Tropical milkweed for Monarch butterflies – the Asclepias curassavica controversy

By James C. Trager, Ph.D. / Restoration Biologist / Shaw Nature Reserve

Tropical milkweed houseplant blooming with snow outside

Tropical milkweed houseplant blooming with snow outside

An article from the Wild Ones Journal – lays out the issues regarding planting tropical milkweed for monarch butterflies, and suggests a solution to the possible issue of causing increase protozoan parasite infection, namely, mowing patches of this plant down a couple of times a season, so they are not a constant attracting feature in a butterfly garden. I will note that monarch caterpillars often completely defoliate patches of Asclepias curassavica here in the St. Louis area, thereby rendering them unattractive to breeding females, as I have observed year after year in my garden, so this practice may not even be necessary around here.

Based on my knowledge of insects’ ecological/physiological triggers to reproduction and migration, and the fact that the butterflies naturally encounter this plant as a common garden and roadside plant on their way (passing through Texas and all of Mexico) to the overwintering grounds, I lean with Dr. Chip Taylor of Monarch Watch in not having much concern on this one. The plant itself is winter-hardy through lower Zone 7, and like many subtropical plants and insects, is slowly spreading north as people plant them out and our winters get shorter (now by about three weeks compared to when weather records were first being kept). But overall, habitat loss is more important.

I wish to state and have readers clearly understand that while there is a sizable literature on the use of A. curassavica by monarchs (e.g. Google Scholar search results) at the moment, there is not a single, much less multiple-corroborative, scientific study/ies that I have come across to support either the pro or con-curassavica positions. The whole body of interconnected issues needs study.

However, I can certainly appreciate that people want to exercise caution in this matter.

Since we’re in the stage of hypothesizing and opining on it, I have a few additional thoughts:

  • Any large, dense patch of some of our strictly native milkweeds could just as well foment transmission of the protozoan parasite as do people’s garden patches of tropical milkweed. I’m thinking about the sometimes extensive patches of swamp milkweed Asclepias incarnata that one sees this time of year around ponds and in roadside ditches, or late sprouting and blooming patches of orange milkweed A. tuberosa, resulting from mid-season haying or roadside mowing. (Needs study)
  • Habitat loss is a primary issue for all declining species. (Doesn’t need study)
  • Careless or thoughtless and increased herbicide usage, and excessive mowing frequency on rural roadsides are important sources of this habitat loss, but seem insignificant in comparison to the primary factor – the total destruction of existing grassland vegetation by agriculture, to feed the continuingly burgeoning human population, both directly and to feed our livestock and automobiles.
  • The drought of 2012 must have had a huge impact on any migrating species forced to cross the drought-stricken desert that was the 2000 km/1200 mi span from the lower Midwest USA to central Mexico. (Someone must have studied this, but I haven’t read any of the work on it.)

In any case, I hope you all see as many monarchs floating around this September as I have been seeing.

If you have questions or can enlighten me regarding any scientific studies of the matter that I may not have encountered, please contact me.

5 thoughts on “Tropical milkweed for Monarch butterflies – the Asclepias curassavica controversy

  1. We have seen many Monarchs this last month, hopefully due to our neighborhood native gardens. It is heartening to feel a small part of a huge effort…

  2. Is this just another evidence of EVOLUTION? Things never stand still. Evolves as it may, slowly as it is. It seems that the Monarch is on the way to a demise! There may be another come in play.

    I was asked recently by an acquaintance, what good is Monarch butterfly anyway? Good question. I don’t really know. I checked on Google afterward and still couldn’t find a reasonable ecological answer except the beauty of the butterfly. May be you all experts out there can give a reasonable answer. In the mean time, I am still planting ALL milkweeds with a hope that I can help the Monarch. I saw 1 maybe 2 this year.

    • I’ll ask James to weigh in, but here’s my take:

      Monarchs, along with other pollinators, are responsible for helping plants reproduce. According to the USDA, 3/4 of the world’s flowering plants and about 35 percent of the world’s food crops depend on animal pollinators (http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/plantsanimals/pollinate/#sthash.QpxPki5p.dpuf).

      Despite being poisonous to some animals, monarchs are part of the food web. They eat plants, and are in turn eaten by predators – mostly insects, spiders and other invertebrates. Each species adds diversity and makes the web stronger.

      As large, attractive butterflies, monarchs attract attention and serve as an “indicator species” (the canary in the coal mine). Other insects, pollinators, and migrants (including birds) are in trouble too. By providing habitat for monarchs, we are also helping other, less showy and less well recognized animals.

  3. Monarchs have had some troubles, but I don’t think they’re meeting their demise, yet. If they were, that would be extinction, not evolution. If it should happen that the migratory behavior is lost, but the species lives on as a non-migratory one, or if they somehow adapted to surviving our cold winters, that would be evolution, as would other changes in the genetic make-up of the population that are adaptations to a changed world.

    As for the question of “What good are they?” – It is a question almost without meaning. What is the significance of the word good? Good for what? Society? Human well-being? The “environment” (another oft-used term with little meaning)? Prairies? Milkweeds? The food chain? and on and on. I hasten to add that the response given above gave the question some real meaning, and provided a thoughtful response. I could add to it by giving a long list of the multitude of other plants and animals with which monarchs interact, but that’s probably not necessary.

    But I will say, completely from a non-biologist’s perspective, I do think that it is good that human beings look beyond themselves to care for others, including caring for species that most likely will never have the merest concept that we do care for them. The more conscious were are of all life around us, the more truly alive we, ourselves are.

    PS – Just came across this relevant piece in the New York Times.

  4. Thanks everyone for the comments. Good read in NYT.
    There are now 2 Mon. caterpillars chewing on the tropical milkweed in my garden! Joy!!!

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