Tropical Milkweed OK for Monarch Butterflies, “Just Cut the Dang Stuff Down”

“Can well-meaning people sometimes make things worse?”

That was the provocative subhead on an article by Dr. Jeffrey Glassberg, founder and president of  the North American Butterfly Association (NABA) in that organization’s most recent edition of American Butterflies Magazine.

Glassberg, who holds a PhD in biology, a law degree and credentials as an entrepreneur, author and butterfly advocate, challenged the recent scientific assertions made by Satterfield et al  that Tropical milkweed, Asclepias curassavica, is bad news for Monarch butterflies.

Tropical milkweed:  The debate continues.  Photo by Monika Maeckle

Tropical milkweed: The debate continues. Photo by Monika Maeckle

Glassberg challenges the study’s claims about Tropical milkweed’s appropriateness in South Texas, where the North American Butterfly Center operates in Mission along the Texas-Mexico border.

Dara Satterfield

Dara Satterfield  Photo by Monika Maeckle

In case you missed it, Dara Satterfield, a PhD candidate at the Odum School of Ecology at the University of Georgia, and her graduate advisor, Dr. Sonia Altizer, a disease ecologist at Georgia and one of the foremost experts on Ophryocystis elektroscirrha, a Monarch-centric spore driven disease known as OE, suggest in their research that sedentary winter-breeding butterflies are at increased risk of OE. They speculate that Tropical milkweed, Asclepias curassavica, the most widely available but technically nonnative milkweed and favorite host plant of the Monarch butterfly, may be damaging the Monarchs’ abilities to stay healthy, on track and make their way to Mexico.

Native to Central America and Mexico, Tropical milkweed grows well and sometimes year round in Texas and Florida.  Scientists worry that it might be confusing Monarchs, making them skip their migration and reproduce locally.  When they do that, spores from butterflies infected with OE build up on the plant and may transfer the disease to other caterpillars, chrysalises, and later, butterflies, resulting in crippling and even death.  Read the Tropical milkweed fact sheet.

Just to be clear:  Satterfield, et al DO NOT THINK TROPICAL MILKWEED IS EVIL.  In fact, they say exactly that in a statement issued by Monarch Joint Venture and shared via the DPlex, a listserv that reaches about 800 butterfly followers.

“Tropical milkweed itself is not ‘bad.’ (It provides larval food for Monarchs in many places where it occurs naturally, such as across the Caribbean, Mexico, and Central America.),” the scientists said in a statement released in January following the milkweed kerfuffle.

“The truth is that we don’t really know,” if butterflies infected with OE at winter-breeding locations will impact the Monarch population as a whole, the statement said.

So to be fair,  the scientists admit that much is still to be determined about the impact of Tropical milkweed on the Monarch butterfly population.   That’s why they suggest cutting Tropical milkweed to the ground over the fall and winter–so the OE spores can’t build up.

Glassberg takes the Satterfield et al. study to task, challenging the assertions with his own data fueled theories.

Monarchs and other milkweed feeders host on the evergreen Pineneedle milkweed in Arizona and have lower than average OE infection rates.  Courtesy photo via Ladybird Johnson Wildflower Center, Sally Wasorski

Monarchs and other milkweed feeders host on the evergreen Pineneedle milkweed in Arizona and have lower than average OE infection rates. Courtesy photo via Ladybird Johnson Wildflower Center, Sally Wasowski

Nonmigrating Monarch butterflies in Hawaii  and Arizona have lesser-than-average levels of OE infection, notes Glassberg, pointing out that some Monarchs overwinter and sustain themselves on evergreen milkweeds like Fringed twinevine and Pineneedle milkweed.

Such examples “suggest that the level of OE infection might not be as highly correlated with non-migratory behavior and that the presence of an evergreen supply of milkweeds doesn’t necessarily mean that OE levels will be high, as Satterfield et al. conclude,” he writes.

“Perhaps the higher levels of infection that Satterfield et al. found to be associated with Tropical milkweeds were due to temperature effects or other factors not intrinsic to Tropical milkweed,” Glassberg writes, suggesting that global warning and higher temperatures beg the question: what is a native plant, anyway?

Climate change is already making the range for Tropical milkweed creep north and “if and when that happens, wouldn’t it be a good thing for there to be extensive areas in the southern United States that might serve as reservoirs for Monarchs that would then be able to repopulate more northern areas, much as Painted Ladies and American Ladies do now?”

Hardiness zones redefined by USDA

In 2012, the USDA redefined hardiness zones in response to climate change.  San Antonio moved to Zone 9a from Zone 8b.  Will Tropical milkweed eventually rank as “native”?  Screengrab via USDA

When asked about the article, Satterfield responded by email that “We do plan to address why our paper rules out effects of temperature and geography and points to year-round milkweed as the source of the high levels of disease.”

