Butterflies or Bust: The Debate Over Tropical Milkweed in California

By Kate Allhusen, Marine Biology ‘26

Monarch Butterfly on the non-native host plant, tropical milkweed (Image credit: David Levinson).

Known for its vibrant flowers and the charismatic creatures it supports, tropical milkweed (Asclepias curassavica) is becoming a popular plant in many California backyards. Despite its attraction to buyers and sellers alike, the California Department of Food and Agriculture (CDFA) classified it as a “noxious weed” in 2022, allowing county agriculture commissioners to ban its sale. Since then, several counties including Marin, Contra Costa, San Mateo, and Ventura have followed through on the ban, with some agricultural commissioners ordering for the removal and destruction of all tropical milkweed stock.

As more Californians have become interested to support and protect the monarch, tropical milkweed has emerged as a convenient yet controversial player.. Its rising popularity has been met with strong opposition from conservationist groups like the Xerces Society, who “advocated for the Pest Level B rating” which “allows county government to regulate the trade of a plant” Dr. Stephanie Frischie told FieldNotes.

Unlike its native cousins, tropical milkweed retains its foliage year-round rather than dying back during the winter, potentially exacerbating the effects of the parasite OE (Ophryocystis elektroscirrha). In eastern populations, high levels of OE in adult migration have been linked to lower migration success and a decrease in body mass, lifespan, mating success, and flight ability. Xerces also raised concern with the plant's interference in migration and reproduction, claiming that in northern areas, tropical milkweed “confuses” monarchs into breeding during periods when they should be migrating.

Not everyone agrees that the California tropical milkweed ban is a good idea. Opposed to the ban is Dr. Hugh Dingle, a former professor of entomology, behavior, and evolution at UC Davis who has spent nearly two decades researching monarch butterflies and their migration patterns. He claims the ban is a wasted effort and has next to no effect on monarchs. Instead, he argues that climate change has a much more direct hand in the changes in migration patterns. Another distinguished professor of evolution and ecology at UC Davis, Dr. Art Shapiro, agrees with Professor Dingle. In an email to the communications specialist for the University of California Agriculture and Natural Resources, Shapiro referred to the ban as “anti-curassavica propaganda,” deeming it “total hogwash.” The ongoing problem in monarch conservation, he points out, is attributed to the substantial decline in the butterflies population, not a lack of a host plant. “My hunch…” he told FieldNotes, “is that climatic warming has been disrupting the photoperiod-temperature interaction underlying monarch seasonal physiology and behavior. The very conspicuous decline of winter roosting and its accompanying reproductive dormancy says a lot.”

In the ongoing debate over tropical milkweed, the stakes are life and death for monarch butterfly conservation. While advocates for the ban emphasize the risks posed by year-round tropical milkweed, critics argue that these concerns are overstated and detract from addressing larger threats like climate change and habitat loss. As western Monarch conservationists grapple with the implications of the ban, the debate highlights a broader challenge in conservation: balancing scientific evidence with practical solutions that benefit both pollinators and the people who seek to protect them. For now, the future of tropical milkweed in California remains a contested space, reflecting the complexities of ecological stewardship in a rapidly changing world.