The Curious Case of the Disappearing Butterflies
[Close-ups of monarch butterflies, wider views of monarchs flying.]
JESSICA WARE (Curator, Division of Invertebrate Zoology): How do you keep tabs on a butterfly? Monarch butterflies perform one of the most amazing feats in the natural world: some crossing a continent, traveling 3,000 miles from Canada to Mexico…
[An adult monarch flaps its wings next to a monarch caterpillar and a pupa. Thousands of monarchs cluster together on the ground and in trees.]
WARE: …in a multi-generational migration and settling in the same special places year after year after year… even though these individual butterflies have never been there before.
[Thousands of monarchs whirl and flutter high in a blue sky.]
WARE: Unlike most butterflies, monarchs fly high—riding thermals over 3,000 feet above the ground. And during migration, they can speed along at 5-8 miles an hour, maybe a little faster than most of us jog.
[A single monarch floats along with a backdrop of fluffy clouds.]
WARE: So, it can be really hard to follow a single butterfly over their long trip.
[Archival image of a man and a woman writing field notes and holding a monarch butterfly.]
WARE: And it wasn’t until the 1970s that American and Canadian scientists began to understand the full scope of this journey.
[A person sorts through numbered stickers that will be attached to butterfly wings to track them. One is placed with tweezers onto a monarch.]
WARE: To them, it was a mystery where monarchs went during the winter.
[Dozens of monarchs flit through a sunlit-dappled forest. Hundreds cluster on a single branch.]
WARE: But for thousands of years, people in Mexico, and likely in coastal California had known where the monarchs were spending the colder months.
[A single monarch with worn and torn wings flutters on the ground.]
WARE: Now, there’s a new kind of monarch disappearance mystery: Why are their numbers plummeting?
[A few monarchs perch on tree branches. One or two flit through a mostly empty sky.]
WARE: In the Western U.S., monarch numbers have dropped from as many as an estimated 10 million butterflies in the 1980s to less than 2,000 in 2020.
[Close-up of monarchs flapping their wings on a tree branch. A researcher holds a radar tracking tool. A young woman looks through binoculars in a forest grove.]
WARE: To find out what’s happening, researchers are harnessing the power of new technology and community scientists to track monarchs in California.
[Title animation: INSECTARIUM]
[On-screen text: American Museum of Natural History]
[Extreme close-ups of monarch caterpillars eating leaves. Various shots of different butterfly species.]
DIRECTOR [off-camera]: Some people, like me, might have read The Very Hungry Caterpillar at some time in their life, but what’s the scientific distinction that makes a butterfly a butterfly?
[Host Jessica Ware speaks to camera while walking through the American Museum of Natural History’s Vivarium—a room where live butterflies fly around visitors.]
[On-screen text: Jessica Ware, Ph.D. | Curator, American Museum of Natural History]
WARE: So butterflies are in the order Lepidoptera.
[Close-up of a butterfly perched on a leaf.]
[On-screen text: Lepidoptera | butterflies and moths]
WARE: We thought for a long time butterflies and moths had a clear distinction.
[A moth flutters around a porch light at night.]
WARE: But then a lot of genetic information seemed to suggest that instead maybe there isn’t really like a clear scientific distinction between the two groups.
[Various species of day-flying moths visit flowers and perch in the sun.]
WARE: We thought that maybe butterflies flew in the day, but it turns out there's day flying moths.
[Extreme close-up of the scaly surface of a monarch wing.]
WARE: Lepidoptera tend to have scales on their wings.
[Ware speaks to camera in the Vivarium. She stands next to a window where pupae from a variety of butterfly species are developing.]
[On-screen text: Davis Family Butterfly Vivarium | American Museum of Natural History]
WARE: They’re all holometabolous, which means they have complete metamorphosis.
[Close-up of a praying mantis cleaning its forelimbs. Shots of adult and young grasshoppers, their age indicated only by size.]