Glassberg makes a lot of sense here.   His characterization of Tropical milkweed as a “life buoy” for Monarchs until the commercial market for native milkweeds can be developed holds great appeal.  In a recent webinar staged by US Fish and Wildlife Service  on creating Monarch butterfly habitat in the U.S., experts stated that it will take a minimum of five years to create a commercial market for native milkweeds.   That’s a long time for Monarchs to wait around for the perfect locavore food, especially when Tropical milkweed is already on the market, easy-to-grow and very affordable.

Troipcal milkweed:  "Life buoy" for Monarchs and other milkweed feeders until the native milkweeds are available.  Photo by Monika Maeckle

Tropical milkweed: “Life buoy” for Monarchs and other milkweed feeders until the native milkweeds are available. Photo by Monika Maeckle

My approach in the garden includes Tropical milkweed as a foundation, natives preferred, but more challenging to grow.  And I’m not alone.  Plenty of us who follow Monarchs believe the Tropical milkweed debate is bloated and misguided.

Here’s what Edith Smith, one of the most seasoned, experienced and thoughtful commercial butterfly breeders on the planet and owner of Shady Oak Butterfly Farm in Florida, thinks about the focus on Tropical milkweed:   “…They’re so fussy about that plant. If only they’d stop to think, they’d realize that if a couple of treaties had been written a bit different and the southern border of our country had been drawn a hundred miles further south, Tropical milkweed WOULD be a U.S. native.  SHEESH!”

Edith Smith

Edith Smith, Shady Oak Butterfly Farm

She adds:  “As far as it being good/bad for Monarchs … let’s remove all the Tropical milkweed from Mexico and see what happens to the Monarch population in the US. That in itself should answer the question.”

Another Monarch expert suggested everyone just chill on the Tropical milkweed fixation, pointing out that a better investment of time, energy and money would be replenishing the million-plus acres of pollinator habitat lost each year.  Arguing about narrow strips of Tropical milkweed along the coastline constitutes a huge misplaced priority.

“Just cut the dang stuff down at the end of the season–maybe twice. We’re wasting too much time on this issue. There are bigger problems,” said the source.

Monarch caterpillars tropical milkweed

Monarch caterpillars on Tropical milkweed, April, 2015  Photo by Monika Maeckle

Even Catalina Trail, the woman who discovered the Monarch butterfly roosting sites in Mexico back in 1975, plants Tropical milkweed in her Austin garden.  “I would prefer to have native milkweeds in my yard, but they’re impossible to grow,” she said by phone.  “I have two Tropical milkweeds in my yard.”

This website has reported repeatedly on this topic and I am at peace with my stance:   Tropical milkweed fills a gap for Monarch butterflies. Just cut it back.

Both early and late in the season, Tropical milkweed is often the ONLY milkweed available for migrating Monarchs.   The eggs of the caterpillars pictured above were laid in late March and because of our cool spring, no native milkweed was up and out of the ground yet.   My Tropical milkweed from last year, which had been cut to the ground in December as per best practice, had plenty of fine, tender new leaves ready for the hungry critters when they arrived.

Had I not this Tropical milkweed in my yard, the migrating Monarch who laid the eggs that became today’s caterpillars in my yard would have had to keep flying, seeking milkweed that in this cool Texas spring was mostly absent until now.  Who knows where/if she would have found a place to lay her eggs before perishing?

Meanwhile, in the Fall, the only native milkweed I see is Swamp milkweed along the Llano River, and it’s usually in bad shape, ravaged by aphids and the summer heat.   Tropical milkweed is the only food available for late season caterpillars, and the lack of available caterpillar food often results in a caterpillar food emergency, with folks calling around town to friends and local nurseries to see if anyone has clean, chemical free milkweed available.   Some breeders and enthusiasts have taken to offering pumpkin, cucumbers and other “alternative fuels” for late season Monarchs.

Monarch caterpillars eating pumpkin

In the fifth instar, Monarch caterpillars will eat a variety of pumpkins. Photo by Ellen Reid

Making an issue about Tropical milkweed reminds me of the locavore food movement: idealistic, admirable, but now always practical.   The caterpillars have to eat.

Imagine you’re driving cross-country with your family and you and the kids find yourselves famished. Sure, you’d prefer to stop at a local diner where good food was whipped up from scratch from local organic ingredients, responsibly harvested, lovingly prepared, delicious, nutritious and affordable.

But that’s not always possible.   Sometimes you have to hit the drive-through of a fast-food joint because that’s all there is.  And that will get you to the next place.

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13 thoughts on “Tropical Milkweed OK for Monarch Butterflies, “Just Cut the Dang Stuff Down”

  1. I am so glad someone has written to agree with me. I love the tropical milkweed. I have four raised beds full of tropical (4’x10′ each) in my Georgetown, TX back yard. I cut them back in Dec, they are regrowing now. I have been unable to grow other kinds of milkweed. Tropical is getting a bad rap.

