Sonia Altizer: What Can We Learn From The Migration Of Monarch Butterflies?

Apr 30, 2021
Originally published on September 10, 2021 11:39 am

Part 4 of the TED Radio Hour episode Migration

Monarch butterflies fly the longest two-way migration of any insect species. Ecologist Sonia Altizer shares how these intrepid butterflies make the journey — and how it's being threatened.

About The Monarch Migration

Each year, millions of monarch butterflies fly some 3,000 miles, from their summer breeding grounds as far north as Canada to their overwintering sites in the oyamel fir forests of central Mexico. It is one of the Earth's great migrations, and it's also one of the best studied. In recent years, ecologists like Sonia Altizer have been able to better answer how and why these intrepid butterflies make the journey.

The how is owed to a complex suit of navigational and physiological adaptations. Migrating monarchs have internal sun and geomagnetic compasses, and the monarchs that make the longest trek actually grow to be bigger and more powerful fliers, particularly the females. They can even read the skies like a meteorologist, finding and following air currents to quicken their travels. Peering into their genes has also revealed more clues into how they evolved this migratory behavior.

In terms of why they migrate, ecologists think there are three main reasons. The first two have to do with milkweed and temperature. To breed, monarchs need milkweed, and in North America native milkweed dies back with cold winter temperatures. Those same temperatures would kill the monarchs. The fir forests in Mexico offer the ideal shelter and perfect goldilocks temperature while they wait for milkweed to grow back in the spring. The forests in Mexico are warm enough that the butterflies don't freeze but cool enough that their metabolism drops and they can enter a kind of dormant state, called diapause. Altizer has helped establish the third reason why monarchs migrate: to escape a parasite that affects them in their breeding grounds. Infected monarchs are less likely to survive the journey, so the long migration weeds out those who are infected and keeps the population safer. For more on monarchs, check out the Forest Service's monarch FAQs.

How You Can Help Protect Monarchs And Their Migration

First, help protect existing monarch breeding habitat. The Corn Belt produces more than half of the monarchs that migrate to Mexico. As Altizer presents in her TED talk, GMO crops, by design, leave no room in agricultural fields for milkweed and nectar plants to grow. Altizer asks us to see agriculture from an ecosystem perspective and consider using GMOs more judiciously.

Second, help create new habitat for monarchs. Right now, organizations are creating alternative breeding habitat for monarchs and you can support those wide-scale efforts. You can also make monarch and butterfly-friendly habitats in your neighborhood or backyard. If you do, Altizer stresses the importance of planting native milkweed species. Many plant nurseries carry what's called tropical milkweed, and unlike native species it doesn't die off in winter. While that might sound good, it actually increases the spread of the parasite and it sends signals to the monarchs to stop migrating. If you do choose to create native monarch habitat, it's best to plant more than just milkweed. Variety ensures you'll help all kinds of pollinators and wildlife.

Third, consider joining a community or citizen science project. There are many that help track the migration. Dr. Altizer also founded the citizen science group Monarch Health to track the prevalence of the protozoan parasite Ophryocystis elektroscirrha (OE) in monarch butterflies.

About Sonia Altizer

Sonia Altizer is a Professor and Associate Dean of Academic Affairs in the Odum School of Ecology at the University of Georgia. Her main research focuses on how pathogens affecting wildlife are shifting in response to human environmental change.

At UGA, her students run a citizen science project called Monarch Health. In 2015, Altizer co-edited a book titled Monarchs in a Changing World: Biology and Conservation of an Iconic Insect, and participated in high-level task forces dedicated to monarch butterfly conservation.

Altizer received a B.S. from Duke University and a Ph.D. from the University of Minnesota. She did her postdoctoral work at Princeton University and Cornell University.

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On the show today, migration. And up until now, we've been talking about human migration.


ZOMORODI: But, of course, humans aren't the only animals that migrate.

SONIA ALTIZER: There are many hundreds, if not thousands of species of birds that migrate.

ZOMORODI: There's caribou across Canada, wildebeest in Africa.

ALTIZER: There are migratory fish, like salmon, and also a lot of marine animals migrate long distances, like sea turtles and whales.

ZOMORODI: But right now, let's turn our attention to the humble but tenacious monarch butterfly.

ALTIZER: I think of monarchs as the tanks of the butterfly world.

ZOMORODI: (Laughter).

ALTIZER: So they're small. They weigh only a half a gram, but they can travel thousands of kilometers in the wild.