WARE: There are other insects that have incomplete metamorphosis, like grasshoppers. Like, a baby grasshopper kind of just looks like what an adult grasshopper looks like.
[Ware speaks to camera in the Vivarium, indicating pupae that hang next to her.]
WARE: But for holometabolous insects…
[Various shots illustrating the lifecycle of a butterfly—from caterpillar hatching out of a tiny egg, to larger and larger caterpillars.]
WARE: …they have an egg that hatches to a caterpillar. The caterpillar goes through a series of instars, a series of molts to a larger and larger size.
[In a time lapse video, a pupa forms around a caterpillar.]
[Ware stands in the Vivarium and indicates the pupa. An extreme close-up of monarch pupae shows them as light green, capsule-shaped forms with a bold gold stripe and spots at the bottom.]
WARE: Then they go into a pupal stage which is what these are, and in the pupal stage…
[Time lapse of the metamorphosis happening inside a hanging pupa.]
WARE: …there is a complete kind of internal and external rearrangement of their body…
[A pupa cracks open and an adult butterfly emerges.]
WARE: …and then they emerge as an adult.
[Time lapse shot of a monarch forming inside a pupa. Its wings are discernable through the translucent covering. An adult emerges, its wings drying in the air.]
WARE: Because they’re turning into monarch soup inside that pupal case, it makes it particularly difficult to track an individual between life stages.
[Close-up of a monarch caterpillar crawling. An adult with its wings now almost dry perches next to the remnants of its pupal case.]
WARE: You might be able to catch a slower moving caterpillar, but a tag wouldn’t transfer over to a winged adult’s body.
[An adult monarch butterfly is perched on a plant. A tracking sticker is visible on its wing.]
DIRECTOR (off-camera): So, even when you can track one adult, you’d still only get part of the migration story, right?
[Ware stands in the collection of entomology research specimens at the American Museum of Natural History. Next to her are trays of pinned monarch butterflies.]
WARE: Yeah, because for monarchs there's multiple generations that go varying distances along the journey from Canada down to Mexico.
[A view of the sky and tree canopy as monarch butterflies flit between the branches. An adult perches on a stem bursting with flowers. A female monarch lays eggs.]
WARE: The round-trip takes 3-5 generations to complete. And most of those will only live about 2-6 weeks, mating and laying eggs during the summer months.
[A monarch perches on a stem in a browning autumn field.]
WARE: But one “super” generation is born in late summer, early fall.
DIRECTOR (off-camera): OK, so it’s that super generation that does the longest leg of migration?
[Hundreds of monarchs flying around a forest and clustered in huge numbers, hanging from branches.]
WARE: Yeah, and if you get a chance to see them gathered at an overwintering site, it’s truly stunning. From October through March, monarchs aggregate in particular spots. Eastern monarchs make their way to Mexico, and most western monarchs wing their way to spots on the California coast.
[Camera tilts down from tree to sign saying, “Welcome to Pismo Beach.]
[On-screen text: Pismo State Beach Monarch Butterfly Grove | Pismo Beach, California]
WARE: We already know where many of these sites are based on decades of observational data.
[Two women stand near a large sign, decorated with paintings of monarchs. One uses binoculars to look up at the butterflies above.]
[One of the two women from the previous shot speaks to camera.]
[On-screen text: Ashley Fisher | Monarch Overwintering Specialist, Xerces Society]
ASHLEY FISHER (Monarch Overwintering Specialist, Xerces Society): I think it’s really special that they overwinter basically really in your face on the coast of California, with thousands of them hanging out together.
[Fisher and her fellow researcher, Isis Howard, walk over a wooden bridge in the Pismo Beach Grove. They stroll through the grove, talking and looking at butterflies.]
WARE: Conservation biologists Ashley Fisher and Isis Howard are researchers with the Xerces Society, a science-based nonprofit focused on conserving invertebrate species. They’re tracking monarchs during their stay in California to understand changes in populations over time, and what factors might be causing the big declines in their numbers.
