something crazy about these parasitoid wasps?
MOLLY BLOOM: You mean more crazy than hundreds of wasps feeding off a chrysalis and flying out when I expected to see a colorful butterfly?
MARC SANCHEZ: Right. OK. Another crazy thing about these parasitoid wasps, there are more species of these wasps than all of vertebrates combined.
MOLLY BLOOM: So more than birds plus fish plus amphibians plus mammals?
MARC SANCHEZ: Yep, combined.
MOLLY BLOOM: That's a lot of wasp species.
MARC SANCHEZ: Don't worry. I'm not trying to scare you.
MOLLY BLOOM: Too late.
[LAUGHS]
MARC SANCHEZ: These things have been around since before we even kept records, and they haven't overtaken us yet.
ROBOT: Attack of the parasitoids.
MOLLY BLOOM: Robot, let's not get ahead of ourselves.
MARC SANCHEZ: OK, my point is that there are so many different species of parasitoid wasps, they can be hard to identify. Tiny differences in size, and color, and shape can be tough to detect, but they're important.
MOLLY BLOOM: So how did Carl do it?
MARC SANCHEZ: This is where his detective work started. The lab that he works in was trying to figure out what might be killing monarchs during the chrysalis stage. So they found them in the wild and brought them back to the lab.
MOLLY BLOOM: And then?
MARC SANCHEZ: And then they waited.
[MUSIC PLAYING]
CARL STENOIEN: Almost luckily, we had one instance of parasitism that first year of the study, and we said, let's go to the books, look it up. It looks like this really common wasp that's used in biological control, pteromalus puparum.
MARC SANCHEZ: But they had to be sure, so they took the next step.
CARL STENOIEN: We sent it to the Smithsonian. They have a large collection of insects to compare it to, and they responded yet probably is pteromalus puparum. We're not 100% sure, but let's go with that.
MARC SANCHEZ: For some people, being close to 100% sure, that'll do. But for scientists, not quite.
CARL STENOIEN: Then we dug into some really old literature and found a paper from 1888. There was this one record of monarch hosts being parasitized by this one wasp. Then we looked a little closer at the catalogs and realized there were a few differences, like the color of the legs were slightly different, and things like that. And so then we realized that not pteromalus puparum. We actually have this other species pteromalus cassotis that no one's paid attention to in over 100 years.
MOLLY BLOOM: So nobody had seen this wasp since 1888. That's pretty amazing.
MARC SANCHEZ: I know. It had been so long that people had actually forgotten all about it. Can you imagine looking at this little teeny wasp that's about as big as the grain of rice, and you're the only person who knows it exists. Think about that for a second.
ROBOT: That is bananas.
MOLLY BLOOM: Bananas, indeed, Robot.
MARC SANCHEZ: Every time I learn something new that nobody else has ever learned, it's a huge rush. I love it. It's a super exciting feeling, and it-- I can't really describe it.
[MUSIC PLAYING]
MOLLY BLOOM: That was Carl Stenoien from the University of Minnesota monarch lab. Thanks for bringing that story to us, Marc.
MARC SANCHEZ: You're welcome.
MOLLY BLOOM: And Thanks, Robot.
ROBOT: My pleasure.
[MUSIC PLAYING]
MOLLY BLOOM: Do you have any questions for us about parasites, robots, anything else? Send them to us at brainson at M, as in Minnesota, pr.org.
[MUSIC PLAYING]
Right now, we're working to find an answer to this question from six-year-old Rupert Kong from Toronto.
SUBJECT 1: How do volcanoes erupt?
MOLLY BLOOM: Did you catch that? He said, how do volcanoes erupt? We'll have an answer for you very soon. Until then, send us your questions. You can also follow us on Twitter and Instagram at brains_on. And there's one more thing we need to take care of before we sign off. It's time for the mystery sound.
SUBJECT 2: Mystery sound.
MOLLY BLOOM: We've been asking you to send us your mystery sounds, and it's been fun hearing what you've come up with and trying to guess the answers. They're pretty tricky. We're going to see if you have an easier time guessing this one than we did. Ready? Here it is.
[MUFFLED NOISES]
SUBJECT 3: My name is Bert Turtle. And I'm 6 years old, and the sound of that was going down the hill at like 25 miles per hour, basically sledding.
[MUSIC PLAYING]
Well, it also sounds like to me is a friction piano. It's like a bunch of notes at the same time, and it sounds like a full melody, like they're just all playing at the same time.
[MUSIC PLAYING]
MOLLY BLOOM: Thanks Barry for sending that really cool sound, the sound of sledding. I don't know what a friction piano is, but it sounds like a cool invention. Good idea. If you have a mystery sound you'd like to share with us, we'd love to hear it. Just send it to brainson@npr.org. We'll play some of the sounds we get on an upcoming episode.
That's it for now. You can always listen to past episodes at our website, brainson.org, or at your favorite podcast app. Thanks for listening.
(SINGING) Ba ba ba ba ba ba ba ba ba ba ba brains on.
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