Monarchs & Milkweed in Mexico – Pt III

Greetings monarch and milkweed enthusiasts from Oaxaca! This is Part III in my series from Mexico where I am based on sabbatical leave from Cornell (see the second post here). This post follows up on observations here that are laying the foundation for my next research and writing projects, continuing on from my recent book Monarchs and Milkweeds.

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I recently visited Monte Alban, a remarkably intact archaeological site just outside of Oaxaca City. This was the site of one or the largest pre-Columbian cities in the region, established some 2500 years ago.
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Monte Alban was a cultural and economic center of the Zapotecs for nearly 1000 years.
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The nodding milkweed (Asclepias glaucescens) was the most abundant Asclepias on site. This species produces copious latex and ranks relatively high on the scale of cardenolides (the poisonous cardiac glycosides that make milkweeds to most insects).
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Monarch butterflies were abundant, flying, mating, and laying eggs. These are non-migratory monarchs, likely resident in Oaxaca all year. The importance of these non-migratory populations for monarchs overall, and exchange between the populations is largely unknown.
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Monte Alban
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The nodding milkweed, Asclepias glaucescens
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Two other milkweeds are common year-round in Oaxaca. Here, high and dry at Monte Alban, Zizotes milkweed (Asclepias oenotheroides) is the other abundant species. Tropical milkweed (Asclepias curassavica) seems to be always flowering in this region, but favors wet soils, often stream-side, and was not seen at the site.
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What I have observed at several sites across the region is that monarchs are frequently laying several eggs per plant.  As my son points out here, at least 4 eggs were on this plant. This behavior, sometimes termed “egg dumping” is not common in the Midwestern and Northeastern USA, where most plants lack eggs, and there are typically not more than 2 eggs laid pr plant.  Indeed, monarchs usually avoid laying multiple eggs on plants as a means to avoid egg cannibalism.  Monarch caterpillars are mostly vegetarian, but the one piece of meat they will eat is the eggs of their own species. Such egg dumping has also been observed occasionally in the north and in other localities such as Australia where monarchs and milkweeds are introduced species.
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The moment my wife, Prof. Jennifer Thaler, and I realized that we were living aspects of the graphic novel by Peter Kuper, Ruins.
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Ruins is the story of academics on sabbatical in Oaxaca, and their story is intertwined with the monarch butterfly’s annual migration.
bug Lygaeus reclivatus
Other spectacular tropical natural history abounds, some associated with milkweeds like this seed bug, Lygaeus reclivatus, a relative of the small milkweed bug (Lygaeus kalmii) common in the Northeast.  Here drinking nectar of the nodding milkweed’s flowers.
A leaf-mining fly (that’s right, a fly that lays its eggs on milkweed with the young feeding between the leaf layers) on nodding milkweed. I hope study this fly, which very well may be a new species to science.
I had previously reported this bug, a species from the genus Largus, associated with milkweed in Mexico, but I didn’t realize that it could be an important herbivore. Here the brownish adult is about to take off from a nodding milkweed, Asclepias glaucescens.  But it appears much more common on Zizotes (Asclepias oenotheroides). 

In a mixed population with abundant “nodding” and many fewer “zizotes”, Largus seems to aggregate on zizotes. Here the nymphs (juveniles) of Largus are intermixed with the adults, and their feeding appears to be causing leaf necrosis.  The steely blue nymphs with a red dot on their back (not visible here) are reminiscent of other milkweed insects and suggests that they may be chemically defended with milkweed poisons.  Another hypothesis that should be tested!
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Back at the ranch, I am continuing to rear monarchs, primarily on nodding milkweed, Asclepias glaucescens.  The larvae grow well on these cut leaves, with little mortality or evidence of disease.
rearing ptoids
Rearing monarchs from eggs takes time, but usually ends well.  Interestingly, in the field, I seem to only see 1 caterpillar for every 100 or so eggs.  In other words, predation on eggs or young caterpillars seems to be very high.  When I find larger monarchs in the field and bring them in to rear, they seem inevitably infested with parasitoids.  Here one monarch (top) and one queen butterfly (bottom) collected in the field attempted to pupate at a rather small size.  Four fly larvae emerged from each of them and pupated themselves instead.  These parasitoids were laid as eggs on the caterpillars, they hatched and burrowed into their bodies, and fed until maturity. They emerged like aliens.
The milkweed vines continue to befuddle me. Although not in the genus Asclepias, they are very much milkweed vines by virtue of their latex, fruits and seeds, and perhaps by their insect herbivores.  It has been difficult to study their insects because most appear to be at the end of their life-cycle, such as this one in February 2018.  Note the flowers of the tree that the milkweed vines is climbing on.
Opening pods of the milkweed vine.
Spiny pod of a different milkweed vine. Seeds are already gone.
Clearly another species, with highly distinct fruits. Several genera occur here, including Cynanchum, Marsdenia, Funastrum, Matelea, and Gonolobus (with many species in each)!  A tremendous diversity of milkweed vines.
wunk euphorbia
Oaxaca sits at mid-elevation (4500 ft)… In my hikes to look for milkweeds and their insects, I have been hiking up to over 9000 feet of elevation in the hills. In the grasslands just above Oaxaca, this very unique Euphorbiaceae appears to be a common early spring plant.  It is tiny, just an inch wide.
wunk Chiranthodendron pentadactylon
Higher up, one passes through oak forests, which transition to pines.  In and amongst these higher elevation trees, was a massive tree from the Malvaceae, the botanical family with cotton and hibiscus. The flowers are huge with cups of nectar which are apparently visited by hummingbirds (but also buzzing with bees). This species, Chiranthodendron pentadactylon, or Devil’s hand tree, was used medicinally by the Aztecs.
y Butterfly Driver
And finally, some humor.
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Anurag Agrawal, photographed by his son, behind a katydid near San Jose del Pacifico, Mexico.

