Some monarch butterfly researchers are questioning Marin County and other counties’ recent decision to ban tropical milkweed sales as a way to protect the state’s dwindling populations of the iconic orange-and-black insects.
Marin — along with Contra Costa, San Mateo and Ventura counties — prohibited nurseries from selling a species of tropical milkweed known as Asclepias curassavica after the California Department of Agriculture recategorized it as a noxious weed. The “B” classification by the state allows county agricultural commissioners to enact bans on the sale of the plant. Ventura County had initially requested the state to reclassify the plant, according to California Department of Agriculture spokesperson Jay Van Rein.
Milkweed plants are vital for monarch butterflies, which lay their eggs exclusively on the plants. The hatched caterpillars eat the milkweed, imbuing them with the plant’s protective toxins.
The rationale for Marin County’s ban, endorsed by the Xerces Society for Invertebrate Conservation nonprofit group, is that tropical milkweed species do not die off during the winter months as native milkweed species typically do. As a result, the butterflies can be exposed to harmful microscopic protozoan parasites that can kill monarch caterpillars and butterflies, which have had steep population declines in the past few decades.
Tropical milkweed also does not die back during the winter as native milkweed typically does. The Xerces Society and the Marin agriculture commissioner’s office said this works to both prolong exposure to parasites and encourages butterflies to breed during a time when they should be migrating or roosting in their overwintering sites.
Hugh Dingle, a retired University of California at Davis entomology professor who has studied monarch butterfly migration for more than two decades, said the bans are “basically a wasted effort” and that the focus should be on larger threats such as pesticide and herbicide use. All species of milkweed carry parasites that can affect monarch populations, Dingle said.
“Planting milkweed of any kind in your garden — tropical or native — there’s unlikely to be enough of it to do any good, but it’s not going to do any harm,” Dingle said. “If you like monarchs in your garden, go ahead and plant it. I didn’t want the information on tropical milkweed to cause the ladies of Marin to go running out to their gardens and dig out all of their tropical milkweed because it’s supposedly bad for butterflies. It’s not.”
Arthur Shapiro, a UC Davis professor who has studied monarch butterflies for the past six decades, described the rationale behind the bans as “hogwash.”
Migrating monarch butterflies typically overwinter near the California coastline for several months, during which they stop breeding in a process known as reproductive diapause. Shapiro, Dingle and other researchers said winter breeding among monarch butterflies is a relatively new behavior and one influenced by warmer winter temperatures caused by climate change.
Curassavica has been widely planted in coastal southern California for a century, but winter breeding only began about a decade ago. Why?” Dingle wrote in an email. “The experiments need to be done to test the hypothesis that reproductive diapause is being prevented by climate change, specifically the increasingly dissonant seasonal information given by the daylength cycle, which is not changing, versus temperature, which is.”
David James, an associate entomology professor at Washington State University who has studied monarch butterfly breeding and migration in the Bay Area, said there is a case to be made about the tropical milkweed as being a vital resource for the monarchs in a changing climate.
“The Monarch is adapting to our warming climate and in doing so a proportion of the population is now spending winters breeding in urban near-coastal California,” James wrote in an email. “And what host plants are there? Some limited native milkweeds but a whole bunch of tropical milkweed! Taking tropical milkweed away will just make it harder for the population to survive.”
Leslie McGinnis, a UC Berkeley doctoral candidate studying monarch populations and working with gardeners in the East Bay, said the bans take a “simplistic view” of the threats that monarchs face, including the fact that many native milkweed plants supplied to nurseries can also be sprayed with pesticides. The bans, she said, can work to disenfranchise or demonize people who have tropical milkweed who instead could be partners in working to help restore monarch populations.
“This energy could be put into something that could benefit other butterfly species as well by working with nurseries for all of these host plants for butterflies to make sure they’re actually safe for the caterpillars to eat,” McGinnis said.
Once estimated to number up to 4.5 million, the western population of monarch butterflies has fallen by a staggering 99% since the 1980s, according to the state. In the winter of 2020, fewer than 2,000 monarch butterflies were counted in the west, according to the Xerces Society, which oversees the counts. While the 2021 count of nearly 250,000 brought some relief, researchers say the declining trend of the population will likely continue unless threats such as habitat loss, pesticide use and other factors are addressed.
Stephanie Frischie, an agronomist and native plant specialist with the Xerces Society, said there are many purported causes for the decline in monarch butterfly populations. The buildup of pathogens on the tropical milkweed during the winter months is one of these pressures, which can be prevented by having residents plant the readily available native milkweed.
“There is really no single cause and therefore there is no silver bullet to solving monarch decline,” Frischie said. “Where we are right now with current research, there are concerns with tropical milkweed with disease and interrupting migratory behavior. In general, as a conservation organization, we support native plants in their native ranges as habitat for invertebrates.
Mia Monroe, the Marine liaison for the National Park Service who leads local monarch counts, said she has seen first-hand the impacts that pathogens from tropical milkweed have had on monarch caterpillars and butterflies. While she said she understands that researchers may find the bans as a token way to address the larger threats behind the decline of monarch butterflies, she said residents are looking to play their role.
“If we know that tropical milkweed is implicated in the ways that damage the individual butterfly plus interfere with the bigger cycles of migration and overwintering, that is one way I can be helpful is by buying the right plant and putting it in the right place, she said.
Marin County Agricultural Commissioner Stefan Parnay said the goal of the ban is to protect monarch populations and their habitat. The ban does not require residents to pull out tropical milkweed they have already planted, but Parnay said residents are being asked to significantly prune the plants during the winter months.
“I made a decision at the time that I think was the right decision based on the information that I had,” Parnay said. “If information changes and something needs to be adjusted, I would look at that again.”
“We’re doing this because we’re trying to protect a species that is potentially threatened,” he said.
Agricultural commissioners in Sonoma and Napa counties, which have not enacted a ban, said they are still monitoring information.
“We’re interested in learning about and understanding the impact of tropical milkweed on monarch butterfly populations based on information from researchers and experts,” Napa County Agricultural Commissioner Tracy Cleveland said.