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.

Browse Our New Biology 2017-2018 Catalog

In our Biology 2017-2018 catalog you will find a host of new books, from an in depth look at the complex relationship between one of our most beautiful butterflies and a family of poisonous plants to a fascinating exploration of the role of beauty and attraction in sexual selection.

If you will be at ESA in Portland, we will be in booth 703. Join us for a reception with wine and light refreshments to celebrate our new titles and meet our authors at 5pm, Tuesday, August 8th. Or stop by any time to check out our full range of titles in biology and related fields.

In Monarchs and Milkweed, Anurag Agrawal draws on more than a decade of research to bring an unsurpassed account of the coevolution of the monarch butterfly and the milkweed. Monarchs lay their eggs exclusively on milkweed plants, on which they feed in the early stages of their lives as caterpillars. The milkweed has evolved a battery of defensive characteristics to reduce the depredations of monarch caterpillars; in turn, monarchs have evolved their own means of overcoming these defenses. Learn about this evolutionary arms race, and much more besides in Monarchs and Milkweed.


In his theory of sexual selection, Darwin argued that animals have “a taste for the beautiful” that governs their attraction to potential mates. But in what does this taste reside? How does it affect the evolution of physical characteristics in animals? What is beautiful to a frog or a peahen? In A Taste for the Beautiful, Michael Ryan delves deeply into the question of sexual attraction and argues that beauty is in the brain of the beholder.

A Taste for the Beautiful by Michael Ryan

Few people know Darwin’s life and work as well as his biographer, Janet Browne (Charles Darwin: Voyaging, Charles Darwin: The Power of Place), making her the perfect editor for The Quotable Darwin. Drawing on the full range of Darwin’s writings, including his letters to friends and family, and his private notebooks, The Quotable Darwin is an unforgettable picture of the man and his thought in his own words.

The Quotable Darwin by Janet Browne

Find these books and many more in our Biology 2017-2018 catalog.