12 Replies to “Monarchs & Milkweed in Mexico – Pt III”

  1. I’ll be sharing your latest blog entry with my FB page community of people who raise Monarchs in Southern CA.
    Several things stood out that I feel sure they will ask about.
    First, our population here in southern CA is resident, although that seems to be blasphemy to all eastern researchers and organizations. so it was interesting that you think the Oaxaca may be a resident population.
    You also mention that …” at least 4 eggs were on this plant. This behavior, sometimes termed “egg dumping” is not common in the Midwestern and Northeastern USA, where most plants lack eggs, and there are typically not more than 2 eggs laid pr plant. Indeed, monarchs usually avoid laying multiple eggs on plants as a means to avoid egg cannibalism. ”
    Our monarchs in southern CA are always egg dumping. I have many photos of my own, and photos from others of multiple eggs on one leaf. We also have photos of females ovipositing eggs on only stems, or on netting we use to cover over plants that we don’t want eggs on.
    Tropical milkweed is used here, but not exclusively. Many people have a mixed variety of native and non-native millkweed species. Usually it isn’t blooming all year though. The Monarchs here eat it to the stem several times a year. And that is when we are consciously removing Tropical milkweed plants from use in our yards to let them rest, fill back out and breathe before subjecting them to another onslaught of caterpillars. Many on my FB page raise Monarchs from eggs. We cannot leave them on the plant because Tachinid flies are used here as a method of IPM by crop farmers. So just when we think it’s safe out in the yard, we found out that these flies are brought in all year long. Seems this should be addressed considering we are concerned about the welfare of Monarchs.
    Also, the milkweed plants you show on this blog are gorgeous! I have seen photos of them before, but it’s always nice to see them again.
    Sometime, I hope we can talk about the misery that the eastern Monarch organizations have heaped on our heads for working with Monarchs that do not fit their narrow guidelines.
    We did not create this ‘non-migratory’ pattern. We are just supporting a wonderful insect in the pattern they present to us.
    What would you do if a female Monarch laid eggs on the netting that you cover your plants with? This is an established pattern. And trying to force the Monarch into a pattern they haven’t followed for decades seems inappropriate and wrong.
    And, just because it is always brought up, for those of us who test our butterflies and view these tests, our rate of O.e. infestation is just under 5%. Not many of us use any bleach solution on eggs or milkweed leaves. I have been advocating that everyone rinse the eggs and leaves off with water. But that is all.
    And we have a lot of that missing population that is often mentioned. But our population is not included in the overall count. I was actually told that we were doing damage to the population by raising healthy Monarch butterflies.
    That makes no sense, unless you understand that our Monarchs don’t seem to know the “rules” that have been established by the eastern organizations.

  2. The non-migratory monarchs in Mexico, and lack of knowledge about any genetic interchange with the migratory population has my attention. Is anyone studying any aspect of this at present? The dramatic cautions to those in southern US areas about cutting back tropical milkweed to discourage malingerers and reduce OE exposure seem so unnecessary. Your reaction?

    • Thanks for your comment! I personally believe there is strong reason to exercise caution with planting tropical milkweed in the southern USA… and more data and evidence is emerging. My sense from Sonia Altizer’s work is that “natural” year-round monarch populations do have higher disease incidence than migratory populations. It is indeed complicated, but I tend to error on the side of caution with these things.

      • Erroring on the side of caution is detrimental to the monarch butterfly population. You just perpetuated an uneducated opinion to another uneducated enthusiast which wil perpetuate this fake news story further. I’ll let a person who was studying butterflies before Sonia was born and is old enough to be Dara Satterfield grandfather and is considered a foremost expert in the field of butterflies and the food they eat. I’ve actually read the published results of Dara Saterfields self professed incomplete study and ishe doesn’t claim what’s been written by over zealous reporters producing fake news stories. May I suggest next time this issue is raised you post or send this explanation pertaining to tropical milkweed and it’s attributes. Thank you.

