"Tropical milkweed is not the first thing that we need to worry about in monarch butterfly conservation.”
So says UC Davis professor and monarch scientist Elizabeth Crone of the Department of Evolution and Ecology.
Crone is the senior author of the newly published “Neither Source Nor Trap: Urban Gardens as Habitat for Nonmigratory Monarch Butterflies in Northern California, “ appearing in the journal Ecosphere, an open-access journal of the Ecological Society of America
Crone and her co-authors, Emily Erickson of UC Davis and Cheryl Schultz of Washington State University, researched non-migratory, winter-breeding populations of monarch butterflies established in urban gardens in Northern California and studied “whether the winter-breeding monarch butterfly population was primarily supported by the larger migratory one using monthly surveys of monarchs and milkweeds throughout urban gardens in the California East Bay.”
“If the winter-breeding population were a trap,” she and her colleagues wrote in their abstract, “we expected increases in abundance and decreases in parasite prevalence to be timed with monarch migration into our study area. Demographic patterns of winter-breeding monarchs were not consistent with an influx from the migratory population. Population size was highest during summer months, when milkweed density was most abundant, not during monarch migration. Parasite loads were consistently high but increased during fall migration, in direct opposition to our prediction. During summer, monarch butterfly larva:egg ratios were lower than in other months, possibly due to predation by synanthropic species such as wasps, but predation did not prevent population growth. These demographic patterns contrast with recent studies of monarch butterflies in eastern North America. They also illustrate the importance of understanding mechanistically how species persist in urban environments and the potential of urban communities to function in novel ways as opposed to replicating natural habitat.”
The Ecosphere paper is drawing widespread interest, particularly sincer freelance science writer Liana Wait, who holds a doctorate in biology, posted a news story on the UC Davis College of Biological Sciences website. The headline: "City-Dwelling Monarch Butterflies Stay Put: New Study Says Non-Native Milkweeds Don’t Help or Harm Migratory Monarch Butterflies.”
“Some people believe that these resident populations are a major cause of monarch butterfly populations declining, but our research suggests it's a bit of a red herring,” Crone told Wait. “Our results highlight the potential for urban ecosystems to contribute to the conservation of some species. In the West, resident monarchs can persist in urban gardens without impacting fluctuations in migratory monarchs.”
Controversy Over Tropical MIlkweed
Milkweed, genus Asclepias, is the host plant of the monarch, Danaus plexippus, which lays its eggs only on milkweed. Tropical milkweed, native from South America to Mexico, can survive the winter in some climates. Some scientists and conservation groups believe that the presence of tropical milkweed, Asclepias curassavica, in winter interferes with monarch migration and reproduction and leads to population decline. They point to the build-up of the parasite, Ophryocystis elektroscirrha (OE) on the tropical milkweed. Some seek a total ban and eradication of all tropical milkweed in the United States. In 2022, at the urging of conservation groups, the California Department of Food and Agriculture declared the tropical milkweed a “noxious weed," resulting in several counties--including Marin, Contra Costa, San Mateo and Ventura-- banning nursery sales of the plant.
So today we asked Professor Crone for her take on tropical milkweed, which has thrived in California for more than 100 years. It is also a food source for a number of other insects.
“Tropical milkweed is not the first thing that we need to worry about in monarch butterfly conservation,” Professor Crone told Bug Squad. “It has benefits and possibly costs and we don't know how they balance out. One the other hand, we know that it is good to focus on conservation of overwintering sites and inland breeding habitat. For home gardeners, we also know that it is good to plant native milkweeds, plant diverse nectar plants that flower throughout the growing season, and to avoid or minimize use of all kinds of pesticides (insecticides, fungicides, and herbicides). These are the things we should focus on if we want to help monarch butterflies."
