the monarch-milkweed arms race, with cornell’s anurag agrawal
UNTIL RECENTLY, if someone said “monarchs” I would probably free-associate and think “milkweed” and then “migration,” and all those elements are in the title of the new book, “Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant and Their Remarkable Story of Coevolution,” by Cornell professor Anurag Agrawal.
But after I read it, mention of the word “monarch” started bringing another phrase to mind, too. Now I also think “arms race,” because that’s what’s been going on with the butterfly and its host plant over the millennia—a fascinating power struggle that includes impressive chemical weaponry and tactical maneuvers, a story of what seems like survival against all odds.
Dr. Agrawal, from the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell, studies the ecology and evolution of plant-insect interactions. He joined me on the July 3, 2017 edition of my public-radio show and podcast to help us get to know the monarch and milkweed a little better. Read along as you listen, using the player below (or at this link).
monarchs and milkweed q&a, with cornell’s dr. anurag agrawal
Q. I have to say, I already thought I knew a little bit about monarchs, and that they were impressive–but I had no idea just how impressive till I read the book. How long have you been studying them?
A. I started studying monarchs and milkweeds in about 2000, so about 17 years.
Q. Wow. A little obsession is good, right? [Laughter.] Dig deep into something.
So let’s set the scene for people who haven’t read the book yet, and explain why as you say in it that monarchs and milkweeds are having this arms race, and why you say early on that they are “a premier pair to study co-evolution.”
A. To set the stage, maybe the first thing to point out is that all animals, whether they are insects or lions and tigers, ultimately depend on plants for their energy. Plants do the thing: They take sunlight, carbon dioxide and water, and with sunlight turn it into what we call biomass.
Then everything else either eats plants or eats other things that eat plants. A consequence of that is that plants have been under threat for the millennia—everybody’s trying to eat them.
Q. Herbivory! Herbivory! [Laughter.]
A. [Laughter.] Exactly. And so among ecologists and biologists like myself, one of the things we study is the mechanisms by which plants defend themselves, and so the monarch and the milkweed in a way is a microcosm of what’s happening out there in oak trees, or in our lawns, or in agricultural fields.
The monarch is trying to make its living by feeding on the plant, and the milkweed has over time has evolved these defenses to ward off being fed upon. So that’s where I think the monarchs and milkweeds are what I call “royal representatives” of how nature works.
Q. And then they have this special thing going on because of this fierce chemistry involved. This is not just any old plant, this Asclepias.
A. No, that’s right. Most plants have interesting chemistry. Some of it we use for our nutritional benefits; some of them we use as spices, like chili peppers. Milkweed plants have a whole set of toxins in them, the cardiac glycosides being the main ones. The cardiac glycosides of the milkweed are an extreme example of the chemical toxins that are out there in the wild plants.
Q. I think at one point in the book—and I’d have to look at my notes; I took compulsive ones since there is a lot in the book—you give an example of another native plant that insects visit, Solidago, the goldenrods. And maybe 100 species of insects visit the goldenrods—I might have just made that number up—…
A. No, you’re right.
Q. …whereas something like 10 or 11 visit the Asclepias. It’s different.
A. Absolutely. Many things of course come to milkweed flowers and they might sip a little bit of nectar, in part because milkweed has a big structure—at least the common milkweed, Asclepias syriaca, stands out with its broad leaves. Many insects will come sit on the leaves and fly away. In terms of the insects that eat the leaves and make their living by eating milkweed, it is a small community.
Q. And those chemicals are a lot of the reason why—and the fact that they are sap-like. So it’s not only the toxins, but they also have a structure that you could get drowned in, couldn’t you? [Laughter.]
A. Something I have spent a lot of time thinking about is the relative importance of the milky latex that is sticky and exudes, versus the toxins. It’s difficult to separate, because the toxins are concentrated in the latex.
Q. We can go back into that deeper, but first let’s meet briefly the monarch. As I said in the intro, people know that it migrates. But it’s not like a migratory songbird if you live in the North, and it goes away in the fall and comes back and hatches a generation on the breeding grounds, and that generation flies south. It’s not like that—it’s a multigenerational annual migration cycle, yes?
