Save milkweed to help save monarch butterflies
As you trim the wild edges of your property this summer, you might want to be careful of which weeds you whack. Few plants are as valuable to North American conservation as the common milkweed.
Not really a weed at all, the native flower blooms with beautiful pink star-shaped petals. Its pollen is fragrant, its bulbous seed pods are hairy and its leaves are the only food and reproductive habitat of the Eastern monarch butterfly. As milkweed declines, the colorful insect’s migration cycle grows more tenuous.
“I think most people recognize common milkweed when they see it,” said Victoria Pocius, a researcher in the entomology department at Penn State University studying the relationship between milkweed and monarch butterflies. “I don’t know if they’re aware of its value in nature.”
The vibrant orange and black butterfly is a common summer sight worldwide, and the monarch is neither endangered nor threatened.
One subspecies, however, is famous for its annual 6,000-mile round-trip migration cycle starting in a 7-acre forest in Mexico. After an Eastern monarch has flown north about 300 miles, it has to find young milkweed plants that provide an egg-laying habitat. After reproducing, the parents die. The young caterpillars feed on the milkweed until they’re strong, build and emerge from cocoons, fly another 300 miles north and must find another patch of suitable milkweed plants.
It takes several generations of Eastern monarchs to make the trip to Canada. In the fall, a “super-generation” of butterflies bulk up on milkweed leaves and fly the entire 3,000-mile southern route to Mexico.
Industrial agriculture techniques and urban sprawl have reduced milkweed dispersal in North America. When butterflies in the migration cycle fail to find it, reproduction may not occur.
The Eastern monarch has been given the informal conservation status of “endangered phenomenon” because its increasing difficulty in finding milkweed is disrupting the migration cycle.
The Center for Biological Diversity, a nonprofit conservation organization, reported that 53% of the butterfly’s population was lost between 2018 and 2019. Some 225 million to 250 million Eastern monarchs remain.
Of the 140 milkweed species in the United States, 11 are native to Pennsylvania and three — common, swamp and butterfly milkweed — will look familiar to most people who spend time outdoors.
Milkweed thrives in places that are in a constant state of change such as roadsides, feral farmland and wild, undeveloped urban spaces. At Pennsylvania’s latitude, common milkweed blooms from now through July.
Chemical defenses give milkweed a bitter taste that protects the plant from most hungry animals, but the compounds trigger an attraction among monarchs.
“Eating [milkweed] slows down the heart rate of any animal with a backbone,” said Ms. Pocius. “A big animal like a deer might get sick. But monarchs have a special way to process those defenses.”
In research published in 2018, Ms. Pocius found that monarchs prefer to lay eggs on common and swamp milkweed. It is believed that chemical compounds in the leaves provide some benefit. How the larvae and adults actually find the plants is less clear.
“When you’re a caterpillar, you can’t see that well,” she said. “They’ll taste it with receptors on their feet. But there must be some sight and smell identification taking place initially [among adults]. I’ve seen a monarch dive bomb from 50 feet in the air to a milkweed plant inches from the ground.”
Because of its beauty and world-class migration, the Eastern monarch is an “ambassador insect,” a gateway creature that can help generate human interest in wildlife, said Ms. Pocius.
Some well-intentioned gardeners plant commercially available milkweed in hopes of helping the butterflies while pollinating their flowers and vegetables. But unlike bees, she said, butterflies are not built to carry a lot of pollen and their use as a pollinator is limited.
“Some larger chains … offer tropical milkweed, not the native species,” she said. “A study in Georgia found that when the migrating butterflies encounter this plant, they like it so much that it disrupts their behavior. Instead of moving on they stay and just keep eating it.”
Some nurseries sell native milkweed species as seeds or small plants. Gardeners should ask the seller how the plants will spread.
“Common milkweed will spread through underground [connections],” she said. Swamp and butterfly milkweed do not spread that way.”
Redistributing milkweed seeds from Mexico to Canada would help monarchs to stabilize their waning migration cycle, said Ms. Pocius, but it would be a daunting and expensive project. Industries required to reseed following topsoil disruptions are not required to include milkweed or other wildlife foods.
The U.S. Department of Agriculture’s Conservation Reserve Program distributes seed to slow farmland erosion. Milkweed isn’t included in the mix, but users can petition for the seeds to be added.
“People want to help, but they don’t know how,” said Ms. Pocius. “A great way to start: Contact a master gardener in your area, Penn State Extension or Audubon Society chapters.”
Penn State Extension can be accessed at extension.psu.edu. The Audubon Society of Western Pennsylvania recently canceled a volunteer milkweed planting outing due to COVID-19 restrictions, but backyard conservationists can learn more about Audubon’s work with monarchs and milkweed at aswp.org.
The nonprofit group Monarch Watch (monarchwatch.org) and partner nurseries have distributed more than 528,000 free milkweed plants for large habitat restoration projects since the program began in 2015. On a seasonal basis, a flat of common milkweed (Asclepias syriaca) plants can be shipped to the Pittsburgh area for $74.