The thriving milkweed patch found on a lush bank of the San Antonio River is just a short walk south of the Pearl Brewery, the bustling culinary center and architectural centerpiece of the city’s Broadway corridor renaissance. On a Saturday morning enlivened by the city’s most popular Farmer’s Market, locals of all ages wander both sides of the river as a natural gas-powered tourist barge silently plies the placid waters.
The January sun has warmed the afternoon air and a pair of Monarch butterflies flutter by, pausing for nectar on late blooming lantana and a few remaining milkweed flowers. Less visible to the casual observer are other river residents: dozens of Monarch caterpillars in various stages of development nosh on milkweed leaves, fueling up to form their chrysalises.
For residents who only have to reach back several years to remember riverbanks overgrown with weeds, strewn with trash, and home to vagrants, the transformation of the river’s Museum Reach is nothing less than a miracle. And by all accounts, this particular milkweed patch seems to embody that successful transformation, a butterfly habitat that attracts the attention of passersby and butterfly researchers, right in the heart of the city.
For some scientists, however, the San Antonio milkweed patch spells trouble. The milkweed responsible for this butterfly bounty is the nonnative Asclepias curassavica, commonly known as Tropical Milkweed.
Affordable, widely available, and easy-to-grow, Tropical Milkweed sports gorgeous orange and yellow blooms, reaches two-three feet in height and serves as a magnet for Monarchs and other butterflies. It resembles the native Asclepias tuberosa, known as Butterflyweed. In all likelihood, Tropical Milkweed is the very plant on which the storied Monarch butterfly evolved—and it vexes and divides scientists who study Monarch butterflies.
“I would limit Tropical Milkweed to being an inside demonstration project, growing the plants in an enclosed area totally inaccessible to wild monarchs,” says Monarch butterfly expert Dr. Lincoln Brower, who has studied the insects for more than five decades.
Dr. Brower and other scientists worry about undesirable colonies of OE, the unpronounceable Ophryocystis elektroscirrha, a protozoan disease that infects Monarchs and other milkweed feeders. Dr. Brower hypothesizes that resident OE-infested Monarchs will breed with migrating populations, jeopardizing the migration. He is the first to admit, however, that his theory is built on informed speculation since a conclusive study has never been published.
OE is transferred by the butterflies themselves when they nectar and lay eggs on plants, leaving spores behind. It seems to especially flourish on Tropical Milkweed in southern climates late in the year. In colder regions and the wild, milkweeds die off in the winter, apparently purging OE to a large degree or sending it into dormancy.
Just as worrisome, the presence of year-round milkweed appears to knock Monarch butterflies out of their diapause, a state in which they don’t reproduce, but conserve their energy for migrating. “The population-level impacts of this remain unknown,” says Dr. Karen Oberhauser, a conservation biologist at the University of Minnesota who oversees Monarch Lab, an education outreach program that brings Monarch butterflies into the classroom. Oberhauser concedes, though, that “even though we haven’t proven that Tropical Milkweed could lead to harm, we haven’t proven that it won’t.”
Butterfly breeders and enthusiasts liken OE to staphylococcus–always present but causing widespread affliction only under stressed conditions. Some believe OE is simply a part of the evolutionary cycle, killing those butterflies less fit than others.
“If it were as deadly as many people imply, there wouldn’t be an OE issue,” suggests Edith Smith, of Shady Oak Butterfly Farm in Brooker, Florida. Smith has raised hundreds of thousands of OE-free Monarchs for education, celebration, and research purposes. Smith and others posit that OE is just one more Darwinian check that nature employs to keep the Monarch population balanced.
Another troubling aspect to Tropical Milkweed is a sometime tendency to be invasive when not properly managed. All milkweeds are gregariously opportunistic, with the milkweed “fluff” carrying their seeds far and wide, seeking welcoming conditions for germination.
“I’m a little cautious about it,” says Andrea DeLong-Amaya, Director of Horticulture at Lady Bird Johnson Wildflower Center in Austin. “I don’t know if it will stay well behaved. Especially if our climate continues to stay warm.”