“Also, I personally do not believe we should outlaw sale of tropical milkweed. I think it is great to have monarch butterflies in coastal towns and cities, and our research shows that this does not seem to harm the migratory monarch population in the west. It would be great if nurseries could label milkweeds in three categories: Native to California, Native to the USA but not California, and Not Native to the USA. That kind of regulation might be hard to implement, but it is the kind of law I would advocate.”
On our Oct. 10, 2022 Bug Squad blog, UC Davis professor emeritus Hugh Dingle, an international expert on migration including that of monarchs, said that "Migration and the diapause that accompanies it in the fall are determined by shortening photoperiod and temperature (warm temps can override short days hence the issue with climate change). There is no significant influence of food plant. There is not enough tropical milkweed planted to have much influence. Yes, there are parasites on A. curassavica as there are on ALL milkweeds."
UC Davis distinguished professor emeritus Art Shapiro agrees. He has monitored the populations of some 115 butterly species in central and northern California since 1972. His data set, known as the largest butterfly monitoring data set in North America and one of the largest in the world, runs parallel to Interstate 80, with 10 sites distributed from the inner Coast Range, across the Sacramento Valley and the Sierra Nevada, to the western edge of the Great Basin. He maintains a research website at https://butterfly.ucdavis.edu/
"The anti-curassavica propaganda is total hogwash,” Shapiro commented in the Oct. 10, 2022 Bug Squa blog. “I have been saying so for years. Much of the conventional wisdom circulating is hogwash."
Dingle, 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, Shapiro and other researchers say that winter breeding among monarch butterflies is a relatively new behavior and one influenced by warmer winter temperatures caused by climate change.
Ecosphere Discussion
"On the whole, our data are not consistent with the notion that urban gardens have a negative impact on monarch populations in Northern California," Professor Cone and her co-authors wrote in the discussion setion of their paper in Ecosphere. Although continuous breeding behavior is undoubtedly supported through the winter months by evergreen non-native A. curassavica and Gomphocarpus spp. monarchs use both native and non-native milkweeds when both are available. Similarly, the patterns we observed in this study are not consistent with a large influx of migratory monarchs into the winter-breeding population in urban gardens. Although studies in the Gulf states show that the winter-breeding population grows in fall with input from the migratory population (Satterfield et al., 2018), we found that counts of adults in Northern California urban gardens declined during fall migration, were lowest during spring dispersal, and grew throughout the summer. Counts of eggs increased roughly in tandem with the abundance of adults during summer and fall Finally, in 2023, lower monarch butterfly densities were associated with lower OE infection, consistent with expected dynamics in an independent population (Majewska et al., 2022). These data support the hypothesis that the Northern California winter-breeding population expands through reproduction in warmer months, as opposed to being sustained by an annual influx from the migratory population. During summer, monarch butterfly larva:egg ratios were lower than in other months, possibly due to predation by synanthropic species such as wasps, but predation did not prevent population growth. These demographic patterns contrast with recent studies of monarch butterflies in eastern North America. They also illustrate the importance of understanding mechanistically how species persist in urban environments and the potential of urban communities to function in novel ways as opposed to replicating natural habitat."
Statistics show that the monarch population has declined by more than 80 percent since the 1990s from central Mexico, and by more than 95 percent since the 1980s in coastal California. The Xerces Society for Invertebrate Conservation reported only 9,119 butterflies overwintering in 2024 in coastal California, " a sharp decline from the past three years, when more than 200,000 overwintering western monarchs were observed each year. (See Xerces website)
Monarchs certainly aren't the only species of butterflies declining.
Shapiro teamed with 32 other colleagues to produce a major report, “Rapid Butterfly Declines Across the United States During the 21stt Century,” published in the journal Science in March 2025. They found that the nation's butterfly population has declined 20 percent over the last 23 years. “Altogether, we analyzed data on some 12.6 million individual butterflies extracted from roughly 77,000 surveys distributed over 35 monitoring projects, ours being one of the largest if not the largest,” Shapiro recently told the Vacaville Museum Guild. “Of 342 species tracked, 245 declined, 32 appeared stable, and 65 increased."
