Monarchs & Milkweed in Mexico – Part IV


Art, biodiversity, chilies, Danaus, and extremes… that’s what I’ve found on sabbatical here in Oaxaca.This is Part IV in my series from Mexico where I am based on sabbatical leave from Cornell (click here for the 3rd post). I’m following up on my recent book Monarchs and Milkweeds: A migrating butterfly, a poisonous plant, and their remarkable story of coevolution.  For the latest review of M&M in the Quarterly Review of Biology, click here. In this post: corn fields, Oaxaqueño milkweeds, interesting milkweed insects, the end of the overwintering season in Michoacán, and a bonus exploration of a couple of plants in the milkweed family (Apocynaceae).

It’s the dry season… and without irrigation, corn isn’t planted again in Oaxaca until May.
The nodding milkweed, Asclepias glaucescens, enjoying the off-season in this corn field.  My partner Jennifer says the common occurrence of milkweeds in corn fields suggests a long associations between this common food, the milky weed, and our beloved butterfly.
The real pollinators of milkweed. Yes, honeybees visit milkweed flowers and do an okay job, but they are not native.  Yes, monarchs happily drink milkweed’s nectar, but they do a terrible job pollinating the plant.  The best pollinators of milkweed are large hymenopterans (bees and wasps). Bumblebees do a good job.  But also prevalent in many places (from the Midwestern USA to the far west, and throughout Mexico) are pompilid wasps (spider wasps or tarantula hawks!) which feed on, you guessed it, spiders … often bigger then themselves.  They are also big nectar drinkers and very effective pollinators. Above is another giant wasp, I think in the family Scoliidae, which are parasitoids of scarab larvae… as adults, they are also nectar feeders and excellent pollinators of milkweed… see it happen next…
The orange bits attached to these hairy legs are milkweed’s pollinia. As they climb over the flowers, pollinia are pulled out and later inserted incidentally into the slit.  When monarchs are on milkweed flowers, they usually sit on top, sipping the nectar, but typically do not drag their spiny legs in the right spot.
Lygaeus seed bugs. Back-to-back mating and perhaps some competition?
Monarchs abound in this field. This rare look into a milkweed flower bud is provided by a hungry caterpillar that has chewed it open. Note the additional monarch egg laid under the same bud. Also note that the monarch “notched” the stem leading to this flower bud — it cut off the latex supply before eating (latex flows through to the floral parts).
And it’s not the only species, here Asclepias oenotheroides or Zizotes milkweed has also popped up to the right of this old corn stalk.
Up close, the oleander aphid, Aphis nerii, another usual suspect.
Culinarily, maize had its origins in Mexico, but this isn’t corn. Rather it’s escamol, a dish prepared with larvae and pupae of ants in the genus Liometopum (with plenty of garlic and butter).
The tropical milkweed, Asclepias curassavica, is weedy, continues to bloom, and is common throughout the Oaxaca valley; seen here near Tlacolula.  These ever-flowering milkweeds in wet areas support a large non-migratory population of monarch butterflies.
And the aposematic insects on milkweed abound here too.  I have been regularly finding this beetle, Diabrotica nummularis.  It has previously been collected from milkweed, and was feeding on leaves here.  Their larvae are typically root feeders (!), but probably on other plant species… the exact association between this beetle and milkweed is unclear, although it appears to be a regular pest on topical milkweed in Oaxaca.
Up close and personal with Diabrotica nummularis, an aposematic beetle feeding on milkweed, but probably not a specialist insect.
This orange and black color combination just seems to work.  Here is a Mexican species of Oncopeltus, a seed and sap feeding specialist true bug (Hemiptera) on milkweed. 
And even the plants converge.  This Lantana, which is likely native to Mexico, but weedy and invasive elsewhere, is frequently confused with tropical milkweed.  Alas, monarchs are disinterested.
Planted in various gardens around central Mexico, and a bit weedy too (it self-propagates) is an African milkweed, Gomphocarpus physocarpus, sometimes called “hairy balls milkweed” (see below) or balloon plant.  In this garden in San Martín Tilcajete, a monarch fluttered by to lay an egg.
hairy balls milkweed
Gomphocarpus physocarpus flower with monarch egg near by.  A bit unusual in that I typically see eggs on the undersides of leaves on this plant species.
Some weeks later, I traveled to Michoacán. It was late February and time to see the monarchs at their overwintering sites before they journey north.  These are the same individual butterflies that had flown south several thousand kilometers from the Northeast and Midwest in the fall and have been resting for nearly 4 months in the Michoacán highlands!
The three amigos at the Monarch Butterfly Biosphere Reserve, here at Reserva Sierra Chincua.