[Isis speaks to camera, indicating the butterflies all around them.]
[On-screen text: Isis Howard | Western Monarch Community Science, Xerces Society]
ISIS HOWARD (Endangered Species Conservation Biologist, Xerces Society): What we see here are monarch butterflies that have traveled from Utah, Montana, Northern California, Oregon, Washington…
FISHER: Because of the decrease in daylight and temperature, it triggers something in them that makes them want to travel south, basically.
[Monarchs float through the air over a field of milkweed. Various close-ups of monarchs perched on plants and flapping their wings.]
WARE: But not only do these genetic triggers tell monarchs that it’s time to get a move on, things start to change in the butterflies’ bodies. They’ll start to do things like store more fat, and slow down their aging. This allows them to live for 8 to 9 months—enough time to fly long distances and survive as adults through the winter.
[Dozens of monarchs flit around the Pismo Beach grove.]
FISHER: We are really interested and basically everyone wants to know where the monarchs are going during their migration.
[Fisher sits at a picnic table, displaying a tracking device. The device consists of a handheld grip with a digital screen and a long, ladder-like metal antenna projecting from it.]
FISHER: So, the best way we can do that is to track individuals. So, to track individuals, we're using a technique called radio telemetry. And it's actually been around for a really long time to track a lot bigger animals.
[Nighttime trail cam footage of a mountain lion wearing a radio collar.]
[Close-up of a monarch butterfly with a radio tag stuck to its back. It attaches like a backpack with a long thin wire projecting between the wings. Compared to the insect’s size, it appears a bit bulky.]
FISHER: Historically, even the lighter tags have been, you know, 0.15 grams. And for a butterfly, even that is a lot because monarch butterflies, even though they're one of the larger species in North America, they only weigh less than a gram.
[Extreme close-up of slim radio tag, held in Fisher’s hand. It consists of a tiny solar panel and a long “tail” projecting out from the end.]
FISHER: These, on the other hand, are incredibly light. It's just a solar panel that is connected to the tail, which is basically its little antenna.
[A series of still images shows these new, lightweight tags being attached to adult monarch butterflies.]
[An adult monarch flies in slow motion over a field of milkweed.]
FISHER: If we can better understand how individuals travel and how they spend their time in overwintering habitat on the coast, we can better understand what kind of habitat they need to complete their overwintering cycle.
[Various shots of eucalyptus trees and butterflies in the Pismo Beach grove. Hundreds of butterflies gather around a stream bed in a Mexican forest.]
WARE: Monarchs look for pretty specific amenities during their winter vacation: dappled sunlight, high humidity, access to fresh water, and no freezing temps or strong winds.
[Howard speaks to camera at Pismo Beach.]
HOWARD: When monarch butterflies aggregate at the overwintering site, they often form what we call a cluster.
[Hundreds, if not thousands of monarchs hang from tree branches. Their wings are folded up and they’re so numerous, they almost look like leaves themselves.]
HOWARD: When the ambient temperature starts to reach or exceed 55 degrees Fahrenheit, monarchs are then able to utilize their wing muscles and start flying.
[Monarchs clustered on a branch ripple their wings open in a sequence, movement traveling up the branch that looks something like people doing the wave at a sporting event.]
HOWARD: They basically start rippling. And you start to see the flashes of orange and black.
[From a cluster of monarchs at the end of a branch, one butterfly leaps into flight and it’s quickly followed by a dozen more.]
HOWARD: And then eventually one butterfly will make the courageous decision to- to leap out into flight, and then so many will end up following them.
[Various shots of monarchs clustering on branches.]
WARE: While it seems like clustering might be about staying warm, it could also protect them from storms and from overheating. That’s important both so they don’t lose their fat stores too quickly, and to maintain their suspended aging.
[Fisher speaks to camera, standing in the middle of the Pismo Beach grove.]
