Monarchs & Milkweed in Mexico – Pt 1

I am getting settled in Oaxaca, on sabbatical leave from Cornell studying milkweeds and monarchs in Mexico. Here is my initial report on plants and insects from the first couple of weeks. The people, culture, food, and biodiversity have all met my expectations so far. What a great country!  This is providing inspiration for the next chapter of what I would like to research and write about, following up on my recent book Monarchs and Milkweeds. Happy new year! -Anurag

aa pach
January 2018. In downtown Oaxaca city, a mature stem-succulent plant from the milkweed family, Pachypodium… I assume it is P. lamerei from Madagascar.  My kids call it Elephante.
oncopeltus sexmaculatus
In Mazunte, on the coast, about 250 km south of Oaxaca city, seed-eating Oncopeltus, it could be O. sexmaculatus on a milkweed vine. Note the bright red abdomens of these toxic milkweed specialist bugs.
Also in Mazunte (the southern most part of Mexico), another milkweed vine, likely Marsdenia sp. (thanks to Mark Fishbein for the ID).
Marsdenia latex, as distasteful as ever.
The Marsdenia were all senescing, as January is the beginning of the dry season, and the vines die back each year much like most milkweeds.  Despite the lack of many herbivores, there was plenty of evidence for leaf chewing and the typical insect behavior of deactivating latex canals by cutting the veins before feeding on leaf tissue. In the US, this type of damage would be done by a leaf beetle. Here, who knows?!
In downtown Oaxaca city, tropical milkweed flourishes, although only alongside streams where there is plenty of moisture and shade. The afternoon sun, even in here, causes them to wilt.  The oleander aphid, Aphis nerii, a specialist on milkweeds is nonetheless a “generalist” — it eats nearly all milkweeds and is on 6 continents (save Antarctica). Here it was doing its typical thing. A. nerii has almost never been reported to make sexuals — it is a highly clonal reproductive machine.
Surprisingly to me (it shouldn’t have been) monarchs (Danaus plexippus) we abundant in this population of several hundred milkweeds.
At least two adults were flying around, and there were plenty of eggs… relatively few larvae, as is typically the case.  Predation pressure is likely very high: most all eggs and caterpillars are food for others.
The big surprise was this spectacular Diabrotica beetle, likely D. nummularis.  It has previously been collected from milkweed, and was feeding on flowers here.  Their larvae are typically root feeders (!), but probably on other plant species.  Thanks to several coleopterists for help with the ID.
a miner on cur
Asclepias curassavica also had a few leaf miners, likely in the genus Liriomyza (its a fly that feeds between leaf layers).  Although I believe that L. asclepiadis is the only known milkweed feeder, there are likely many undescribed species.  Their host range and evolutionary ecology mostly unknown.  A great PhD thesis waiting to be done.
But tropical milkweed was well-known to me.  From hikes around the city and taxi rides, I have been looking for another somewhat common milkweed, but one that I have only grown (never seen in the wild) (Asclepias glaucescens).  And there it was — from the back seat of the taxi “por favor, para, necesito ver una planta”  Complete with plenty of insect damage and A. nerii.  Interesting to me that  milkweeds, several thousand miles removed from Ithaca, appear to be subject to the same consistent selection by similar, if not the same, insect pests. 
On a jog through the city, towards the edge of town, Jennifer and I found a farmer’s field with tens of A. glaucescens (aka nodding milkweed) . This primarily Mexican species is widely distributed, has a tall upright structure, and is VERY waxy.  It looks purple-ish, no leaf hairs to speak up, and has big creamy flowers. Very latexy.
And quite predictably, a familiar caterpillar was munching away.  But upon closer inspection, it was not familiar to me… not a monarch, but its sister species, The Soldier, Danaus eresimus. Although long-thought to be more closely related to the Queen butterfly (D. gilippus), recent work shows that it is closer to monarchs.  Related milkweeds, related butterflies, the whole community repeats itself.























The classic vein drain followed by feeding in V form… in the US this type of damage would most often be done by the four-eyed Tetraopes beetles. This is is not how a monarch typically disarms milkweed.  Its sister, the soldier, has found a very similar, but distinct counter-ploy to milkweed’s latex.

Until next time, all my best, -Anurag

10 Replies to “Monarchs & Milkweed in Mexico – Pt 1”

  1. I hope you and your family are enjoying your sabbatical!
    It surprises me very much that there are adult monarchs flying around there in Mexico and also laying eggs. Are these monarchs not part of the migration and do they not overwinter in a state of relative inactivity? Are these local Mexican monarchs that just stay put and breed all year round?

    • Thanks for your note. My sense is that there are year-round non-migratory monarchs in much of Mexico. In the USA, the main place there is this same phenomena is Florida. I am interested to study the extent of exchange between the over-wintering butterflies in Michoacán and those in the rest of country. All my best.

        • Dear Dr. Shaw,

          Great to hear from you… something we should talk about in person… but my sense is that this isn’t speciation by migration divergence, but I love the idea! I hope you are well back in the frozen north. Jenn and I had a great day hiking around Oaxaca.

  2. Best wishes to you. My daughter, Sara Hospador sent me your post. We used to live off Juniper. Now my husband and I live outside Seattle. (Sara lives in Seattle). Your work looks extremely interesting. BTW, my husband is an ee graduate from Cornell. I hope things continue to go well for you, (and I believe your family, from what I gather in your post). Jane and Andrew hospador

  3. Hi Anurag,

    I’m enjoying your reports from Mexico! I’m headed down there on Feb. 19, flying into Mexico City, with visits to Sierra Chincua, El Rosario, and possibly Cerro Pelon. I’ve not been to the latter and want to get there very much. I’m embarrassed to say that I’ve not yet finished reading your book, but you can be sure I will.

    Best regards from frigid Ohio!


  4. Thank you for this series! We are here in Oaxaca on a botanical trip and have run across that species several times, in bloom at Monte Alban yesterday. Your series got me straight to an ID!

  5. Hi Anurang, I have also witnessed similar monarch’s pattern in Seattle. I am going through your book but didn’t finished it yet.

    Best Regards

Leave a Reply

Your email address will not be published. Required fields are marked *