Milkweeds but not monarchs in Europe: natural and cultural history (and a modest proposal)

Why declining?

What to do about the decline of the monarchs?  The recent Thanksgiving count in California was dire. The spiral in eastern north America is no better.  The monarchs need many things, and milkweed is absolutely essential.  It is the only plant their caterpillars eat.  And as described in my book, Monarchs and Milkweed, the two share an intimate and antagonistic evolutionary history.  While monarchs continually evolve to keep up with milkweed, milkweed too constantly evolves better and better defenses.  But, in Europe, where the common milkweed (Asclepias syriaca) was introduced over 400 years ago, there are no monarchs.  What follows below is part travel log, part natural history diary, and part historical journey regarding milkweeds and their bugs in Europe.  At the bottom, I end with a modest proposal for what we might do.  Here we go.

The genus Asclepias has about 140 species (in the botanical family Apocynaceae), with >90% from temperate North America, and the lion’s share of them in Mexico and the SW USA.  Click here for a visual smattering of milkweed floral diversity. Only a handful are tropical, from Central America and northern S. America. Although sometimes controversial, the sister to Asclepias,Gomphocarpus is a genus of >200 old-world species, all in Africa.  But, Asclepias does have a history in Europe, an unnatural history that starts >400 years ago.  But I am getting ahead of myself.  My exploration of Asclepias in Europe started with research for Monarchs and Milkweed, continued with a research project on evolution of introduced milkweed in Europe, and most recently culminated with a trip to Hungary.  Let me start with the last first.

In August 2018, I headed to Budapest to attend the International Society of Chemical Ecology‘s(ISCE) annual meeting, where I was a featured speaker, having just been awarded the Silverstein-Simeone Award.
As you might expect, I gave a lecture on Monarchs and Milkweed, although it was slightly unnerving to have the ISCE executive board with me on stage as I gave my talk.
Perhaps the most special part of the conference for me was meeting up with four past post-doctoral scholars, all highly successful chemical ecologists: Jared Ali (in the back), and left to right in front: Sergio Rasmann, Tobias , and Georg Petschenka.
Hungary is famous for is paprika and other preparations of peppers (which are native to central America)
Georg and I in front of the Hungarian Parliament Building on the Danube.
The other highlight was a field trip to look for America’s common milkweed, Asclepias syriaca, an introduced plant which is abundant (but not “invasive”) in the Hungarian countryside.
Remarkably, milkweed grows with a common friend, also native to N America, tall goldenrod (Solidago altissima), and these meadow plants grow among corn and sunflower agricultural fields here outside of Budapest.
We couldn’t translate it, but it didn’t seem good.  And rather un-European!
With Georg and my friend and colleague John Maron from the Univ. of Montana
Pentatomid bugs (stinkbugs) are common on milkweed in N. America and Europe.
The European milkweed fauna has a familiar face, here is Lygaeus equestris, a close relative of our N. American L. kalmii. In Europe, L. equestris, typically feeds on Cynanchum species (also in the milkweed family).
And here they are, a familiar tri-trophic interaction: milkweed, the oleander aphid (Aphis nerii), and a ladybug. Aphis nerii is a aposematic (brightly colored) aphid that eats nearly all milkweeds and is found on six continents.
Flowers were sparse mid-August, as most plants had already gone to seed. There are no monarchs in Europe… although one seems to make it across the Atlantic every few decades.  So far, their populations are not self-sustaining here, although there are established monarch populations in nearby Spain.
Two other cool plants from our jaunt in the Hungarian countryside. First, a remarkably spiny herb from the carrot family (Apiaceae) in the genus Eryngium.
Tobias also found a cardenolide-containing mustard (A wallflower in the genus Erysimum) … although the bulk of plants that produce these toxic heart-impacting compounds are in the milkweed family, several other groups have evolved a small set of plants with them.