FISHER: One of the factors we think is really important for determining where monarchs like to cluster in a grove is wind.
[Close-up of a device that looks something like a small boat propeller hangs high in the Pismo Beach tree branches. It has a propeller that turns in the wind and a rudder-like tail fin projecting out behind.]
FISHER: We want to figure out what wind speed is a threshold that starts breaking up monarch butterfly clusters.
[Kyle Nessen, a man in a bright orange safety vest, stands on a ladder leaning against a tree and tugs on a pulley rope to lower the wind speed sensor.]
FISHER: So, there’s a graduate student from Cal Poly named Kyle Nessen, and he put up sensors all around our grove.
[Close-up of Nessen’s hands as he checks the wind speed sensor. He attaches the sensor to his laptop and downloads data.]
KYLE NESSEN (Graduate Student, California Polytechnic State University): I'm going to check the batteries right now and just make sure everything is good to go.
[Nessen speaks to camera in the grove.]
[On-screen text: Kyle Nessen | Graduate Student, California Polytechnic State University]
NESSEN: And then once I'm done, it will record the average wind speed and the maximum speed every minute for the next couple of months.
[Nessen displays a graph of wind speed data on his laptop, and speaks to camera.]
NESSEN: Right now I'm collecting actual wind measurements throughout the grove at different points.
[Various shots of the Pismo Beach Grove trees and environment.]
NESSEN: And then the second half of this will be coming here with a LiDAR drone and we’ll fly this whole area with really high resolution 3D data and recreate the forest structure in the computer.
[Nessen makes adjustments to the wind speed sensors.]
WARE: That data could tell researchers things like where planting extra trees might create a better spot for monarchs to hang.
[Sign at Pismo Beach reads, “Welcome! Current Count: 10,470 as of 1/16/24.” Below that, a white board displays annual counts at Pismo Beach from 1990 to 2023. In 2020 there is a massive drop in numbers.]
WARE: There are plenty of mysteries we’re still trying to unravel when it comes to monarchs, but what we already know is that monarch populations have fluctuated widely over the past decade.
[An animated graph titled “Western Monarch Count” shows number of monarchs on the y-axis, and years on the x-axis. From 2018 through 2020, there’s a massive decline—from numbers in the hundreds of thousands to less than 25,000, and in 2020, the number drops all the way to the very bottom of the graph. The total count rebounds in 2021 to between 200,000-250,000, but less dramatic fluctuations in recent years show that numbers are still not stable.]
WARE: In 2020, when the western monarch population dropped to 2,000, that seems like too much.
[Fisher and Howard sit on a bench in the butterfly grove and speak to camera.]
FISHER: And they were close to what we would call an extinction vortex, which is where there's so few numbers that there's no way that they could mathematically recover. Luckily, we had a really big bounce back and really good years. If you compare it to the population numbers we saw in the eighties, we are still in a long term decline. The one good year does not necessarily make up for-
HOWARD: Dozens of-
FISHER: Decades. Yeah many decades of not good years.
[Various shots of monarch caterpillars chowing down on milkweed leaves.]
WARE: With a vulnerable population, protecting habitat can be key. Monarch caterpillars have evolved to dine almost exclusively on a plant called milkweed. Milkweed contains toxins called cardenolides that monarch caterpillars can sequester in their bodies and retain as adults, a defense against would-be predators.
[Close-ups of milkweed plants and adult monarch butterflies.]
WARE: They need these plants to survive. But due to habitat loss, milkweed breeding areas are in increasingly short supply. That, plus pesticide use, and climate change are all big threats facing these butterflies.
[Fisher and Howard talk and observe butterflies at Pismo Beach.]
FISHER: It's so great that we have monarchs in our communities, but it's also pretty unfortunate that where they choose to overwinter is on the coast of California, which is pretty prime real estate.
[Large machines like forklifts and dump trucks are parked at a construction site just across the street from the Pismo Beach butterfly grove.]