A. That’s absolutely right, and I think both types of migration are spectacularly interesting. And just what you said: from the bird’s perspective, because it’s individuals that make the round trip, I think the really compelling and interesting piece is that they may be using their memory to return to very specific areas and locations.
Q. I know; amazing. [Laughter.]
A. For monarchs, the memory is not possible, because it’s the grandchildren or the great-great grandchildren that are returning to the sites that their great-grandparents were at, so they’ve never seen those locations before. That’s what makes that a really remarkable part of that story.
Q. What time of year shall we begin to go through a sort of brief life cycle?
A. We can either start in January or in June, given that we are in June now. [Laughter.]
Q. I don’t know, you’re the teacher and have probably tried to explain it to many people—what makes more sense?
A. In the book I started with January, the calendar year. So it’s winter, and it’s frozen north here where we are, and the butterflies [of the monarch population that migrates to the Midwest and Eastern U.S.] are sitting in Mexico in trees, at about 10,000 feet of elevation. They’re in fir trees.
They’ve been there for about two months waiting out the winter, and they will be there for another two months. Toward the end of February-early March, individual butterflies will fly from Central Mexico to the Gulf States, mostly Texas and Oklahoma and Louisiana, so that’s about an 800-mile journey. By the time they get to those Gulf States, those monarch butterflies are about eight months old. They are tattered; they are faded. They get to the Gulf States and lay eggs on the milkweeds there in March and April, and most of them perish; they die there.
But they’ve laid eggs, so that’s the beginning—the birth—of the new cycle. The eggs hatch, they turn into caterpillars, they feed, they turn into the chrysalis, they turn into the butterfly. In May they then take flight from the Gulf States and fly North, to the Midwestern U.S. or the Northeast, more or less separated by the Appalachian Mountains.
It’s then in the North that they have two to three more generations, meaning those butterflies lay eggs-caterpillar-chrysalis-adults, eggs-caterpillar-chrysalis-adults. And then mid-August, something changes and the days are getting shorter, the nights are getting cooler, and the milkweed is getting tattered and old and tougher itself.
All of those conditions—the temperature, the daylength, and the milkweed being older—conspire to lead the monarch in late August to not mature its reproductive organs. A butterfly typically comes of out its chrysalis and the first thing it does is mature its reproductive organs. But it does not mature its reproductive organs, so it orients South and starts that unbelievable journey.
And so those monarchs beginning in September will fly around 3,000 miles, from the upper Midwest and Northeast all the way back to those overwintering grounds in the highlands of Central Mexico. [There is also a Western U.S., population, centered mostly in California, that takes a different migratory route.]
Q. Where their grandparents or great-grandparents were the previous January.
A. That’s correct. That’s exactly right.
Q. Now along the way you mentioned laying eggs, and they are laying eggs on milkweed specifically, yes?
A. Yes, the only food plant for the caterpillar stage is milkweed. The adults have what we entomologically call a proboscis—they have a tube-like mouthpart, and as adults they can only drink. They can drink water and flower nectar. But that’s a very important part of the story—the separation of the caterpillar-phase milkweed eaters and the adult water and nectar feeders.
Q. I was kind of blown away in the book when you mentioned that the butterfly will often lay one egg on one plant—not a whole cluster. Sometimes with insects we see these aggregations, like tiny beads, on the underside of a leaf or something. But one?
A. There are a couple of interesting pieces about that. You are absolutely right that insects have two kinds of strategies over all: to lay their eggs singly, like the monarch typically does, or to lay these rafts. There is another milkweed insect, the milkweed tussock moth, and it lays 200 to 400 eggs [laughter] at a time.
Q. Just in case; covers its bases. [Laughter.]
A. The monarch main reason we think it lays one at a time, and avoids plants that already has monarchs on them: The dirty little secret of the monarch butterfly is that they engage in a little bit of cannibalism.
Q. Oh, dear.
A. But the only other monarch that a caterpillar will eat is a monarch egg. If you have two caterpillars, they won’t eat each other. If you have a caterpillar and an adult, it won’t eat it, but a caterpillar will eat the egg of another monarch caterpillar.
