San Antonio’s Tropical Milkweed Patch Raises Monarch Butterfly Questions

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Sunflowers draw butterflies in our converted Lavaca yard

Sunflowers draw butterflies in our converted Lavaca yard

Monika MaeckleGraduate student Dara Satterfield came to town in late January for the second time in 12 months to take the pulse of the Monarch butterfly population at the San Antonio River Museum Reach Milkweed Patch.

There, dozens of Tropical milkweed plants play year round host to Monarch butterflies.

Satterfield works closely with local volunteers like Mary Kennedy and Mobi Warren of the Monarch Larvae Monitoring Project (MLMP), a citizen scientist program, to monitor milkweed patches far and wide for egg, caterpillar, chrysalis and butterfly action that might shed light on the state of the Monarch butterfly migration.

Monarch on Tropical Milkweed

Monarch butterflies LOVE Tropical milkweed. But is it appropriate to plant it? Photo by Monika Maeckle

Satterfield, like other scientists, believes the increased availability of  Asclepias curassavica, commonly known as Tropical milkweed, coupled with our warmer winters, may have an unhealthy impact on Monarch butterflies and their migration.   The science is undetermined on that question.  San Antonio's Museum Reach Milkweed Patch boasts about 60 Tropical milkweeds.

A native of  Marietta, Georgia, Satterfield attends the University of Georgia where she is a PhD candidate whose dissertation focuses on the relationship between migration and infectious disease in wildlife.  Monarchs are her species focus.  Satterfield works closely with Monarch scientist Dr. Sonia Altizer, the foremost expert in the country on Ophryocystis elektroscirrha, or OE. The unpronounceable protozoan disease infects Monarchs and other milkweed feeders, often resulting in butterfly crippling or death.

Dara Satterfield

Dara Satterfield checks Monarch butterflies at the San Antonio River Museum Reach Milkweed Patch for OE.           Photo by Monika Maeckle

As winters get warmer, Asclepias curassavica – the only milkweed species commercially available – is less likely to die. Some scientists hypothesize that A. curassavica entices Monarchs to forego migration and winter in the U.S. This could create an unhealthy hotbed of lingering OE spores for caterpillars and butterflies that remain in the local area. Since the spores transfer from creature to creature via physical contact with each other or the plants on which they rest or eat, scientists worry that local OE-infested Monarchs will infect and breed with populations that are passing through, possibly jeopardizing the migration.

The situation exacerbates an already serious decline in the Monarch migration.  Drought, genetically modified crops, late summer wildfires, and generally inhospitable conditions pose multiple threats to Monarchs and their migration.   It's likely that 2012 will be the worst year, numbers wise, in Monarch butterfly migration history since records have been kept.

"Monarchs are famous for this migration so when we see what appears to be a break in their migratory pattern, we want to know why and what the implications are," said Satterfield.

OE Spores with Monarch Butterfly Scales

Eeeew! OE spores look like little footballs next to Monarch Butterfly Scales--photo courtesy of MLMP

But butterfly breeders and enthusiasts argue that OE is like staphylococcus–present in the general population and becoming a threat only under stressed circumstances. Organizations like the International Butterfly Breeders Association promote best practices for limiting OE in captive environments through education of its membership and seminars on managing and limiting its presence.  And yet others believe that OE is simply a part of the evolutionary cycle, killing those butterflies less fit than others.

For butterfly hobbyists and gardeners, the "curassavica question" presents a quandary:  should we be planting Tropical milkweed or not?

"This is a very sensitive subject in the Monarch world,"  said Satterfield.  "We just don't have the data right now."

It will take three or four more years to complete Satterfield's research.  She advises that natives are always better--which is true, IF you can find them and meet their persnickety growing needs.  Organizations like Monarch Watch and the Xerces Society's have launched milkweed restoration campaigns, but wide availability of native stock is still a dream.

If you DO plant curassavica, many scientists suggest cutting it to the ground in winter--unless yours is a research site like the Milkweed Patch on the San Antonio River Museum Reach.  Scientists encouraged the City of San Antonio and San Antonio River Authority to leave our milkweed patch alone as an experiment.

