Q&A about tropical milkweed (Asclepias curassavica)

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Hello,

I am a retired physician, neurologist and recent master gardener. I am a fellow butterfly enthusiast and have just returned from Mexico where I saw the wintering monarchs. I have been planting many types of milkweed and observing for several years.

I planted tropical milkweed Asclepias curassavica and have been told it is terrible to do that because of the parasite OE being perpetuated within the plant. I live in New England so my Asclepias curassavica dies to the ground outside and I take the stems indoors in a pot where it usually rises again in the spring.

In your book you talk about this milkweed as high in cardenolides and preferred by monarchs infected with OE. That it may lessen the infection in new caterpillars.I found that fascinating. I also read that in warm climates because it does not die, this milkweed confuses the monarchs preparing for migration. I did see one plant at Cerro Pelon in Mexico.

I wanted to ask you what you recommend about tropical milkweed?  I thoroughly enjoy your book.It is both authoritative and readable. I am in the process of writing about my personal butterfly experiences.

Thank you very much.I appreciate any reply you may give.  –Sandra


Hi Sandra,

Thank you for your email and kind words. So glad you enjoyed the book! The literature on tropical milkweed is getting large, growing, and a little confusing 🙂 In general, the tropical milkweed is highly desired by adult egg-laying monarchs, and larvae tend to do very well feeding on it. We think this is because of the low amount of latex, thin and soft leaves, and appropriately high levels of cardenolide toxins that the caterpillars are sequestering for their own defense.

The tropical milkweed has been found to be “medicinal” in terms of improving parasite resistance to OE, and as you know, butterflies that are infected tend to prefer the tropical milkweed even more than usual, suggesting that they are medicating their offspring. This was really great research by Jaap de Roode’s lab at Emory University. The context of this work was comparing uninfected or infected female butterflies, and their relative choice of the tropical milkweed over some other species. In other words, the effect is certainly real, but it may be subtle in nature, and it does not mean that the tropical milkweed is overall the best food.

The other point you bring up is mostly unrelated to the medicinal aspects above. Because the tropical milkweed is very attractive in general, and in gardens it tends to flower as long as it’s warm enough and watered, the tropical milkweed is often available as an egg laying plant in seasons with other milkweeds are not available. This is especially true during this southern migration in the gulf States from Florida to Texas. In other words, when northern monarchs are flying south, they are typically in reproductive diapause and not ready to lay eggs. Nonetheless, if butterflies encounter flowering tropical milkweed they tend arrest, mature their reproductive organs over a week, lay eggs, and become sedentary… all of which disrupts the southern migration. It is unclear how often this happens, but it has been documented. This is the context for which research has shown that disease levels can increase in a population, caused by local tropical milkweed populations in the southern USA.  ((update: As pointed out in a comment below, it is important to note that OE does not infect the plant, but rather the spores sit on the surface.  OE is infective on the surface of all milkweeds))

diabrotica
Diabrotica beetle on tropical milkweed (Asclepias cuassavica) in Oaxaca, Mexico

It is unclear whether this is also a problem with tropical milkweed planted in the northeastern USA and Canada, in September, for example, when the southern migration is just beginning. I think probably what you do currently is just fine. In the southern USA, I strongly recommend cutting back the tropical milkweed in September such that when the southern migrating butterflies are flying through, there are no flowers or foliage for them to lay on. In the north, simply bringing them in at the end of the season is probably just fine.

I too grow some tropical milkweed just because it is easy and a beautiful plant. Nonetheless, I always do get concerned when people promote planting lots of it to help the butterflies. From the perspective of conservation, I simply recommend general habitat protection and of planting native species.

All my best, -Anurag

9 Replies to “Q&A about tropical milkweed (Asclepias curassavica)”

  1. Hello,
    I think that you neglected to answer the fundamental question posed by Sandra, and that is this “I planted tropical milkweed Asclepias curassavica and have been told it is terrible to do that because of the parasite OE being perpetuated within the plant.”
    I have been finding more and more people seem to think that the OE parasite is actually infecting the Tropical milkweed when in fact it is just covered with the spores. At least that has been my understanding. OE spores will be on all types of milkweed not just Tropical. Now the fact that it stays growing for longer in the southern region means it may in fact have more OE spores because of that is not the same as saying the tropical milkweed is infected with the living parasite. Please clarify this point so that others don’t get confused and start spreading the idea that Tropical milkweed is infected internally with the OE parasite.
    Thank you and I too enjoyed your book.