ZOMORODI: This is Sonia Altizer.

ALTIZER: I'm an ecologist at the University of Georgia. So I study the ecology of animal migration.

ZOMORODI: And Sonia says monarch butterflies are different because their migration is multigenerational.

ALTIZER: So the same monarch never makes the journey twice. It's their grandoffspring and great-grandoffspring of the migratory generation that will migrate again the following year.


ZOMORODI: Sonia's specifically talking about a migration path east of the Rocky Mountains. These monarchs travel thousands of miles across international borders every year. Ecologists think they're looking for the precious milkweed plant.

ALTIZER: You know, inarguably, the most important driver for them is food and especially milkweed plants where the females can lay their eggs.


ZOMORODI: Another reason why they migrate is to ride out the winter in the Sierra Madre Mountains near Mexico City.


ALTIZER: So there might be 10 million butterflies or more in a single colony. And these colonies would be densely packed butterflies that are hanging in these beautiful fir forests. And so they're carpeting the trunks of trees. And it's almost like the butterflies spend the winter in the refrigerator. And then the temperature does warm up, you know, especially as the overwintering season progresses into the spring. And these clusters will sort of burst open, almost like orange confetti fluttering through the sky.

ZOMORODI: Does it make a sound when they burst open like that?

ALTIZER: It does. So it's almost like a very gentle wind or rustling of the leaves.


ALTIZER: And, sometimes, the air is so thick with butterflies that it might be hard to see a person standing 50 meters away just because there's so many butterflies flying through the air.

ZOMORODI: By early March, it's time to procreate. So the butterflies leave the mountains for northern Mexico and Texas to lay their eggs on milkweed, the only plant that their caterpillars will eat.


ALTIZER: But by this time, they're really old, so they've been alive for about nine months. And eventually, you know, they die. And then it takes time for their offspring to develop.

ZOMORODI: But by May, this new generation is ready to continue the journey north.

ALTIZER: In the part of the United States that we refer to as the Corn Belt - so Iowa, Wisconsin, Minnesota and even farther north into Michigan and southern Canada.

ZOMORODI: There, the butterflies have enough milkweed, nectar and sun to stay put and cycle through one, even two more generations.

ALTIZER: But then the last generation at the end of the summer - it's the shorter day lengths and the cooler temperatures that signal to those butterflies, that generation, that it's time to get ready to migrate. And so instead of producing eggs and mating and, you know, hanging out in milkweed patches, those butterflies instead tank up on nectar. They build up their fat reserves, and they head south towards the overwintering sites in Mexico. And so they have to be in a special physiological state to be able to successfully make that migration.

ZOMORODI: Huh. So they keep the species going. But it's this - I mean, I'm sorry, but describing a butterfly as fat is like - I've seen fat caterpillars, but I've never seen a fat butterfly (laughter).

ALTIZER: Yeah, they are butterballs in the fall and winter.

ZOMORODI: (Laughter).

ALTIZER: And it's important that they build up those fat reserves because they not only need the energy to fuel the migration, but they have to live off of their fat reserves for five months at the overwintering sites and also use them to fuel that journey partway back north again.

ZOMORODI: Here's Sonia Altizer on the TED stage.


ALTIZER: Now, this migration of monarchs is one of the Earth's last great migrations. But around the world, a lot of these great migrations have disappeared or are disappearing due to things that we as people are doing to them and their habitats. Their loss has changed the entire ecology of ecosystems, and they're impossible to replace. Like these other migrations, monarch migration is declining, too. In fact, the last three consecutive years have been the lowest numbers of monarchs ever recorded in Mexico - so low, in fact, that scientists estimate migratory monarchs have declined by 90%. So if monarchs were people, this would be like losing every person living in the United States except for those in Ohio and Florida.

Now, what are the causes of this monarch decline? Well, unfortunately, there's a lot of different challenges facing monarchs, ranging from climate change and drought to deforestation and illegal logging in in even car strikes along roads during the fall migration. One of the more ominous threats has been the loss of milkweed plants and agricultural habitats due to shifting agricultural practices. So it might surprise you to hear that what we eat affects food that's available to the monarchs.

ZOMORODI: So you actually link the monarch's well-being to how we humans grow our food. Can you just explain what that link is, what the connection is between the two?

