Why some monarch butterflies are skipping migration
Monarch butterflies are icons of long-distance migration, drifting each autumn from inland western states to sheltered groves on California’s coast.
Yet some in the growing butterfly population now skip the journey, living and breeding year-round among neighborhood gardens.
New research from the University of California, Davis has found that these “stay-at-home” monarch butterflies represent a largely self-contained group – neither helping nor harming their declining migratory cousins.
Why some monarchs stay at home
Historically, western monarchs spent spring and summer in places such as Arizona, Nevada, and Idaho, then flew to coastal California for the winter.
Over the past decade, winter counts of those migrants have collapsed, sparking alarm among conservationists. At the same time, sightings of monarchs flitting through San Francisco Bay Area streets in January and February have multiplied.
One suspected driver is the popularity of tropical milkweed in urban yards. Unlike California’s native milkweed species, which die back in winter, the evergreen tropical variety offers continuous foliage and flowers.
The tropical milkweed is a tempting resource that can encourage butterflies to breed instead of migrate. Critics worry the practice fosters parasite outbreaks and disrupts the century-old monarch butterfly migration.
Success story or a disaster?
Elizabeth Crone is the study’s author and a professor in UC Davis’ Department of Evolution and Ecology.
“Although in many ways monarchs seem to be doing just fine in cities, there are a lot of concerns that these non-migratory populations build up high levels of parasites, even to the point that some counties have outlawed planting the non-native evergreen milkweeds that monarch butterflies use in winter,” said Professor Crone.
“We were curious about whether the establishment of these resident populations was a conservation success story or a conservation disaster.”
Monarch surveys in cities
To test the connection, Crone’s team walked fifteen three-mile routes across East Bay suburbs every month in 2022 and 2023.
They recorded milkweed plants – native and non-native – in yards and roadsides, counted monarch eggs, caterpillars, and adults, and briefly netted butterflies to check for a debilitating protozoan parasite.
Surveys showed milkweed abundant year-round, peaking in summer. Monarch butterfly activity matched an urban breeding cycle rather than seasonal migration.
Adults were most numerous in summer and scarcer in winter; heavily infected butterflies mirrored that pattern, with high parasite burdens in late summer and autumn but far fewer during the colder months.
Little overlap with migratory monarchs
“These resident monarchs in urban areas are almost unconnected from the migratory population, so helping resident monarchs likely doesn’t help or hurt the migratory population,” said lead author Emily Erickson. She completed the work while a postdoctoral fellow in Crone’s lab.
Because the two groups show minimal exchange, local bans on tropical milkweed may be misplaced, the researchers argue.
“I think it’s reasonable to focus on planting native milkweeds, but ripping out all of the milkweeds in urban gardens in the Bay Area or in Los Angeles does not need to be the immediate focus for monarch conservation,” Erickson added.
Urban gardens help all
The study suggests resident monarchs bring other advantages. “Having people see monarch butterflies every day makes them want to invest in the conservation of butterflies, and in conservation in general,” Erickson said.
“And no matter what milkweed you’re planting, native or non-native, it supports a lot of things beyond monarchs.”
Crone likewise noted that urban habitat can complement broader recovery strategies. “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,” she said.
“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.”
Tech joins butterfly research
Funding for the project came from Google, which has supported monarch habitat initiatives since 2021. “We are really grateful to Google,” Crone said.
“It was a unique opportunity – it’s the first time in my 28 years of being a professor that a corporation has reached out to us and said that they’re interested in butterfly conservation and asked how they can help.”
While the Bay Area’s resident butterflies won’t by themselves rescue the declining migratory population, they illustrate how adaptable some wildlife can be in city landscapes.
Urban gardens and migration
The authors emphasize that large-scale monarch recovery will still depend on restoring native milkweed and nectar sources across inland breeding grounds and coastal overwintering sites. Yet urban gardens can play a complementary role, sustaining local biodiversity and fostering public engagement.
Thus, while some people see tropical milkweed and resident monarchs as villains, data shows they’re not the cause of migratory decline.
Instead, they offer an unexpected bright spot: proof that cities, when planted thoughtfully, can support thriving butterfly populations. This happens without undermining the grand monarch migration scientists are working so hard to save.
The study is published in the journal Ecosphere.
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