  2. Please also consider that it frequently escapes from gardens into the wild. The wind can carry the seeds far and wide.
    This happens in southern and northern states.
    If you really think it is important to do the cutting back and bringing indoors, who do you think is going to that for escaped plants?
    It has all the attributes needed to become an invasive species.
    Thanks

  3. Agreed certainly for the southern states. The plant completely dies in the winter up north. Do you know if the seeds can survive the winter (and germinate in spring)? I would have guessed not.

  4. I posted the question to DPLEX: Do A. curassavica seeds left outside germinate in your area? Does it re-seed itself in your garden? The answer was a resounding YES from South Texas to Minnesota to Connecticut. It is root hardy north to Zone 8b which includes all of Texas south of Waco, almost all of Lousianna, half of Georgia and coastal South Carolina and even small portions of coastal North Carolina. It is a ticking time bomb and the fuse has already been pulled.

  5. Humm ! On the North side of the Rio Grande tropical is designated non native and grows year around in areas where it doesn’t freeze. On the South side of the Rio Grande, a couple hundred yards away, it’s designated native and grows in the lowlands where it doesn’t freeze year around all the way to the overwintering sites and beyond throughout the country. I haven’t read of any gringos venturing down into Mexico and doing research into how many eggs and cats they find on the tropical all winter long south of the river or any Mexicans researching it either. It’s over 300 miles to the overwintering sites from the Rio Grande. Tropical is an annual in 95% of the eastern range in the U S and everywhere and a perennial in 100% of the lowlands of Mexico. You guys better inform the Mexicans they need to go out and cut down all the Tropical milkweed throughout their entire country so the monarchs don’t break diapause and decrease the migration or maybe we just shouldn’t get all excited about it. Seems I read years ago scientists believe monarchs existed on tropical in the tropics before there was a migration.I would n’t worry to much about it blowing up John.

    • For folks who have never driven from central Texas to Mexico City, it is understandable that there is some confusion how tropical milkweed can be native to “Mexico” but not native just across the border in the US . Basically once you get a bit south of San Antonio, you enter dry, near desert, scrub-lands on both sides of the border for about 250 miles to Monterrey, Mexico and about 650 miles to Mexico City. This area is devoid of native milkweed and is incapable of supporting growth of tropical milkweed due to a lack of necessary moisture, with a couple exceptions. Obviously the thin east-west ribbon of the Rio Grande river bottom-lands support a couple native milkweed. Another exception is the north south mountain ranges west of Monterrey that are capable of supporting some milkweed species at the higher elevations. (Migrating monarchs follow these mountains on their way north and south.) Add to this the prevailing winds and you have a very effective blockade keeping tropical milkweed in its native range* well south of the US. *There is some disagreement about the exact native range of Tropical Milkweed. Some sources claim the Caribbean, some South and Central America into southern Mexico, and some include south Florida. It was never native across what we humans call the United States of America where it is commonly planted today.

      For years, I have been very concerned that Tropical Milkweed is becoming a non-native invasive species (with devastating effects on the Monarch butterfly due to OE). My concern is based upon 20 years growing it in Austin (before I realized the issues), observing its growth around Texas, and asking lots of questions about others experiences (on DPLEX).

      My fear is that our attempts to “help” the monarch butterfly are going to doom it.

      Consider this, why did the Monarch Butterfly evolve the migration adaptation? No really, stop and consider, why? Could it have been to escape the buildup of parasites like OE on Tropical Milkweed? Maybe. Probably, that and a huge natural abundance of milkweed across the great plains. Whatever the reason it began migrating, the result was leaving the tropical milkweed behind in Mexico and the elimination of most infected monarchs during the return migration.

  6. Oaxaca? 767 miles from San Antonio as the crow flies (per Google measure distance). You’re proving my point. Find it, native, somewhere in the 250 miles of near desert thorn scrub between San Antonio and Monterrey.

    And with that I’ll stop responding.

    Folks will do what they want….facts, rational arguments, reasonableness be damned.

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