So if there is already one egg on the leaf, whether by this given mother or a different one. The egg that is likely to hatch first will eat the other egg. [Laughter.]
Q. Good that you find it funny. [Laughter.] One has to, I think; all these crazy things in nature.
So my mother has laid the egg, and I have hatched, and I am somewhere alone theoretically on this Asclepias plant somewhere. And I am tiny, and I come out of my little shell—and I think I eat my shell first maybe as my first meal, is that what I do?
A. That’s absolutely right.
Q. And then it gets even more dangerous because I’ve got to eat this plant that’s full of these toxins and goo. Tell us how I go about that.
A. The egg hatches just like you said; you eat the eggshell, which has a good number of proteins in it. The first barrier to feeding are what botanists call trichomes. They are leaf hairs; they make the leaves fuzzy. In proportion, those leaf hairs on a common milkweed are taller than a monarch caterpillar’s head when it first hatches.
So a monarch has a pretty difficult time getting down through the hairs so that it can use its mandibles—its teeth—to start chewing the leaf. To cope with those trichomes or leaf hairs, the caterpillar shaves them.
A. It cuts them one at a time, and it doesn’t eat them—it’s just getting them out of the way. So the trichomes are out of the way due to the shaving.
Next it punctures the leaf with its mandibles, and that’s when it really encounters the latex. In milkweed plants, the latex is stored under pressure, so that when the leaves are broken—either by one of us as we pass by and pluck a leaf, or by the caterpillar that bites into the leaf, that latex exudes with some force.
Q. We’ve seen that.
A. So there is a sort of back and forth to the plant having a barrier to feeding, like the trichomes; they’re shaved, and then it presents the latex. And the monarchs, if they’re lucky and they survive the latex, as they get bigger they will drain the veins—they’ll cut the canals that deliver that latex, so that they can feed on the leaves without being burdened by the latex.
Q. So I don’t want to be downhill when I do it, because I don’t want to get dripped on. But I am there without my mother—no one is teaching me. [Above, one defensive strategy is to cut a circle trench in the leaf.]
A. That’s correct.
Q. And this is what’s so incredible. I’m developing—or I’m not developing, but I have from the generations before me, I guess, these co-evolutionary adaptations that have happened. I shouldn’t use the word taught; I shouldn’t say that they have taught me as this little caterpillar to do this. But this behavior came from thousands of years of the generations doing it before. So is this where the arms race is?
A. That’s exactly it. It’s interesting for me to think about as a biologist. The arms race occurs over the millennia. It occurs just what you were saying, over thousands and millions of generations. Each generation, most of the caterpillars will perish—and those that survive are the ones that have traits that are slightly better adapted to the environment.
So the really amazing thing to me about monarchs and milkweeds is that in those first few days of life after the caterpillar hatches, you can see the evidence of that long-term evolutionary arms race, because the hairs are there and it has to be shaven. The latex is there and it has to be drained. And then the toxins are there but the caterpillars are sequestering them away in their own body.
You’re absolutely right that that back and forth that we see—there’s no teaching there, and there’s very little evolution happening in any one generation. But what we see is the products of evolutionary and co-evolutionary interactions over the millennia. [A primer by Dr. Agrawal on co-evolution.]
Q. It’s amazing. You use the word “sequester,” and that is a big word in the monarch’s life. What’s sequestering?
A. Sequestering is packing away of toxins by the caterpillar for its own benefit. It’s a little bit of a twist and the turn in the story, because the plant is producing these toxins to ward off the herbivores—the caterpillars—but the monarch is so specialized; it only eats the milkweeds. At least for some of the toxins that the milkweed makes, it packs them away in its own body.
In the adult monarch butterfly they are most concentrated in the wing, in the scales on the wings.
Q. Oh, I didn’t know that.
A. And the reason we think the monarch is black, orange and white, and that it has these toxins packed away in its wing scales is to advertise to its predators—the birds, mostly—don’t eat me because if you do you are going to get a dose of this nasty toxin. So it really takes things to the next level.