        • Craig, I just want to understand who you are responding to…
          Dara Satterfield did what she needed to do to get past that hurdle for her Phd. She knew the reports banging around the internet and in print were not accurate, but she tried once to clarify. It made it worse.
          I won’t speak for Sonia Altizer. But I know that when I sent my results of tests taken from Monarch butterflies that I raised from eggs, fed tropical milkweed leaves that were rinsed in water, I was not aware of the difference between ‘wild’ caterpillars vs. “captive’ caterpillars. So my results, which ended up being 100% dense O.e. spores as viewed by a grad student at Altizer lab, were used for their report. I was contacted and told about it. But those results could not have been correct. 100% densely covered O.e. spores? No…I raised Monarchs in the same way since, and view my own test samples, and I don’t even come up to 5% with O.e. disease. And I know a lot of people who ended up doing the same thing.
          And I later became aware that Dr. Altizer possibly/probably used data collected by her grad students, not the data sent to her lab.
          Her numbers and her methods need to be looked at.

      • Dear Admin, What do you base ‘normal’ on? For goodness sake, we just learned of their migration route in 1975. We just learned by an error made by researchers in Florida about O.e. on Monarchs. (Don’t know that story, find the McLaughlinMyers 1970 paper…hopefully you have the ability to access this paper. I found it in references on a Wikipedia document. )
        Nothing should be written in stone. They are creatures of nature, and adapt to nature as they experience it.
        And the fact that Tropical milkweed has been labeled as a ‘carrier’ of O.e.? That was one of the most ridiculous statements a researcher could have made. Any plant that a butterfly with O.e. spores flies over or nectars at will get equal shares of O.e. spores. And as for Tropical milkweed ‘changing’ the ‘normal’ migration pattern, who really knows what that pattern should be at any given time? Our weather is changing so rapidly, and our ‘natives’ don’t necessarily grow here as well now. As for which native milkweed we should be using, that’s hard to say since none grows well. And we were told by Monarch Watch which ones to use, except they aren’t really from our region, the seeds were just more readily available.
        People need to stop quoting “normal” and start actually talking with people about what they are seeing and doing in regards to raising Monarchs. Otherwise, you are just repeating gossip.

    • Val, I agree with you that the cautions about cutting back tropical milkweed in the southern areas of the USA are unnecessary and I also agree with Craig they are detrimental. Why? We already know that some of the winter breeding monarchs along the Gulf coast and in the lowlands of northern Mexico migrate north into the central USA in March and April and lay eggs on the native milkweeds. This means some of the winter breeding monarchs are migratory and helping to repopulate the summer breeding range of the eastern migratory monarch. And we already know that the seasonal monarch migrations in New Zealand and southwestern Australia are just like our own and 100% sustained by evergreen tropical milkweeds that were introduced to those countries from south Africa in the 1800’s. And we already know that despite the greatly increased plantings of evergreen tropical milkweeds in home gardens and parks along the southern and central California coast during the past 15 years, the OE parasite infection rate of the western migratory monarchs since the mid-2000’s has been lower than it was in the 1970’s – 90’s before such plantings were popular with residential gardeners.

      • Good to hear from you Paul!
        Ironically, one researcher here in CA finally saw Native milkweed plants that were green and doing well during the winter. So it’s not just Tropical milkweed plants that are around all year long.
        Still, I’d like to emphasize that any plant, especially milkweed plants, must be well tended in order to remain O.e. spore free.
        In rinsing with strong stream of water, the O.e. spores will rinse off. And I think we prove that by having a low O.e. rate of under 5%.

      • Yep…those nature/nurture conundrums cry out for attention, and without study provide fodder for different opinions. I recall some study long ago that observed, anecdotally, a difference in the genes of migratory monarchs, suggesting no physical way resident monarchs could ever “step-up” and become migratory. Perhaps they also have a weakness/susceptibility to OE? To be repetitious, why not study genetic differences in these two populations, as well as OE residue on tropical milkweed in Florida and the Gulf Coast? Currently visiting south Florida and observing resident monarchs totally consume scattered Mexican Milkweed…no need to cut it down.

  3. I’m eagerly reading this series as I moved to Jalisco in June this year, 2022 from Southern Oregon. Quite a learning experience trying to learn how to garden and protect pollinators in a totally different climate and environment. I’m trying to just observe nature and take note of species and bloom times right now while I stuggle to keep what used to be common plants alive. I didn’t actually know it was possible to kill geraniums, but I have proof! Thank you for this really interesting series. Oaxaca is rather different from Jalisco, Lake Chapala area, but I have seen monarchs and I know they live here!

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