Marin bans sale of tropical milkweed to protect monarch butterflies

West End Nursery’s Elizabeth Blackstone, left, chats with customer Ann Brenner of Ross while Brenner selects a pair of native milkweed plants Tuesday, Aug. 30, 2022, at West End Nursery in San Rafael. (Sherry LaVars/Marin Independent Journal)
A monarch butterfly lands on a plant at the West End Nursery in San Rafael, Tuesday, Aug. 30, 2022.  Marin County prohibits local nurseries from selling tropical milkweed to protect the declining population of monarch butterflies.  (Sherry LaVars/Marin Independent Journal)
A monarch butterfly lands on a plant at the West End Nursery in San Rafael, Tuesday, Aug. 30, 2022. Marin County prohibits local nurseries from selling tropical milkweed to protect the declining population of monarch butterflies. (Sherry LaVars/Marin Independent Journal)

Marin County has become one of the first states in California to ban the sale of a weed that researchers say threatens the survival of the declining western monarch butterfly.

The decision by Marin Agriculture Commissioner Stefan Parnay this month to ban the sale of tropical milkweed, known as Asclepias curassavica, comes after the California Department of Agriculture recently classified the species as a noxious weed known to cause environmental or economic harm. The designation allows county agriculture commissioners to prohibit the sale or distribution of the plant.

“We want to thank the orphanage staff for helping us protect monarch butterflies,” Parnay said in a statement announcing the ban this month. “Their customers can play an important role by purchasing native milkweed and native nectar plants that will improve pollinator health. Planting native plants is the way to go.”

Marin joins Ventura County, which enacted a similar ban earlier this summer.

Milkweed is critical to the survival of monarch butterflies because it is the only plant on which they will lay their eggs during the long year-long migration from Baja California to British Columbia. Hatched caterpillars eat the leaves, which infuses them with the plant’s protective toxin.

However, tropical milkweed, native to the Caribbean, Mexico, and Central America, can carry a microscopic protozoan parasite that can affect the migration success and lifespan of monarch butterflies. Additionally, tropical milkweed does not die back in winter like native milkweed, which can confuse butterflies and cause them to reproduce when they need to migrate or hibernate at that time of year.

California Department of Agriculture spokesman Steve Lyle said tropical milkweed was not considered a pest until this year. Lyle said it was reclassified this year because “planting helps concentrate and spread a deadly bacterial disease of monarch butterflies.”

Marin Assistant Commissioner of Agriculture Scott Wise said that while the county has the authority to issue a ban, it does not want to impose an economic hardship on businesses if it can be helped.

“In this case, there are alternatives such as local milkweed. It’s kind of an easy transition,” Wise said. “They are all there for the butterfly at the point of sale as well. This is a charismatic insect. People love him. I love it.”

Some local nursery owners have said they support the ban. Dave Stoner, president of Sloat Garden Center, said starting next year it will stop selling tropical milkweed at all of its stores, including San Francisco and Contra Costa counties, not just its four Marin locations.

“We have no problem with that,” Stoner said. “We’re not going to hold it in other counties, regardless of their policies on that.”

Chris Untermann, owner of West End Nursery in San Rafael, said he stopped selling tropical milkweed before the ban.

“I think it’s a good thing,” Untermann said, “Everyone loves the Monarchs. We’re happy to do whatever we can to support it.”

As a native herb garden, O’Donnell’s Fairfax Nursery in Fairfax works to educate residents about the benefits of planting native and tropical milkweed. Bayley Elenzweig, nursery outreach manager, said this is the best time of year for residents to replace tropical milkweed in their yards with native milkweed.

A native milkweed plant sits on a table at West End Nursery in San Rafael, Tuesday, Aug. 30, 2022.  (Sherry LaVars/Marin Independent Journal)
A native milkweed plant sits on a table at West End Nursery in San Rafael, Tuesday, Aug. 30, 2022. (Sherry LaVars/Marin Independent Journal)

Native milkweed can take about one to two years to establish, but it is relatively easy to care for and has thrived with monarch butterflies.

“There’s kind of a false pretense that selling tropical milkweed helps monarchs,” Elenzweig said. “It comes from good intentions, but when you realize they’re not, it’s pretty easy to get people involved in restoring that ecosystem.”

Researchers have sounded the alarm about the collapse of monarch butterfly populations in recent years. Once estimated to number 4.5 million, the western population of the iconic orange-and-black-winged beetle has declined by a staggering 99% since the 1980s, according to the state.

Fewer than 2,000 monarch butterflies were counted in the west in the winter of 2020, according to the Xerces Invertebrate Conservation Society, which monitors the counts. Although the number of around 250,000 in 2021 is a sigh of relief, researchers say the population will continue to decline unless threats are addressed.

Pesticide use, degradation of overwintering habitats, loss of nectar and milkweed plants that the butterflies use for food and breeding, and climate change have been cited as reasons for the decline.

Only 141 monarch butterflies were observed in Marin during the winter of 2020-2021, down from the record 38,700 recorded in 2015. The count for 2021-2022 showed a slight improvement with 180 butterflies.

Several public land managers in Marin are more than a year into a project designed to enhance monarch butterfly habitat. The work includes studying arboretums in Bolinas, Stinson Beach, Muir Beach and Fort Baker, where monarch butterflies congregate and rest during the cold winter months. An inventory of the county’s wild milkweed is also being completed to find the best places to plant more.

Mia Monroe, Marin coordinator for the National Park Service, said residents are doing their part by planting more nectar and milkweed plants. He and other researchers reported seeing large numbers of monarchs breeding this spring and summer.

“People are planting the right milkweed in their yards and nectar plants,” Monroe said. “It restores habitat for monarchs and they find it. We have to wait and see if they will move to the coast and pass the winter.”

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