Anurag Agruwal: Great News for Monarch Overwintering Population

The estimates of the monarch butterfly overwintering population were announced today (Wed. Jan. 30th 2019) by WWF Mexico.  The butterflies are so dense at their dozen or so mountain-top clustering sites that overwintering butterflies cannot be counted individually.  Instead, the area of forest that is densely coated with butterflies (at about 5,000 butterflies per square meter looking up into the canopy) is estimated as a measure of monarch abundance.  Butterflies arrive to Mexico around the day of dead in November and stay until March each year as part of their annual migratory cycle.  Butterflies have been declining over the past three decades, and the annual announcement is a welcome addition to our understanding of the long-term dynamics of our beloved monarch.

The annual multi-generational migratory cycle of the monarch butterfly. The southernmost red dot indicates the high elevation overwintering grounds in central Mexico where populations are censused. North pointing arrows indicate the spring and summer generations that migrate, breed, and eat milkweed. Learn more in my book Monarchs and Milkweed.

This winter season (2018-2019), there were approximately 6.05 hectares (nearly 15 acres) of forest occupied with dense monarchs in the Mexican highlands (somewhere in the neighborhood of 300,000 million overwintering butterflies).  The monarchs end up congregating in a tiny area, with the bulk of the butterflies concentrated among twelve mountain massifs (clusters of peaks) within three hundred square miles (eight hundred square kilometers), an area smaller than New York City. In other words, most of the monarchs from eastern North America, from Maine to Saskatchewan, and south to Texas, probably covering two million square miles, funnel down and overwinter in a location 0.015 percent the area that they occupy in the summer!  Unbelievable. This year’s estimate is well over double compared to last year, great news for monarchs!

Where does this leave us?  The good news is that this year’s population was huge in the summer months throughout the USA and Canada, and the resulting migration and overwintering population in Mexico was the highest in 12 years, higher than predicted by many.  The season started with a very early spring and a far reaching northern migration.  As I have previously argued, there is often a disconnect between summer breeding populations of monarchs and the overwintering population — that seems to not be the case this past year.

With 26 years of data, there are various ways to plot and assess the trends.  Below I have plotted four year averages for seven periods working backwards (so the first average on the left is only for 2 years).  Any way you slice it, the trend has been negative, and the population is not what it was.  Nonetheless, the extreme downward trend seems to have bumped up in the last period of four years.  Is this the new norm, a winter population hovering between two and five hectares?  How dangerously low are these numbers? And what can be done to continue to reverse the trend and buffer the population?  I have recently written more about this issue in a scientific article as a well as my book.

For now, let’s celebrate. The government is open, and thus the Fish and Wildlife Service will be deciding on the petition to list monarchs as threatened under the Endangered Species Act this summer. Looking forward to seeing butterflies and their caterpillars once Ithaca, NY thaws in spring. Thanks for reading! 

Anurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed and lives in Ithaca, New York.



Anurag Agrawal: Monarchs & Milkweed in Mexico

Greetings monarch and milkweed enthusiasts from Mexico. This is Part II in my series from Oaxaca, where I am based on sabbatical leave from Cornell (see the first post here). This post follows up on the inspiration I am gaining on sabbatical and is an entryway for my next research and writing projects, following up on my recent book Monarchs and Milkweeds. I expect that the next post will be a detour from this series, as the overwintering numbers of monarch butterflies will soon be announced by the World Wildlife Fund – and this will provide an opportunity to reflect on monarch population trends (are they still declining?) (see last year’s post on the population here).

Oaxaca city is a bustling cultural and culinary capital, tropical, yet it sits at mid-elevation around 5000 ft above sea level.  January is the beginning of the dry season, with many trees beginning to lose their deciduous leaves.  And the “weedy” milkweed of Mexico, analogous to the common milkweed (Asclepias syriaca) of the eastern USA, is blood flower, sometimes called tropical milkweed, Asclepias curassavica.  This species, however, needs moisture and protection from the intense, constant and direct sunlight loved by many other milkweeds.  Here in the middle of the city, where a stream provides both moisture and shade, blood flower is abundant and apparently always in flower.  Hundreds of plants are along this corridor.

Although hundreds of millions of butterflies are currently overwintering in Michoacán (within the Monarch Butterfly Biosphere Reserve), here in Oaxaca, monarchs have a year-around (and apparently non-migratory) population. Adult butterflies were abundant in this stream corridor in January as were many eggs and large caterpillars.

This caterpillar found a quite spot away from milkweed to molt (change its exoskeleton to the larger size).

Butterflies fluttered, twirled around in mating behavior, and females could be seen curling their abdomens in preparation for laying an egg. The fact that non-migratory monarchs are abundant in the city of Oaxaca gives me pause.  I am not sure how long this has been the case.  Are these butterflies a reservoir for the declining migratory butterflies?

In the mid-elevation grasslands on the edge of town, this rare spring flowering milkweed was blooming early. Formerly known as Asclepias rosea, and now renamed to Asclepias senecionifolia (“the milkweed with leaves like Senecio”), it is pretty small and has feathery fine leaves.