Steven Schauer of SARA confirmed that all pest and weed removal at the Patch is done by hand. Interestingly, the San Antonio patch brags a lower incidence of OE (15%) than in other monitoring sites observed (47%) by the Monarch Larvae Monitoring Project.   The 47% figure is SIX TIMES the rate of OE in Monarchs that migrate to Mexico.

Satterfield suggested that a deluge of milkweed beetles at the Patch this winter decimated not only the foliage, but OE spores.  The hungry orange-and-black bugs pruned much of the diseased growth, creating a better balance for the butterflies. Volunteers showed up one Saturday morning and removed many of the bugs by hand, but apparently Mother Nature's plan was pretty effective in managing the OE problem.

Milkweed beetles have defoliated the Milkweed Patch. But aren't they cute?

Mother Nature on the case: Milkweed beetles defoliated the Milkweed Patch last spring, cleaning out OE spores in the process. Photo by Monika Maeckle.

"This may have removed any OE spores on the milkweed plants, which probably helped to keep the prevalence of OE low this year," said Satterfield. She added that since our Milkweed Patch is further inland and enjoys cooler temperatures than coastal situations in Houston and Florida, the Monarch population was lower and perhaps more hearty.

In the meantime, gardeners are left to make their own calls.   Is it better to have a questionable milkweed source in your yard to provide Monarchs with nectar and host plant, or not?

Let's see: Tropical milkweed is easy to grow, widely available, a prolific bloomer, favorite host plant for Monarchs and a great nectar source for all butterflies.   I know where I come down on that.   You'll see Tropical milkweed in my yard.   But I'll be sure to keep it out of wildscapes and ranch situations, and slash it to the ground in the winter.

Monika Maeckle writes about gardening, butterflies, conservation and the Monarch butterfly migration at the Texas Butterfly Ranch.  She covers nature in the urban environment for this website.  You can reach her at or follow her on Twitter @monikam.

Related Stories on the Rivard Report:

Monarch Butterflies Coming our Way in What Could be Worst Migration in History

Beetlemania at the San Antonio River Museum Reach Milkweed Patch

Monarch Butterflies Make Milkweed Patch on San Antonio Museum Reach Home

Here they Come! Monarch and Other Butterflies Passing Through, Laying Eggs and Sipping Nectar


4 thoughts on “San Antonio’s Tropical Milkweed Patch Raises Monarch Butterfly Questions

  1. I wonder how long it will be before climate warming allows tropical milkweed to spread north “naturally” and render this whole discussion moot?

    Is there any research proving that swamp milkweed beetles (in the picture but not correctly labelled) actually consume oe spores?

    • I believe Dara Satterfield is looking into this exact question. As she says, the science isn’t in yet. All we can do is stay tuned.

  2. Sarah, these are excellent questions.

    I think you are right that climate change may make conditions more favorable for tropical milkweed growth in the southern U.S., either now or in the future. My guess is that it would still not naturally disperse this far north from its native range (e.g., Costa Rica) without some human help, though.

    As for the swamp milkweed beetles, I am not aware of any direct studies to test if they are consuming the OE and re-distributing the spores elsewhere — but they certainly must be eating these microscopic OE spores if spores are present on the milkweed leaves, which they apparently eat voraciously.

  3. Monarch enthusiasts in New Zealand and Australia would never entertained the thought
    of cutting back their evergreen Gomphocarpus milkweed to near the ground as Dara Satterfield recommends. Why? The consequences to their monarchs would be devastating. How?

    1) Thousands (collectively) of monarch eggs / caterpillars / chrysalids that were eating those milkweeds at the time of cutting during the winter would be killed outright or subsequently starved.

    2) The small number of female monarchs that develop eggs during the middle of winter and the huge number that develop eggs in late winter would find a greatly deminished biomass of milkweed foliage available on which to lay their eggs due to the midwinter cutting. Plus there would be alot less milkweed foliage available for the caterpillars to eat. Thus the monarch migratory population would greatly decline.

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