q b
Now, on to some of the earlier history.  To understand how the common milkweed got its scientific name “Asclepias syriaca” we must travel back in time over 400 years… and examine plants that were transported from eastern Canada to Paris.
q2 Asklepios by Segio Rasmann
In the 1700s, Carolus Linnaeus named the milkweed genus “Asclepias” after the Greek god of medicine Asklepios, son of Apollo (photo by Segio Rasmann). Linnaeus gave this name because he was well aware of milkweed’s medicinal properties.  But the story of common milkweed’s name is a little complicated…
In his 1753 opus Species Plantarum, Linnaeus recognized that A. syriaca was a New World plant native to eastern North America, but he decided to keep an old species name (probably for consistency). The “syriaca” epithet was given because the original namer, Jacques-Philippe Cornuti made a mistake in 1635. Here in the Paris Botanical Garden, Cornuti examined the plant we now call common milkweed (imported from eastern Canada few years earlier) and concluded that it was the same species then called “Beidelsar” a plant known from the Middle East that had been classified as Apocynum syriacum by Carolus Clusius in 1601.
Common milkweed in the Paris Botanical Garden 2018… not many bugs, just some aphids and ladybugs.  Where are the monarchs?
w linnaeus on asclepias syriaca
A page from Species Plantarum. Linnaeus could have straightened things out, had he realized that Cornuti had confused the two plant species as one. But, Linnaeus was unaware of Cornuti’s mistake, and unaware that Clusius had already classified Beidelsar as Apocynum syriacum. In my humble opinion, Linnaeus should have kept the species name syriaca for Beidelsar, retaining some historical precedence and a geographically correct name, but he apparently did not check his sources closely enough 😉
x calotropis1
Calotropis gigantea, photo taken in Madagascar
x calotropis2
Calotropis gigantea, had Jacques-Philippe Cornuti seen the flowers, presumably he wouldn’t have made the mistake.  Interestingly, monarchs can grow and develop on Calotropis, and it is used widely as a ornamental as well as rearing plant for monarchs in Florida. Across much of Africa, Calotropis is fed upon by the “African Queen” Danaus chrysippus a close relative of the monarch.
world monarchs
So where does this leave us?  Monarchs are declining precipitously in their native range, in eastern N America and in the semi-independent population in California. Yes, where milkweeds have been introduced elsewhere, typically as ornamentals, monarchs have found them and established self-sustaining populations. In Spain and Hawaii (and Florida, where they are native), monarchs are non-migratory.  While in other newly introduced places such as New Zealand and Australia, monarchs have established new migratory patterns.  Who knew!
A modest proposal. So what about it? …. Perhaps we should introduce monarchs to Europe.  Maybe there too monarchs could establish on the milkweeds that were introduced over the past 400 years.  Extra insurance for our beloved and declining butterfly? But what would the ecological consequences be?  Unintended side effects?  Could this insurance policy actually help reverse the decline.  Almost certainly not. Most of the proposed band-aids, mass rearing and the like, ignore the real problem.  If as many of have speculated, monarchs are sentinels for the health of our continent, then we should take their decline as a wake up call, not as a call to arms to fix this one small problem.
UP 2018 1040 CALS Award Ceremony
Thanks for reading!  And to leave you, here is a picture from the awards ceremony (October 2018) where Amy Hastings, the Research Support Specialist in my lab received a college-wide award for all of her excellent contributions over the past 10 years! Congratulations Amy, so great to work with you!

11 Replies to “Milkweeds but not monarchs in Europe: natural and cultural history (and a modest proposal)”

  1. When I was doing research for my own book, I came upon milkweed honey. Of course, I had to have it, but it was nowhere to be found, commercially anyway. It seemed only to be produced in Hungary, where as you know, Asclepias isn’t native, but grows very well in certain areas. To make a long story short, a friend of a friend of a friend just happened to be a milkweed honey producer in Hungary. They can only sell it to members of the EU, but friends can do remarkable things. The path two pints of milkweed honey took was Hungary>England>Texas>Ohio. I got my honey, and yes, it’s everything it’s lauded to be.

  2. It seems a bit strange to say that there are no monarchs in Europe, but there is an established population in Spain, which is, I think, generally considered to be a European country. However, from the map, I see your point. Monarchs are not able to cross the Pyrenees.

    Do you suppose this is simply a difficulty caused by altitude? Or a problem due to a lack of some resource in the mountains?

    The map suggests a similar situation in the US, with the California population isolated by the Rocky Mountains. Could it be the same problem?

    As for the signs in Hungarian, Google Translate says the sign with the gun image has the text “Armed Area”, which I suppose means “There be guns here.” The sign above it says “Protection area for drinking water”.

    • Tim, I cannot believe you actually read this thing! Thank you. Of course Spain is in Europe, but more remarkable is your insight! Especially in Europe, there are probably no milkweeds in the mountains, whereas feral milkweeds abound in the southern lowlands. There very well be some climate stuff going on as well. I think in the Rockies there is milkweed to 7000 or so feet, but not above.

      And thanks for the translate… A friend from Toronto, Don Davis, wrote this: So happens my nephew’s wife is from Hungary. So….the signs you photographed and posted in your blog reads: “City water works protected property, Potable water wells protected area, Property guarded by armed security”

      • “Property guarded by armed security” makes more sense than “Armed area”, but Google Translate did okay, I think, with the other sign.

        As for the mountain barrier, it makes sense that the monarch would not expand its range incrementally across a region without milkweed.