This individual has the oleander aphid on it, feeding away (see yellow bugs in the center) and 5 (!) butterfly eggs.  I wasn’t sure which species of butterfly, until I noticed the plant next door.

This mature caterpillar is not a monarch, and I cannot quite tell if it is a “Soldier” Danaus eresimus or a “Queen” Danaus gillipus. Some of the closest relatives of our beloved monarch. Note that although the monarch has two pairs of tentacles, the soldier and queen sports three pairs, likely to sense the sounds, vibrations, and other aspects of its local environment.

As noted in the first post, the nodding milkweed, Asclepias glaucescens, a robus and waxy milkweed can be common in the region.

The creamy white (and large!) flower of the nodding milkweed, Asclepias glaucescens.


Leaves can be green to purple, and the prominent venation shows the canals that hold pressurized latex.

Break a leaf and experience the thick and toxic goo.  Both this species and the A. senecionifolia above exude large quantities of latex.

Before I encountered butterflies or caterpillars on the nodding milkweed, I came across this “bug” (a true bug, as it were, in the insect group Hemiptera). Note the highly contrasting red and deep blue, a classic advertisement of the toxic milkweed insects. Colleagues Georg Petschenka and Jürgen Deckert helped identify this as an immature of Largus species. This reminded me that I had previously found a Largus species associated with Asclepias linaria in northern Mexico about a decade ago.  We currently don’t know the extent to which Largus is specialized on milkweed (versus eating other plants) or whether it gains toxicity by sequestering milkweed’s toxins. Perhaps a good PhD dissertation for one my students!

Egg dumping!  This nodding milkweed had, count them, over 9 eggs, an unusual phenomenon for monarchs.  But I didn’t know if these were monarch or solider/queen eggs.  I suspected soldier because I had found solider caterpillars nearby (see above). Nonetheless, after they hatched a few days later, it was clear, they were monarchs!

Nonetheless, I found another soldier/queen caterpillar on a neighboring plant. Note the notch in the leaf made by a caterpillar (the vein drain!)

The soldier (or queen!) in all its glory.

Among the 25 or so nodding milkweeds I found, I came to realize that more than one Danaus species were coexisting. Several caterpillars of each species intermingled.  It is unclear the extent to which butterflies and caterpillars recognize each other as similar or different species, and what ecological consequence this has.  Do they compete?

And lastly, on the nodding milkweed Asclepias glaucescens, were these green aphids.  Not the oleander aphid found on A. senecionifolia above. Some ants collected their sugary excrement, but they were not so heavily tended.  The species identity and relationship with milkweed are unknown.

Later last week, I went on a hike to San Pablo Cuatro Venados, a close by community that sits near the top of a ridge (around 9000 ft above sea level) that forms the western wall of the Oaxaca Valley.  It was chili at the start of the hike, with thin mountain air along the 10 km trek on a dusty dry dirt road.  Just as we were turning around, I noticed this dusty purplish plant.  I instantly knew it was one of the rarer highland milkweeds.

The spectacular but confusing Asclepias melantha. Confusing only because I did not expect to see it flowering until the beginning of the rainy season in June or July.

Note the petals of Asclepias melantha. Although milkweed petals are often “reflexed”, or pushed back to give the flower a rocket-like appearance (see A. senecionifolia above), these petals form a cup around the rest of the flower.  The flowers of Asclepias glaucescens above are not reflexed either, but are less cup-like.

Back at lower elevation in the grasslands, this Asclepias oenotheroides (“looking like Oenothera”) had multiple butterfly eggs.  This species, known as zizotes milkweed, is common the south-central USA and all the way down here in Oaxaca.

Flowers of zizotes milkweed, Asclepias oenotheroides.

A large monarch caterpillar munching away on zizotes milkweed, Asclepias oenotheroides.


Alas, I thought I would have a butterfly, but here, as in so many other places, parasites like this fly larva got the best of the caterpillar! These flies are “parasitoids” who lay an egg in a caterpillar and then eat them from the inside out.


Anurag Agrawal (photographed, left, with a Malagasy elephant milkweed [Pachypodium] in downtown Oaxaca, Mexico) is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University.


He lives in Ithaca, New York. For more information, see his blog, publications, and multi-media on monarchs and milkweed.

John Tutino: Mexico, Mexicans, and the Challenge of Global Capitalism

This piece has been published in collaboration with the History News Network. 

TutinoMexico and Mexicans are in the news these days. The Trump administration demands a wall to keep Mexicans out of “America,” insisting that undocumented immigrants cause unemployment, low wages, and worse north of the border. It presses a renegotiation of the North American Free Trade Agreement, claiming to defend U.S. workers from the pernicious impacts of a deal said to favor Mexico and its people. Meanwhile U.S. businesses (from autos to agriculture) work to keep the gains they have made in decades of profitable cross-border production and marketing. Their lobbying highlights the profits they make employing Mexicans who earn little (at home and in the U.S.), and by their efforts subsidize U.S. businesses and consumers.