Not all scientists consider Tropical Milkweed a threat to the Monarch butterfly migration, however.
“Lots of opinions, but very little data,” says Dr. Chip Taylor, founder of Monarch Watch, a citizen scientist program based at the University of Kansas at Lawrence devoted to the study of Monarch butterflies. “In the grand scheme of all things Monarch, this is a trivial issue. We are losing more habitat per day than can possibly be replaced by gardeners everywhere planting Asclepias curassavica,” says Dr. Taylor.
Monarch Watch estimates that 2.2 million acres of potential milkweed habitat are lost each year because of development, genetically modified and herbicide tolerant crops, and pesticide use. The organization also launched a “Bring Back the Monarchs” native milkweed restoration campaign 2010.
Texas has been dubbed “the funnel” of the Monarch migration since the creatures must pass through the Lone Star State coming and going to their migratory roosts in the forested mountains of Michoacán state in Mexico. Dr. Taylor has even called Texas the migration’s “most important state.”
“If there’s no milkweed in Texas, the Monarch population won’t grow,” he says, adding that as temperatures rise over the next 30 yrs and other milkweed species continue their decline, Tropical Milkweed will become naturalized in parts of Texas and “may assume a larger role in supporting the US monarch population.” Recent USDA revisions to plant hardiness maps, which moved many cities to warmer zones, seem to reinforce his notion.
For Texas gardeners aiming to support the butterfly population, we have little choice when it comes to buying milkweed plants. On the rare occasions that native milkweed seedlings are available—at plant sales and specialty nurseries—transplanting often results in failure. To further complicate matters, Tropical Milkweeds are often identified as natives.
“It’s frequently mislabeled,” says Kip Kiphart, a volunteer and trainer at the Cibolo Nature Center’s Monarch Larvae Monitoring project in Boerne. “I see it all the time,” he says, adding that sellers are likely misinformed or don’t care.
Even when available, native milkweeds are notoriously persnickety to cultivate. George Cates, “seed cleaner extraordinaire” at the highly regarded native milkweed supplier Native American Seed Company in the Hill Country town of Junction, explains: “Native milkweeds simply don’t do well in containers. They require a very specific set of conditions and have an extremely long tap route, making containerization untenable.”
Planting milkweed from seed can also be complicated, requiring stratification, moist conditions for 45 days, specific soil conditions, and alternate dry and wet periods.
The cost and challenges of commercial milkweed production combined with consumers’ penchant for bold color and easy care make a difficult case for nurseries to invest in native milkweeds, says David Rodriguez, Bexar County Agent for Texas Agrilife Extension.
“Consumers want the ‘wow’ factor, they want pretty, pretty,” says Rodriguez. “The demand for native milkweeds just isn’t there.”
Milkweed and Monarch evangelists at the Monarch Joint Venture and some specialty growers are working to make native milkweed seeds and plants more available. But until growers recognize a market for native milkweeds and perfect its commercial production, the only practical option for Texas gardeners is Tropical Milkweed.
Even ecosystem specialists such as Dr. Mark Simmons of the Ladybird Johnson Wildflower Center view Tropical Milkweed with pragmatism.
“Take it out of the context of plants, and just consider it butterfly food,” says Dr. Simmons. “I grow potatoes in my garden and they’re not native. But they fill a need.”
Over the next few weeks, Monarch butterflies will travel through Texas. Females will lay their eggs on whatever milkweeds they can find. Caterpillars will hatch into their signature black, white, and yellow- striped skins, consuming 200x their weight in milkweed leaves before forming a chrysalis.
In the Hill Country they’ll eat Antelope horns. Along rivers and streams, they’ll consume Swamp milkweed. And elsewhere in Texas, Common, Green, Butterflyweed, Zizotes and other milkweeds will sustain them.
In most Texas butterfly gardens, however, the milkweed buffet will consist of nonnative Tropical Milkweed.