The Vanishing Flights of the Monarch Butterfly

February 20, 2019By

Monarch butterflies east of the Rockies typically start the journey in Canada and the
upper Midwest, aiming for the Oyamel-fir forests in Mexico’s Transverse Neovolcanic Mountains.

 

Walking around my family’s property in upstate New York last summer, I noticed something I hadn’t seen in years: scores of monarch butterflies flying around the milkweed that rings the perimeter of the yard. Six months later, at the end of January, biologists attending the Trinational Monarch Science Meeting, in Mexico City, confirmed what I and many others had been seeing throughout the summer and fall: the eastern monarch population was a hundred and forty-four per cent larger than it had been a year earlier. The announcement offered a modicum of hope amid dire warnings of mass extinctions and ecological catastrophe. Just two weeks earlier, the Xerces Society for Invertebrate Conservation had issued a finding that the monarch population west of the Rockies dropped by around ninety per cent in the past year and is on the verge of collapse.

Monarch butterflies migrate. Though they weigh less than a gram, they travel thousands of miles each fall to overwintering sites that provide the right microclimate to enable them to survive for months with little food or water. To track these migration patterns, citizen scientists have been gluing small tags on monarchs’ wings since the nineteen-fifties. The data from recovered tags is continually overlaid on maps that show where the butterflies are going and where they are coming from. That’s how we now know that monarchs east of the Rockies typically start the journey in Canada and the upper Midwest, aiming for the Oyamel-fir forests in Mexico’s Transverse Neovolcanic Mountains. In the spring, the butterflies lay eggs in Texas before dying; successive generations move northward in a kind of relay race that follows the proliferation of milkweed, their host plant. Monarchs that begin their journey west of the Rockies do something similar: after wintering on the coast of California, shielded by stands of eucalyptus or Monterey pine, they move inland to the Central Valley, but also north to Washington State and southern British Columbia, and to Nevada, Arizona, Idaho, and possibly Montana.

One day in July, walking around our pond as my dog hunted for frogs, I watched two monarchs mate overhead, and, when they separated, I followed the female as she laid an egg on the underside of a nearby milkweed leaf. Milkweed is the monarch’s host plant because it contains a toxin that is poisonous to the butterfly’s predators. Monarch caterpillars chew the leaves and ingest the poison, which will protect the butterflies that they eventually become. I took the leaf with the egg and brought it inside.

Four days later, the egg, which was white and approximately the size of a grain of sand, was gone, and in its place, barely visible, was the tiniest yellow-black-and-white caterpillar, so small that my husband had no idea where it was when I showed him. A few days, a few molts, and a few milkweed leaves later, the caterpillar was about a half-inch long and fattening up, following a timetable nearly as reliable as a Swiss watch. Another molt followed, and the caterpillar unzipped, then discarded, its striped jacket. Underneath, there was an identical one, one size larger. This phase of the metamorphosis happened to coincide with the death of Lincoln Brower, an emeritus professor of zoology who the Times, in its obituary, rightly called the “champion of the Monarch butterfly.” I attached a small sign, which read “The Lincoln Bedroom,” to the enclosure that I’d made for the caterpillar. I started to refer to the caterpillar as Lincoln, too—an anthropomorphism that I hoped Professor Brower, with whom I’d travelled to Mexico a number of times to see the butterflies, would have forgiven.

In 1995, Brower, who was then at the University of Florida, published a paper in the Journal of the Lepidopterists’ Society, explaining that while the monarch butterfly was not an endangered species—it can also be found in Hawaii, the Caribbean, Spain, Portugal, Australia and New Zealand—its North American migration patterns east of the Rockies were at grave risk of disappearing. In that paper, he called it an “endangered biological phenomenon.” Less than two decades later, in 2013, that danger appeared to have come to fruition, when that winter’s monarch population covered less than three acres of the Mexican forest, down from around twenty acres ten years earlier.

At the recent Monarch Science meeting, in Mexico City, when researchers reported that this year’s migration covered six hectares, or nearly fifteen acres, “people were cheering,” Chip Taylor, an emeritus professor of ecology and evolutionary biology at the University of Kansas, said. This year was not only a banner year for eastern monarchs, it was one for tagging as well. According to Taylor, who, since 1992, has run Monarch Watch, the most prolific monarch-tagging program in the world, four hundred thousand tags were distributed in 2018, one and a half times as many as the year before. Lincoln Brower’s great friend and occasional collaborator, the Mexican poet Homero Aridjis, another monarch champion, told me in an e-mail that he was seeing aggregations of monarchs in the mountains near his boyhood home of Contepec for the first time in a decade.

Though this is all encouraging, a single year is not a trend, and scientists remain concerned about the viability of the migration. “We should celebrate the fact that we go up to this six-hectare number, and people who are living in areas where monarchs breed really noticed them this summer,” Karen Oberhauser, the director of the University of Wisconsin-Madison Arboretum, who researches monarchs, told me. “It illustrates the fact that they have this incredible potential.” But a population swing in the opposite direction can happen, too, she said. Oberhauser and her research associates have found that there is an underlying threat from habitat loss. (One study estimated that two hundred and twenty three million milkweed stems were lost in the Midwest between 2008 and 2016.) According to the World Wildlife Fund, six hectares is just enough to sustain a viable population in North America.