The integration of Mexico and the U.S., their workers and markets, is pivotal to U.S. power, yet problematic to many U.S. voters who feel prejudiced in a world of globalizing capitalism and buy into stereotypes that proclaim invasive Mexicans the cause of so many problems. Analysts of diverse views, including many scholars, often imagine that this all began in the 1990s with NAFTA. A historical survey, however, shows that the integration of North America’s economies began with the U.S. taking rich lands from Texas to California by war in the 1840s, driving the border south to its current location. U.S. capitalists led a westward expansion and turned south to rule railroads, mining, petroleum, and more in Mexico before 1910—while Mexican migrants went north to build railroads, harvest crops, and supply cities in lands once Mexican. The revolution that followed in part reacted to U.S. economic power; its disruptions sent more Mexicans north to work. While Mexico struggled toward national development in the 1920s, displaced families still moved north. When depression stalled the U.S. economy in the 1930s, Mexicans (including many born U.S. citizens) were expelled south. When World War II stimulated both North American economies, the nations contracted to draw Mexican men north to work as braceros. Mexico’s “miracle” growth after 1950 relied on U.S. models, capital, and labor-saving technology—and never created enough work to curtail migrant flows. The Mexican oil boom of the 1970s tapped U.S. funds, aiming to bring down OPEC oil prices to favor U.S. hegemony in a Cold-War world. By the 1980s the U.S. gained cheaper oil, helping re-start its economy. In the same decade, falling oil prices set off a debt fueled depression in Mexico that drove more people north. NAFTA, another Mexican collapse, and soaring migration followed in the 1990s. The history of life and work across the U.S.-Mexican border is long and complex. Through twists and turns it shaped modern Mexico while drawing profits, produce, and Mexicans to the U.S.

The Mexican Heartland takes a long view to explore how communities around Mexico City sustained, shaped, and at times challenged capitalism from its sixteenth century origins to our globalizing times. From the 1550s they fed an economy that sent silver, then the world’s primary money, to fuel trades that linked China, South Asia, Europe, and Africa—before British America began. By the eighteenth century, Mexico City was the richest place in the Americas, financing mines and global trade, sustained by people living in landed communities and laboring at commercial estates. It’s merchant-financiers and landed oligarchs were the richest men in the Americas while the coastal colonies of British America drew small profits sending tobacco to Europe and food to Caribbean plantations (the other American engines of early capitalism).

Then, imperial wars mixed with revolutionary risings to bring a world of change: North American merchants and slave holders escaped British rule after 1776, founding the United States; slaves in Saint Domingue took arms, claimed freedom, destroyed sugar plantations, and ended French rule, making Haiti by 1804; insurgents north of Mexico City took down silver capitalism and Spain’s empire after 1810, founding Mexico in 1821. Amid those conflicts, Britain forged a new industrial world while the U.S. began a rise to continental hegemony, taking lands from native peoples and Mexico to expand cotton and slavery, gain gold and silver, and settle European migrants. Meanwhile, Mexicans struggled to make a nation in a reduced territory while searching for a new economy.

The Mexican Heartland explores how families built lives within capitalism before and after the U.S. rose to power. They sought the best they could get from economies made and remade to profit the few. Grounded in landed communities sanctioned by Spain’s empire, they provided produce and labor to carry silver capitalism. When nineteenth-century liberals denied community land rights, villagers pushed back in long struggles. When land became scarce as new machines curtailed work and income, they joined Zapata in revolution after 1910. They gained land, rebuilt communities, and carried a national development project. Then after 1950, medical capitalism delivered antibiotics that fueled a population explosion while “green revolution” agriculture profited by expanding harvests while making work and income scarce. People without land or work thronged to burgeoning cities and across the border into the U.S., searching for new ways to survive, sustain families, and re-create communities.

Now, Mexicans’ continuing search for sustainable lives and sustaining communities is proclaimed an assault on U.S. power and prosperity. Such claims distract us from the myriad ways that Mexicans feed the profits of global corporations, the prosperity of the U.S. economy, and the comforts of many consumers. Mexicans’ efforts to sustain families and communities have long benefitted capitalism, even as they periodically challenged capitalists and their political allies to keep promises of shared prosperity. Yet many in the U.S. blame Mexico and Mexicans for the insecurities, inequities, and scarce opportunities that mark too many lives under urbanizing global capitalism.

Can a wall can solve problems of dependence and insecurity pervasive on both sides of the border? Or would it lock in inequities and turn neighboring nations proclaiming shared democratic values into ever more coercive police states? Can we dream that those who proclaim the liberating good of democratic capitalism may allow people across North America to pursue secure sustenance, build sustaining communities, and moderate soaring inequities? Such questions define our times and will shape our future. The historic struggles of Mexican communities illuminate the challenges we face—and reveal the power of people who persevere.

John Tutino is professor of history and international affairs and director of the Americas Initiative at Georgetown University. His books include The Mexican Heartland: How Communities Shaped Capitalism, a Nation, and World History, 1500-2000 and From Insurrection to Revolution in Mexico: Social Bases of Agrarian Violence, 1750–1940.