        It still puzzles me that individual monarch females haven’t accidentally crossed the range, either carried on storm fronts, or driven by zugunruhe, frequently enough to establish a permanent population north of the Pyrenees, as they have on the other side of the Rockies. Apparently this happens occasionally, but not sufficiently. There must be some reason beyond the absence of milkweed in the mountains that prevents the establishment of a population in southern France.

        The probability of a successful establishment must be much lower than I think. The difficulty of establishing permanent populations of biocontrol agents suggests that I am too optimistic. The minimum ecosystem requirements for most species are probably more complicated than we imagine, that is, species are embedded in their habitat more precisely than we suppose (a major theme in Kim Stanley Robinson’s SciFi novel “Aurora”).

  3. The eastern USA monarch overwintering population has actually been stable for the past 8 years: And the fall migration of 2018 was spectacular at times in the Midwest USA like this: Video shot at sunset as the butterflies were coming down from high altitude migration:

    Until 2018 the California overwintering population had been stable for about 17 years: Although 2018 has turned out to be anomalously record low year in California, the western USA population has reliably recovered from deep lows in the past.

    • Just like Earth’s climate. Completely stable. I guess all the talk about Mexico developing land the monarchs hibernate in was much ado about nothing, too.

      The only stability I see is the encroachment of the burgeoning human population and its rapacious ever-growing appetite for per-capita consumption.

      That is the one thing one can count upon: Greedy ignorance.

      Both of your data charts showcase precipitous drops. Stability whilst circling the drain is cold comfort.

  4. Of course weather patterns, mostly catastrophic drought determine wildlife populations although the native plants that go dormant have to recover or very much life on the planet never would have existed. We know by fellow Austini, Bill Calvets studies of fireants in the 90s spreading across the south what a detriment fireants are to wildlife including monarchs. Since most milkweed and common specifically is actually invasive and spreads on it’s own during times of normal participation many times more milkweed stems exist now than even before GMO. This , of course is because more people are letting it exist in non cropland areas especially in the summer breeding grounds. Increase predation is most likely the hard to study culprit in holding monarch populations down but that won’t stop the tax dollar and grant seekers from continuing to promote planting invasive weeds. Have some fun and raise some monarchs. At least you can feel like your making some kind of legitimate contribution even if monarchs and other butterflies disappear completely

    • Everything I have read from legitimate sources has shown that milkweed populations in North America have shrunk. This isn’t just because of the relentless onslaught of “development” but also because of Roundup and the fact that American farmers, at least all of those in my area (Indiana and Ohio) are clueless about beneficial ecological practices like beetle banks. All the university “extension” pages are all about which chemical to dump on the land to solve every problem and every plant that’s not agricultural or merely decorative is a weed that establishes “infestations”. Is it surprising that practically every forb in the US that isn’t from Europe has “weed” in its name?

      Not only are there fewer milkweeds, the populations are more fragmented. Some are on the verge of extinction. Mead’s milkweed, which the government has been trying to reestablish, exists with only 11-12% of its plants on protected land and politicians these days are moving toward privatizing/exploiting protected lands. Most of the plants are mowed so they can’t go to seed. Some populations are so genetically non-diverse that they have difficulty producing viable seed. Some Mexican species are barely documented. For example, good luck finding a source for melantha seed, or Mead’s, or many other Asclepias species.

      I also find your post a bit baffling because you seem to be claiming that A) there are more milkweed plants now than there have been before and B) there isn’t much point in trying to help the monarch because it’s likely to become extinct.

      It is likely to become extinct, becoming added to the very long and ever-lengthening list of our species’ slaughter — “for jobs and profit”. That list will eventually have our name added to it. Yes, I am extremely pessimistic (aka realistic) but I also plant milkweeds and support the idea of establishing biological control populations. Fiddling while Rome burns is lazy and a bit dull. If we fight the law (the law = money talks) the law will win. But, it’s better to keep busy than to throw in the towel.

      Syrica milkweed is not invasive. Corn is invasive. Soybeans are invasive. Humans are invasive. Syrica milkweed is eaten, simultaneously, by so many insects that it’s a wonder it manages to survive at all. No plant in my area is more of an insect haven.

      • Nice to meet you Charles. If you Google “Craig the Butterflyman” you can find out a little more about how I contribute and if you like you can join my 2000+ member Public Facebook group “How to Raise Monarch Butterflies” and learn what the people I teach are doing to actually add to the monarch butterfly population instead of “wondering”. Or like most scientists prefacing all their comments with “could” or “maybe” Yes, what I teach Is ecologically correct and approved by many biologists and butterfly experts including U S Fish and Wildlife biologists and once multiplied and released they feed the entire wildlife food chain through breeding in the wild.

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