Taylor has a one-word explanation for why 2018 was an especially good year for eastern monarchs, and why that good year likely portends bad ones to come: temperature. In March, when the monarchs arrived in Texas from Mexico, the mean temperatures were 5.4 degrees Fahrenheit above normal, which should have blown the butterflies all the way to Kansas.The butterflies couldn’t go farther north, though, because it was too cold in northern Texas. “I looked at the weather every day and I’d say, ‘damn, that’s good. Keep your babies in Texas,’” Taylor said. In May, the temperatures were optimal as the butterflies moved north, and they were ideal in the summer as monarchs spread out across their breeding range. Then, in the fall, as the butterflies headed south, those numbers were favorable, too. “What we had this year is everything was positive in every one of those stages,” Taylor told me. “This is why I say this is not likely to happen again—because you’re not likely to see a pattern like this again.”

West of the Rockies, the number of monarchs overwintering in California is estimated to be around twenty thousand, down eighty-six per cent since last year, which “isn’t completely unprecedented,” Emma Pelton, a researcher with the Xerces Society, told me. “I’ve looked back at data between 1980 and today, and there have been other instances where we’ve had a single-year drop of this magnitude. But the difference is that this is all in the context of a ninety-seven per cent decline since the nineteen-eighties.” The current situation, Pelton added, is considered “a quasi-extinction.” There may not be enough butterflies to repopulate the range. If that happens, there will still be western monarchs, but they will be increasingly hard to find. “That would be like losing all the rhinos, except the rhinos in the zoos,” Pelton said. “Really, the threat is that we’re going to lose migration.”

In many ways, the stressors compromising western monarchs are the same as those affecting monarchs east of the Rockies: habitat loss both in the overwintering sites and along the migratory pathways; the proliferation of herbicides and insecticides, especially in wide swaths of the agricultural Midwest and the Central Valley; and increased temperatures due to climate change. But added to these are a protracted drought, enormous wildfires, and the propagation of non-native, tropical milkweed that carries a deadly parasite. Monarchs are “a very resilient species,” Taylor said. “It survived long periods of negative conditions.” The question now, in the western United States, is if the butterflies are resilient enough to rebound from such a precipitous decline.

Robert Michael Pyle, the founder of the Xerces Society and an expert on the western monarch migration, thinks that they may be, but for an unlikely reason. The western migration may be an accident of happenstance, and its fate may be tied to the fate of the eastern monarchs in the Mexican mountains. “I have long thought that the Californian wintering monarchs may be an epiphenomenon—a more or less ephemeral state, subject to radical fluctuations, perhaps even extinction and recolonization,” Pyle wrote in an e-mail. It wasn’t until 1816 that the existence of monarchs in California entered the written record, when they were observed by Russian naturalists on an expedition to circumnavigate the globe. Earlier accounts of the region, dating back to the seventeenth century, by Native Americans, early explorers, and priests sent by the Catholic Church to establish missions, make no mention of the butterflies, leading Pyle to suggest that the western migration “might not even be that old.”

“Lincoln Brower and I have hypothesized that it may have blinked on and off in the past, having been refreshed by chance incursions of Mexican monarchs pushed westerly in the spring by shifting winds,” Pyle wrote. Between 1990 and 1997, he added, the population of California monarchs crashed nearly as severely as it did in 2018, and then recovered. “So, while I cannot say whether the current crash is systemic and lethal or stochastic and recoverable, I suspect the latter,” he said, “And I am very hopeful that the higher numbers in Mexico this winter might be propitiously poised to aid in Californian recovery next spring. It all depends on which way the wind blows.”

The wind was not blowing at all inside the Lincoln Bedroom as the caterpillar chewed through milkweed leaves and grew proportionately. Outside, in the yard, the butterflies and caterpillars continued to multiply, though predators were quick to find the larvae, and the mortality rate, by my informal daily observations, was high. Then, about two weeks after emerging from his egg, Lincoln climbed to the top of the enclosure, secreted a white gooey liquid, attached himself to it, and then hung like a solitary letter “J.” Days later, he appeared to turn himself inside out and then disappeared into a green, gold-flecked chrysalis. He was gone, though it was not clear where, exactly, he’d gone to.

And then I waited, checking the chrysalis obsessively, even though I knew it would be at least a week before a butterfly emerged. Ten days later, I awoke to find the chrysalis transparent, and the black and orange of the monarch’s wings visible through it. Rebirth was imminent, or so I thought. More waiting, more watching, some worrying—I’d had monarchs die before, hanging stillborn in their cradles. But, on the fourteenth day, I witnessed Lincoln break through what now appeared to be the thinnest of membranes, clinging to it for quite a while, his wings still soft and folded. And then, as if someone were pumping them up, they unfurled and began to take shape. As they did, I noticed from the absence of pheromone patches, that Lincoln was a she, not a he.

I asked Oberhauser, who has devoted most of her professional life to monarch butterflies, why anyone should care if the monarch population crashes, in Mexico or out West. “We can look at a species and say it’s important because it’s a pollinator or it’s a keystone species, and if we pull it out of an ecosystem it’s going to have all these harmful, cascading effects,” she said. “Monarchs probably don’t have big-picture ecosystem importance. But just the fact that they connect people to nature is reason enough to make us care.” At one point, she told me, “When we lose these strong connections with nature, from the point of view of conservation, we lose one of our most important strengths. When people stop caring, things are going to get worse a lot faster.”

Once Lincoln’s wings hardened, I carried her and her bedroom outdoors, opened the enclosure, and got ready for her to fly away. She didn’t. We sat peaceably for a couple of hours, both of us basking in the sun. She would flap her wings a few times and then stop, like a car engine that could not quite turn over. And then it did, and she took flight, landing on the branch of a nearby tree. Minutes later, she soared above the house and disappeared.

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Sue Halpern, a contributing writer covering politics and technology, has been writing for The New Yorker since 2005. She is the author, most recently, of the novel “Summer Hours at the Robbers Library.”

 

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