Why no tropical milkweed at the Cockrell Butterfly Center plant sale this year?


March 21, 2015
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Asclepias_curassavica_(Mexican_Butterfly_Weed)_W_IMG_1570

Aslepias curassavica

We are sorry to disappoint monarch enthusiasts, but the Cockrell Butterfly Center has decided not to sell tropical milkweed (aka Mexican milkweed, Asclepias curassavica) any more. Instead, we will have a limited quantity of native milkweeds for sale. Recently, biologists studying monarchs have discovered that tropical milkweed may be a factor in the spread of a parasitic infection that attacks monarchs. The infection is called Oe (short for Ophryocystis elektroscirrha) and is transmitted by spores that fall from an infected female’s body onto the hostplant when she lays her eggs. The hatchling caterpillars eat the spores along with the leaves, and become infected themselves. After a generation or two or three, the infection level becomes so high that the butterfly dies (sometimes in the caterpillar stage, sometimes in the pupal stage, and sometimes as the adult).

This could happen with any milkweed – the problem with the tropical species is that it does not senesce (die back) in Houston’s mild winters but is perennial, growing throughout the year. In contrast, native species die back to the ground in the winter, and when they regrow in the spring they are spore free – so the infection cycle is broken.

Also, researchers have found that some monarchs in the southern part of the USA don’t bother to migrate if they have milkweed available. These year-round residents have been found to have very high levels of Oe infection, because they are mostly using the tropical milkweed species generation after generation. While this probably doesn’t greatly impact the migration as a whole, we don’t want to contribute to the local spread of the disease.

If you do already have tropical milkweed, one solution is to cut it back severely a couple of times a year. Even better is to remove the tropical variety and switch to native milkweed species. Unfortunately, so far these are not widely available in the nursery trade and are not as easy to grow as the tropical variety!

Aslepias viridis

Aslepias viridis

We are all learning and struggling to do our best for the butterflies. This year we will have a limited quantity of two native species at our spring plant sale: Green Milkweed (Asclepias viridis) and Antelope Horns Milkweed (Asclepias asperula).

 PlantSaleSpring2015_Facebook_cover

Our next plant sale will be Saturday, March 28, 2015 from 9 a.m. to noon, or until we sell out. It will be located in its usual spot on the 7th level of the Museum parking garage. We hope you will try growing native milkweeds, and please let us know how it goes for you!

Soni
Authored By Soni Holladay

Soni is the Greenhouse Manager and Horticulturist for the Cockrell Butterfly Center. Her job consists of maintaining the support greenhouses for the CBC, organizing plant sales, leading outreach programs, assisting with the butterfly rearing program, and spreading enthusiasm for butterfly gardening. She earned a B.S. in Horticulture from Texas A&M University and she got her first experience at the Museum as a summer intern for the CBC.

5 responses to “Why no tropical milkweed at the Cockrell Butterfly Center plant sale this year?”

  1. Denise Bossarte says:

    I recently purchased a package of milkweed seeds from
    Native American seeds:
    http://www.seedsource.com/catalog/detail.asp?PRODUCT_ID=7082
    It does not contain Aslepias curassavica but I am wondering if any of the other varieties are considered tropical? see list below.
    If none are tropical, then this local business in Junction, Texas could be a great source for milkweed seeds for the spring plant sale!

    Sustain the Migration Kit :
    Antelope Horns (10 seeds)
    Green Milkweed (10 seeds)
    Butterfly Weed (10 seeds)
    Common Milkweed (10 seeds)
    Showy Milkweed (10 seeds)
    Swamp Milkweed (10 seeds)

    Thanks, Denise

  2. Nancy Greig says:

    This is a reply to Denise – Hello – yes, I saw those packets for sale at Buchanan’s nursery here in Houston (they were expensive, $7.00 for 10 seeds!) and bought some for us to try. All or at least most of those varieties are appropriate for our area – it remains to be seen how easy they are to grow. In our experience they are NOT easy. I hope that buying them directly from Native American Seed you did not pay as much as I did! Good luck! Let us know how they do for you.

  3. Amy says:

    I am in the Northern part of the country, where the tropical dies back each year and I reseed for the following year. Is it okay for me to stick with the tropical variety? Thanks!

  4. Cathy says:

    I plant tropical milkweed for the butterflies every year and they prefer it over 4 other kinds that I’ve tried here (Common, Purple, Swamp, and Tuberosa/Butterfly Weed). I personally don’t see a problem with planting it this far north since it dies back in the winter. I don’t let the seeds fall willy nilly though because I left them go a couple years back and ended up with hundreds if not a couple thousand tropical milkweed plants in my garden. I do see where they could possibly become a problem, but I’m aware of that and manage accordingly.

  5. Mary says:

    From an article I took this excerpt. “This could happen with any milkweed – the problem with the tropical species is that it does not senesce (die back) in Houston’s mild winters but is perennial, growing throughout the year. In contrast, native species die back to the ground in the winter, and when they regrow in the spring they are spore free – so the infection cycle is broken.”
    Link to article: http://blog.hmns.org/2015/03/why-no-tropical-milkweed-at-the-cockrell-butterfly-center-plant-sale-this-year/

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