HOWARD: The plain fact is that if we lose the overwintering sites along the California coast, we're going to lose the monarch migration as well.
[Ware speaks to camera in the Vivarium.]
WARE: We know more about monarch populations perhaps than any other type of insect that's out there. And yet we still don't really have enough information to know the magnitude of what this kind of massive drops, and increases and drops, mean in terms of long-term survival for monarchs. I think people should be worried.
[Hundreds of monarchs flit around the butterfly grove and perch on branches. Howard wanders through the grove trails, looking for butterflies.]
WARE: But there are amazing efforts underway to keep tabs on monarch numbers. Isis works with volunteers for the Western Monarch Count, a community science-driven initiative that’s been collecting data since 1997.
[Fisher speaks to camera in the grove, standing next to a sign explaining the function of the wind sensors.]
FISHER: We're putting a lot of brainpower and a lot of money into conserving habitat for the monarch butterflies to try to save their population. I think we care because it's just such an incredible phenomenon that a lot of people feel a lot of emotional connection to.
[Various shots of butterflies and insects native to the California coast.]
HOWARD: The declines in monarch butterflies, especially on the West Coast, really represent and reflect the declines of other beneficial native insects and pollinators. So, our efforts to provide habitat and protect habitat for monarch butterflies is really benefiting more than just these butterflies.
[Howard speaks to camera in the grove. Various close-ups of monarch butterflies.]
HOWARD: Monarchs have touched most of our lives in one way or another. So, they really represent this symbol and icon that can not only bring people together over pollinator conservation, but biodiversity conservation at large.
[Monarchs clustered on a eucalyptus branch ripple and flutter their wings in front of a beautiful blue sky.]
[Credits roll]
[Ware speaks to camera in the Vivarium.]
WARE: As you might imagine, for people who lived in the area where monarchs arrived in tens of thousands of individuals, there was a significant event that happened every year.
[Día de Muertos decorations, including large, colorfully painted skulls, giant monarch butterflies, and a skeleton sporting large monarch-like wings.]
WARE: The monarchs’ migration to the southwestern United States and Mexico coincides with Día de Muertos in early November. To some people, the butterflies represent the souls of deceased loved ones who come back to visit. Preserving a species can help us preserve cultural heritage, too.
[A sign at Pismo Beach reads, “Want to Help Monarchs? Plant a Pollinator Garden”. Various shots of milkweed plants.]
WARE: So what can you do to help monarchs? Well, you can plant species of milkweed that are native to your area.
[People plant flowers in a community garden.]
WARE: You can plant nectaring flowers for adult butterflies to enjoy.
[People tag butterflies with numbered stickers, and look through magnifying lenses and binoculars.]
WARE: You can reduce pesticide use and find a community science project in your area. You don’t need radio tags or high tech scanners to make a difference!
[Quick shots of the ocean, sharks, and marine biologists.]
WARE: Before you go, if you’re a fan of underappreciated animals (and if you’ve made it this far that seems likely) we wanted to tell you about SHARKS UNKNOWN WITH JASMIN GRAHAM. It’s another awesome animal series right here on Terra, which follows a diverse team of shark researchers as they study these amazing, yet often misunderstood creatures. There’s a link in the description, we think you’ll love it. Thanks!
Why are monarch butterflies disappearing? You may know them for their amazing transcontinental migration, but over the past few decades, monarch butterflies have been part of a vanishing act that has scientists worried. Conservation biologists Ashley Fisher and Isis Howard show us what it takes to track monarch butterfly populations at one of their favorite overwintering spots.
Join our host and museum curator Jessica Ware, Ph.D. for a close look at this iconic insect, its incredible lifecycle, and its multigenerational migration. Then, follow scientists who are working at a monarch butterfly roosting site in the western U.S. to figure out what’s behind the monarch butterfly population plunge. The series is produced for PBS by the American Museum of Natural History.