  2. Thanks, Monika, for another sensible article on this subject. Mike Quinn advised me to cut my tropical milkweed back twice a year to help prevent OE, so that’s what I do. Like so many others, I’ve tried and totally failed at growing the “native” milkweed. Of course, I’m totally open to anything that will help our Monarchs (including yanking out all the tropical) and have closely followed the debate about tropical. All I know is that in my little yard there are at least 25 thriving cats with zero signs of OE.

  3. During the past 10 days there have been multiple sighting reports on Journey North of brightly colored young monarchs in the central USA such as this one: https://www.learner.org/jnorth/sightings/query_result.html?record_id=1429566644 So that means these butterflies had to have grown up in the southern States or in Mexico on tropical milkweed and then migrated north, thus helping to repopulate the central States. Also means that a policy of cutting tropical milkweed down during the winter would reduce the production of new generation spring migrants that help repopulate the central States.

  4. Hi Monika,
    Nice posting. It’s unfortunate that tropical has managed to so distract us from the real problem of habitat loss. The solution of cutting it down is so simple, a suggestion based on inferred relationships in the Satterfield et al. paper. There seem to be two points in the paper. 1. OE infection rates are pretty high in monarchs collected from around the Gulf of Mexico. This overlaps with the range of tropical milkweed. That’s clear, although the infection rates are variable from site to site. 2. This correlation is used to infer that winter breeding created this high infection rate. That’s where it gets a little fuzzier. As Glassberg points out, their citation supporting winter breeding is not very robust. Okay, perhaps a better citation could have been found (like the unpublished Satterfield 2013 personal observation; should get that data out), but it still leaves the problem of did winter-breeding drive the high OE infection rate. An excellent test of this would have been to collect and eclose monarch caterpillars from the tropical milkweed. After all, that would leave no doubt as to their origin. An alternative hypothesis is that OE weakened monarchs preferentially abandon migration and collect around nectar sources. Migrating birds alter their behavior with the availability of resources, why not butterflies? There are many winter blooming plants in the nursery trade (my creeping purple lantana and rosemary bloomed all winter long in coastal Texas, much more so than the tropical milkweed every did). Many of them have their roots in tropical lands for the same reason tropical milkweed is so good. They keep blooming until a frost kills them. Sure, tropical milkweed would be a good place to look for monarchs, but is it what really created the high infection rates in non-migrating population? Can we exclude winter blooming landscaping in general as a refuge for OE weakened monarchs? Certainly, more work needs to be done on this question of what alters the fall migration behavior.
    Regardless of this, I think the bottom line of cutting back tropical milkweed in the fall is prudent. It’s an excellent host plant in the spring and until the market can provide alternatives, there is little else most gardeners can do to aid monarchs. But remember that this is a DISTRACTION from the real problem of habitat loss. If we continue to lose a million acres a year of monarch habitat in the midwest (Chip Taylor’s estimate), then it will really not matter what we do with tropical milkweed.

    • Tropical milkweed does not contribute to habitat conservation, on the contrary. This is why organizations as the Xerces Society and the Monarch Joint Venture recommend only the use of native milkweeds, particularly milkweeds that are native to your region. Please, read my other comment and the links I provide.

  5. Tracy, in Arizona some fall migrants also stop migrating when they arrive in the lowland areas with trees and flower nectar (e.g. Rio Salado Park in Phoenix, AZ, http://3.bp.blogspot.com/-_NGu5dv7wkA/TV_RKwoNECI/AAAAAAAAAbw/AOKA4d5rhas/s1600/SAM_3242.JPG parks near the Colorado River in or near Lake Havasu City, AZ http://1.bp.blogspot.com/-Q302DsW5Knw/UuU2vRFeqdI/AAAAAAAABD0/UL8SvGDYnyg/s1600/MonarchBuckskinStatePark012014.jpg and OE sampling by the Southwest Monarch Study has determined those fall migrants have very low OE spore loads. So we already know that fall migrants that stop migrating long before they reach the main overwintering sites in central Mexico and along the California coast are not necessarily “OE weakened”. We also know the fall migrants that stop migrating and lay eggs on tropical milkweed in the Gulf coast states and northern Mexico in the fall produce offspring in late winter and early Spring, some of which migrate as evidenced by these recent reports on Journey North of young, brightly colored monarchs appearing in Virginia https://www.learner.org/jnorth/sightings/query_result.html?record_id=1430236899
    https://www.learner.org/jnorth/sightings/query_result.html?record_id=1429566644
    North Carolina: https://www.learner.org/jnorth/sightings/query_result.html?record_id=1429833712 Tennesee: https://www.learner.org/jnorth/sightings/query_result.html?record_id=1429666337 Therefore the policy of cutting tropical milkweed down to near the ground during the winter would reduce the production of new generation spring migrants that help repopulate the central States, especially if that policy was extended into the lowlands of northeastern Mexico. The policy would also force pregnant female fall migrants to lay eggs on the cut stems of tropical milkweed which in turn would cause most of the subsequent caterpillars to die of starvation.

