On the sprawling grounds of Hermann Park in Houston, about 100 tropical milkweed plants bloom bright red and orange. Adding their own vividness to the scene are the eastern monarch butterflies that live here year-round, sipping nectar from the flowers and laying eggs among the leaves. When most monarchs are roosting on the oyamel fir trees in the mountains of central Mexico, these local beauties spend the winter right in Houston.
That’s because, for a monarch, finding a perpetually blooming patch of milkweed is like discovering an oasis in the desert. Milkweed, the cornerstone of monarch breeding and survival, typically sprouts in the spring and dies in the fall. But the non-native milkweed Asclepias curassavica), or tropical milkweed, grows all year long in southern coastal states—and therein lies a problem.
“One of the concerns is that tropical milkweed can support year-round breeding in southerly and coastal mild climates. The risk is that it will break the normal migratory cycle.” says Sonia Altizer, a monarch expert at the University of Georgia. In some places, like Houston’s Hermann Park, this has led to the establishment of year-round resident colonies of monarchs. “And if too many butterflies stay and breed,” Altizer adds, “there won’t be enough milkweed to support them.”
Tropical milkweed, like the crowded cluster in Hermann Park, survives the mild southern winters from Texas to South Carolina. The plant’s popularity has soared in recent decades, driven partly by the urge to help monarch butterflies and partly by tropical milkweed’s availability, beauty, and hardiness. However, according to the scientists who study these ethereal insects, this particular milkweed’s longevity poses problems for monarch migration and health and could further endanger a beloved species that’s already in decline thanks to habitat loss and rampant pesticide use.
Every year, the eastern monarch butterfly makes a multigenerational journey from Mexico to southern Canada and back again. Their breeding cycle kicks off in spring as they depart Mexico and follow the milkweed northward as it blooms. The journey can take up to two months and three or four generations (each with a life span of about five weeks) to complete the spring leg of the tour. Then in the fall, a special generation hatches and metamorphosizes. These butterflies live a whopping eight months as they flutter the 2,500 miles back to Mexico to spend the winter and produce the next generation the following spring.
This migration is an epic natural wonder and crucial to the species’ symphonic life cycle. But tropical milkweed could be disrupting this ancient rhythm.
“I remember coming to the Houston site and finding the tropical milkweed plants almost completely naked of leaves. The caterpillar density was so high they had run out of food,” says Dara Satterfield, a fellow at the Smithsonian Migratory Bird Center at the National Zoo in Washington, D.C., who studies how environmental changes affect the monarch migration. “It was disheartening to see because I knew most of those caterpillars wouldn’t survive.”
Then there’s the issue of disease. Tropical milkweed hosts the protozoan parasite Ophryocystis elektroscirrha, which travels on monarch bodies and wings and collects on milkweed leaves. Because tropical milkweed doesn’t die back when the weather cools, the parasite accumulates on the plant, and monarch caterpillars then ingest the parasites while munching the leafy greens. The parasite may arrest chrysalis development or stunt a butterfly’s growth and life span, along with its ability to reproduce and migrate.
Monarchs that breed on tropical milkweed year-round face a higher risk of infection compared with migratory monarchs, according to a 2015 paper coauthored by Satterfield and Altizer. In 2015, infection rates were up to 90 percent in the Hermann Park butterflies.
“It’s very, very rare to see high infection rates on a native milkweed patch. The plants tend to be more abundant when they are around, so monarchs don’t seem to feel the need to dump their eggs in one place,” explains Satterfield, thus avoiding “a high density, high disease situation.”
Perhaps more concerning is the ease with which the parasite infiltrates new populations. A follow-up study examined whether resident and migrant monarchs are coming into contact with each other. The answer is yes, especially in parts of coastal Texas. Texas serves as the migratory flyway for monarchs traveling to and from Mexico. The study concludes that if migratory monarchs coming from Mexico lay eggs on the milkweed contaminated with parasites from resident monarchs, it could increase infection risk for migrants’ offspring.
Still, not all monarch experts lose sleep over tropical milkweed. “I don’t see it as a serious threat to the monarch population,” says Chip Taylor, head of Monarch Watch, a nonprofit education, conservation, and research program based at the University of Kansas. “It is a tiny issue relative to the annual loss of two million acres of monarch habitat a year due to agriculture and development.”
Habitat loss does indeed exact the greatest toll on monarch populations. In the late 1990s, farmers across the cluster of midwestern agricultural states known as the Corn Belt started growing Roundup Ready corn and soybeans, which are genetically engineered to tolerate glyphosate, the key ingredient in the herbicide Roundup. The genetically altered plants allowed farmers to treat entire fields with Roundup, killing off all “weeds,” including milkweed, without harming their crops. (Think of it as the bottom trawling of weed management.) In the following two decades, the average overwintering monarch population plummeted to half its size.
While the monarch population benefited from ideal weather conditions last year, milkweed—and more of it!—is essential for the butterfly’s ongoing survival. “The monarch is one of our most well-known butterflies,” says Sylvia Fallon, senior director of NRDC’s Nature Program. “A lot of people grew up doing science projects related to monarchs and their amazing migration. It’s a natural wonder, and it would be a huge loss for the world if we don’t save them.”
Fallon says the most important message for gardeners and landscapers—in Texas and beyond—is to plant milkweed that’s native to their area. (Here’s a helpful map of native milkweed species for each region.) In places where tropical milkweed already grows, the plant should be cut back within six inches of the ground every fall to stop monarchs (and their parasites) from setting up residence. Then, come spring, even more monarchs may drop by.
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