Q. So among the other tactics that it has actively figured out over the generations and generations is the ability to sequester what would otherwise be poison to other insects. As we said, there are only like 11 or 10 who even use the milkweed in some way like this, or eat it.
So they not only figure out how to eat around it strategically or tactically, but they can store it up and use it to their defense—they have turned it into something to their benefit. But what is the poor milkweed get out of it, except for chewed up? [Laughter.]
A. The milkweed gets very little out of this relationship. [Laughter.] You know, plants are the original energy makers, as we started off by talking about, and I think they are just trying to make their living, and for that they need sunlight, carbon dioxide and water.
Many plants need pollinators also, and milkweed is one of those that needs pollinators, but even that is not something that the monarch butterfly provides the milkweed.
Q. That surprised me; duh, I just figured—but it’s not its pollinator.
A. It’s hard to know, because the monarch is sitting there typically on milkweed flowers, it’s drinking the nectar—so it’s perfectly reasonable to assume that they would be pollinators. But the way that they sit on the plant and their large, clumsy size, they don’t really grab the pollen grains.
Q. They don’t get at the sexy bits and do the whole job. [Laughter.]
Q. See how scientific I am, Anurag? I have my layperson’s approach to understanding all of this.
I want to ask you about what gardeners and others reading newspapers and magazines the last few years especially will have read. The headlines exhort us to “to plant more milkweed, plant more milkweed,” as if that’s the fix against monarch decline that is so well known and much written about. But I think you have a more textured perspective on what’s going on.
A. I should say my perspective has been a little controversial, and not everybody agrees or likes it.
Q. I like agitators, it’s OK. [Laughter.]
A. I have to say, I have planted a lot of milkweed in my time. My front yard is full of milkweed right now. I found four caterpillars there last week and I am showing them to my kids. So I think there is nothing wrong with planting milkweed.
Q. Can’t hurt.
A. Can’t hurt; we should do it for several reasons, one of which is to teach ourselves about nature and biology. And like you say it can’t hurt.
But the point of departure I have with some others is that planting milkweed will reverse the decline or save the monarch. From my perspective it’s really important to get the science right if we’re going to ask citizens to participate in science and conservation, and I think many people want to help.
We have to get the science right. My analysis of—a lot of it is from citizen-science data—of the population trends in monarchs do not reveal that milkweed is what’s limiting the population or that planting milkweed will reverse the decline.
And part of it is that at the beginning when I started doing that work, I didn’t know what we would find. But the decline has been so steady over the last 20 years in Eastern North America, and the last 40 years where there is actually data in California, that the change in milkweed hasn’t been kind of linear and incremental over that many decades. Something else is going on, at a very large scale.
[Read Agrawal’s May 2017 column in “Scientific American” on what beyond milkweed supplies may be at work in monarch decline.]
Q. I loved the book. It’s way over my head in some parts, but I plowed through, and I felt like I came away with such a richer picture of these incredible butterflies and this incredible plant.
more from dr. anurag agrawal
- More about the arms race
- From “Scientific American,” about the monarch’s decline
- A primer on co-evolution
- Dr. Agrawal’s blog, Herbivory.com
- The book, on Amazon
- The Facebook page for “Monarchs and Milkweed”
enter to win ‘monarchs and milkweed’
I’LL BUY A LUCKY READER A COPY of the new book, “Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant and Their Remarkable Story of Coevolution,” by Cornell professor Anurag Agrawal, published by Princeton University Press. All you have to do to enter is answer this question in the comments box at the very bottom of the page below the last reader comment:
What’s your monarch story? Do you see caterpillars or butterflies in your garden, and are you growing any species of Asclepias for them? If not, any other butterflies you want to tell us about in your backyard?
I’ll pick a random winner after entries close at midnight on Tuesday, July 11, 2017. No answer, or feeling shy? Just say something like, “Count me in,” and I Will, but an answer is even better. Good luck to all; U.S. and Canada only.
prefer the podcast version of the show?
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the July 3, 2017 show right here, and also use the buttons below to subscribe to future shows, free.
(Photos courtesy of Princeton University Press, used with permission; videos from Dr. Agrawal. Disclosure: Purchases from Amazon affiliate links yield a small commission.)