ALTIZER: Well, so monarchs need milkweed. Milkweed isn't the only resource that they need. They also need nectar plants. But milkweed is the key resource that monarchs need to reproduce. And it's an agricultural weed. And so you would find it along roadsides, even country roads or gravel roads. It would be growing in and around cornfields, in and around other row crops and orchards. And so one thing that has become popular since the late 1990s are crops that are genetically modified to resist common herbicides, like Roundup. And the herbicides can be sprayed on crop fields of soybean or corn, and the crops do just fine, but milkweeds and other agricultural weeds that would be providing nectar for monarchs would die.

ZOMORODI: So you suggest that one way to help stem the decline is to buy non-GMO food. But GMOs have been around for - what? - nearly 30 years now. Is that even possible anymore?

ALTIZER: That's an interesting question. I mean, certainly, we can use our purchasing power as consumers to buy sustainably sourced crops or, you know, agriculture. So buy local. Buy organic. It's probably too late to turn the clock on GMO crops. And it is a controversial topic. So the technology itself isn't, you know, harmful or evil. It's just the way that these crops have been deployed and the scale at which they've been deployed. It means that we're growing food now in a way that doesn't leave room for other biodiversity. And so these agroecosystems have become, you know, really almost ecological deserts, if you will.

ZOMORODI: So is there anything else we can do, like, I guess, plant monarch-friendly gardens, plant more milkweed?

ALTIZER: Definitely planting milkweed but especially native milkweeds is something that people can do to help them. And you're - again, being aware that it's not just milkweeds that monarchs need. It's nectar plants and other resources, too. And if you plant habitats and gardens for monarchs and other pollinators, you'll be helping dozens of other species, as well. And so it's realizing that monarchs are part of these complicated food webs that involve birds and spiders and ants and other plant species, even parasites that attack them. And certainly, milkweed is a critical part of that, and there are other parts, too.


ALTIZER: You know, one of my dreams is to be able to take my kids to the overwintering sites in central Mexico to let them be able to see what it's like to stand in a forest full of millions of butterflies. And so to see that declining, to see those migrations unraveling does make me sad. At the same time, they are resilient, and they can acclimate or adapt to a wide range of conditions. And so they do exist in places in the world where they don't undergo long-distance migrations. So there are native resident monarch populations throughout Central and South America and the Caribbean islands. And monarchs more recently colonized the Pacific Islands. They've also recently crossed the Atlantic.

ZOMORODI: They've crossed the Atlantic, like, literally, do you think?

ALTIZER: Yes, they have. And so one interesting fact about monarchs is that in England, people used to call them storm fritillaries in historic times because they would occasionally blow over with big storms.

ZOMORODI: Wow (laughter).

ALTIZER: You know, people thought that they maybe naturally just blew across the Atlantic in storms.


ALTIZER: It also seems likely that monarchs have hitched a ride with people to different places around the world on trade ships, for example. But in a lot of these places, monarchs breed year-round and don't undergo long-distance migrations. And so how these tiny insects can show such a wide range of behavioral responses to different environments is fascinating to me. And so, you know, I think a lot of us are trying to figure out what's going to be the new normal.

ZOMORODI: Yeah, I mean, the new normal sounds like it's not great for these butterflies. There's a lot we humans keep doing to cause problems for them. So does that mean that in addition to studying them, we also need to start enacting laws to protect them?

ALTIZER: You know, one of the great challenges with protecting migratory species is that they don't see or respond to or respect geopolitical boundaries. And so we need to think about ways of engaging in conservation that cross these boundaries, which, you know, are really just artificial constructs of people and nations and really reflect on the fact that for most of life on Earth, movement is not only a part of their life. It's essential to the persistence of these species.

ZOMORODI: That's Sonia Altizer. She's an ecologist at the University of Georgia. You can learn more about her research and what we humans can do to help the monarch butterfly at

Thank you so much for being with us this week to talk about migration. To learn more about the talks on today's show, go to And to see hundreds more TED Talks, check out or the TED app.

Our TED Radio production staff at NPR includes Jeff Rogers, Sanaz Meshkinpour, Rachel Faulkner, Diba Mohtasham, James Delahoussaye, J.C. Howard, Katie Monteleone, Maria Paz Gutierrez, Christina Cala, Matthew Cloutier and Janet Woojeong Lee, with help from Daniel Shukin. Our theme music was written by Ramtin Arablouei. Our partners at TED are Chris Anderson, Colin Helms, Anna Phelan and Michelle Quint. I'm Manoush Zomorodi, and you have been listening to the TED Radio Hour from NPR. Transcript provided by NPR, Copyright NPR.