Due to storms in the past couple of years, the forests on the way, walking to the butterfly colonies, were very bright and open. Fallen trees had been removed for timber and the under-story was uncharacteristically blooming with many flowers.
When it’s warm, they fly.
When it’s cool, they cluster.  This image is from a walk up to the top of Cerro Pelon.
Atop Cerro Pelon… but the altitude was too much for me. We started at 8000 ft and went up to 11500 to the peak in 3 hours…. The hike was tough and I puked on the way down ;-(  Somebody said that I went to commune with the monarchs and ended up like Brower’s barfing blue jays.
Monarch mating is best described as coercive, but this has generated some controversy in the scientific community (see Chapter 4 of my book). Some mating occurs at the end of the overwintering season (beginning late February) as seen here.  The male (with black spots showing on the hind wing) is nearly completely attacked to the female (back-to-back)…. then, he will fly with her passively coming along.
A pile of dead monarchs… compiled on my way down the mountain, heading back to Morelia.
On the big screen for a science lecture at the Ecosystems and Sustainability Research Institute at the National Autonomous University of Mexico (UNAM), Morelia Campus.
After the talk, an original Michoacány beer
Also on the Morelia campus, a week later, was the 5th annual monarch symposium.  Present were several political leaders (of Michoacán and Mexico States) as well as community leaders, scientists, etc.
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An evening keynote lecture that focused on the knowns and unknowns of the monarch decline and possible means for conservation. Ealier this week, the overwintering numbers from 2017-18 were announced by WWF Mexico.  Monarchs were recorded at about 15% lower than last year.
Back in Oaxaca city, one last stop on this journey, to the downtown Jardín Etnobotánico, a spectacular oasis in a bustling city.
The garden is home to a tremendous diversity of the Oaxaqueño plant diversity.  As always, I had my ears peeled for milkweeds and monarchs.
This is Plumaria, sometimes called frangipani, a group of a dozen or more species native to Mexico and further south.  These shrubs and small trees in the milkweed family (Apocynaceae) have familiar seed pods, with a twin pair shown above.
Flowers are without nectar, but apparently attract pollinators by fooling them with scent.
And although monarchs don’t feed on Plumaria, adults were certainly flitting about in the downtown garden.
Plumaria is also a common ornamental in the tropics and is sometimes defoliated by a large sphinx moth (but I have not seen this in Oaxaca yet)
On the Oaxaca coast, near San Agustinillo, milkweed vines (not Asclepias per se, but plants in the family, Apocynaceae) abound. This is a species of Marsdenia.
Pods of another Marsdenia milkweed vine, next to the Pacific Ocean.


z anurag
Thanks for reading! 












9 Replies to “Monarchs & Milkweed in Mexico – Part IV”

  1. These are beautiful photos, and there is much of interest to read in your comments. You didn’t say whether or not you tried the dish of ant pupae and larvae — it didn’t look so bad, and I’m guessing it’s a sustainable way to eat. Also interesting to read about the milkweed plant from Africa growing there. Enjoy the rest of your sabbatical.

    • Thank you! Yes, I did eat the escamol… we shared it as an appetizer. In Oaxaca, grasshoppers are also very commonly eaten (with salt and spice after being pan fried, I believe) (see this recent feature on ESPN) — Chapulinas! Here people also eat puffed ants and dried caterpillars that infest agave. Pretty amazing.

  2. It has been a pleasure following your sabbatical adventure. I am amazed at the plethora of milkweed species! Is OE a problem in Mexico where the permanent resident (non-migratory) monarchs are active?

  3. Beautiful photos, especially on the insects associated with milkweed.
    My question, tho, is concerning this assumption that Female Monarch butterflies lay eggs ‘mostly’ on the underside of a milkweed leaf.
    Does this assumption come from your own experience?
    The female Monarch butterflies in many areas lay eggs on any part of the milkweed plant they can get near to.
    Sometimes, they even lay them on the netting we use to keep the females from laying eggs on recovering milkweed plants. They lay them in the flower & buds, on the stem, on leafless cuttings in water, on the glass of the cuttings, and even on the mesh of a container that I put over recovering milkweed plants. Yes, I have photos of all of these sites.
    And they lay multiple eggs, not just the one often mentioned by researchers.
    So I’m curious to know where your assumption of where the eggs are laid by the female monarch butterfly.
    thank you!

    • Good question and good points. You are right, even in my area, a substantial number of eggs are laid on the upper side, flower buds, and apical meristems. If I had to guess, perhaps at least half are laid on the underside in my area (on common milkweed, A. syriaca). Again, in my area, i is nearly always one egg per plant, unless it is regrown plants following mowing later in summer. In other places and on other milkweed species, the numbers and locations can be quite different. Thanks for pointing this out! All my best, -Anurag

  4. What non-milkweed nectar sources did you observe? My immigrant midfle school science students in Las Vegas are very interested in Monarch ecology in Mexico!

    • They are real flower generalists… will go to nectar on many flower types, but they do like sitting on Asters. Best wishes, -Anurag

  5. How interesting. Thanks for your report. I live in Mexico City and am trying to decide whether or not to plant A. curassavica in my garden. Is there any proof that this would disrupt the migratory pattern of monarchs (as I have read elsewhere) and that the problem of OE exists here? Considering the plant is native to Mexico and (as seen in your photo of the plants next to the brook in Tlacolula) doesn’t seem to die back, I am unsure. Thanks.

    • From what I have read, I believe A. currassavica disrupts monarch migrations when it is planted in the US or Canada, because it lasts late into the season, and the monarchs stay for its food, rather than going south. It is the native milkweed in Mexico, and has no disruptive properties south of the border, because the Monarchs in Mexico are already in their winter home. Plant it!

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