    • Hi Paul,
      Those are interesting observations, but much more data is needed before we can conclude that these single observations are representative of a general pattern. I would in no way advocate cutting tropical milkweed in northern Mexico. This is only for regions around the northern Gulf of Mexico. I can’t really address the comments about the populations in Arizona or California. Until a study with extensive sampling is conducted that is designed to test specific hypothesis about migrants and tropical milkweed in these areas, it is hard to know what point observations mean. Perhaps they are relevant to what happens along the northern Gulf of Mexico, perhaps not. At the moment, it is largely speculation unconstrained by data.
      Most fall migrants have low OE spore loads (although that may mean different things to us), so that’s not a surprise. The question is whether some property of the habitat along the migration route in the east is accumulating contaminated butterflies in the areas that were examined. I think it is also important to remember that in biological systems, truth is rarely absolute or binary. We tend to want to make things black and white, but biology is messy and often has multiple factors in play at once and these factors change in importance with both time and space. It’s why the classical idea of how science progresses as defined by physics is a lousy model for ecology. There may well be both preferential trapping and winter-breeding going on.

      • I have been raising monarchs for about 10 years or more. I now have some plants that are two or more years old. I believe I have some cats with O.E. Is there anyway I can tell this by looking at the black cocoons and seeing some butterflies lying on the ground, kind of wriggling?

  6. A few key points to keep in mind in the context of this article: 1) the Satterfield et al. paper focuses on a relatively small geographic range for these recommendations. The concerns with non-native tropical milkweed increasing the prevalence of OE are only in areas where the species can grow year round, allowing the parasite spores to build up on the plants. In areas where tropical milkweed dies back with a hard freeze, these disease concerns are no longer. 2) All species of milkweeds have the ability to spread OE if they have been visited by infected adults, who leave dormant spores behind for future caterpillars to eat. The issue arises when they do not die back naturally, as most native species do. 3) In all of this, the best recommendation put forth so far is to cut back tropical milkweeds in fall/winter (where they grow year round) to minimize disease spread. Researchers have documented that OE is occurring at a higher rate in the winter breeding populations in the southern U.S., and cutting back the tropical milkweed foliage during the winter could help to mitigate this issue. While it may seem relatively small compared to other issues that monarchs face, it is something that we can begin to address and alleviate yet another threat that monarchs are facing. They all add up, especially with a population that is small and vulnerable to begin with. We cannot simply forget about the “small” problems. Whenever possible, try to add/replace natives to your garden whenever they become available. 4) Lastly, this work is not complete. As with all scientific research, efforts to track this phenomenon will be ongoing and more will be learned and published in future years. Right now, researchers are going with their best recommendations for the information that is available. To expect that years of data be at our fingertips and all questions thought of an answered surrounding this issue is unfair to the scientific process. It is important to discuss these questions or methods that might help in future research, but do so in a way that is constructive. There might be limitations to that question, or perhaps just a need for more citizen science volunteers to gather more samples. Let’s be constructive, not critical!

  7. We need to save entire habitats, not just an individual species. Organizations engaged on conservation of monarch butterflies as well as conservation in general list only native milkweeds. It is important to use not just native milkweeds, but those native to your area. The following organizations provide lists of milkweeds and information on local suppliers and regional distribution of milkweed varieties. None of them list tropical milkweed for very good reasons.
    Xerces Society Milkweed Seed Finder (25 native species and list of regional suppliers), http://www.xerces.org/milkweed-seed-finder/
    Monarch Watch Milkweed Market (15 native species and regional maps), http://monarchwatch.org/milkweed/market/
    Monarch Joint Venture (about 20 native species by region), http://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/documents/MilkweedInfoSheet.pdf

  8. Thank you Monika! I’ve been in email contact with Dara Satterfield as well. They did do some follow-up PR to try to correct the misconception of the appropriate use of Tropical milkweed. But the damage was done, and now we need to keep posting these updates and clarifications of the use of Tropical milkweed.
    I’ll be posting this on my facebook page for our community to see.

    • What it boils down to is that if you want to preserve ecosystems, not just the flagship species, you have to plant local species of milkweeds. Follow the directives of the Xerces Society and the Monarch Joint Venture. They recommend only the use of native milkweeds, particularly milkweeds that are native to your region.

  9. Pingback: Fall Butterfly Garden Tips | Attracting Butterflies

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