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
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))
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