Numbers of monarch butterflies wintering on coast fall to new lows

Critically low numbers of monarch butterflies are coming to the California coast during the winter, according to an annual Thanksgiving count.|

The iconic monarch butterflies have some dedicated friends in Sonoma County. And it looks like they’ll need them.

Two million years or so ago, scientists say, the regally colored orange, black and white insect we call the monarch was a tropical butterfly. Gradually, it spread across North America, following abundant fields of its essential food, milkweed. But the northern climes also held deadly cold winters, so the fragile monarch adapted an unusual survival strategy: after feeding and breeding in northern fields all spring and summer, it beat wings south in vast numbers to safely wait out the winter in protected, warmer locales.

The butterfly now migrates distances of hundreds and even thousands of miles each year.

Historically, one western population of monarchs has unerringly headed to California from as far away as Idaho and Washington state to concentrate in select, sheltered spots along the coastline. A few dozen special groves from Mendocino to Baja have annually hosted up to many millions of traveling monarchs. Safe from freezing temperatures and fierce winter storms, the butterflies rouse again in early spring to begin their northward trek once more.

But not this year. According to the annual Thanksgiving monarch butterfly count, after years of steady decline, only 2,000 returned to California in 2020, a population decline of 99.99% from counts taken as recently as the 1980s.

Of the 12 historically active monarch overwintering sites in Sonoma County, none were occupied when volunteers checked them this winter.

“Everyone’s pretty heartbroken about the count,” Noelle Johnson said last week. Johnson is a conservation planner with Gold Ridge Resource Conservation District and program manager for its Sonoma Coast Monarch Overwintering Site Protection and Enhancement Project. She is one of a dedicated corps of local scientists, conservationists and volunteers working with the newly funded project to try and head off the butterfly’s extinction.

Few good groves

Finding and protecting monarch overwintering sites is one of the goals of the Gold Ridge district’s monarch project.

Most of the known overwintering sites in Sonoma County are on public land — in state parks, county regional parks or Wildlife Conservation Board-owned preserves. One site, a grove of eucalyptus trees at the northern edge of the Bodega Dunes Campground overlooking Bodega Bay, has been a reliable destination for butterfly watchers to find clusters of monarchs, until this year.

Despite the missing butterflies, there may be a silver lining, according to Mia Monroe, a Bay Area volunteer with Xerces Society, an international nonprofit focused on the conservation of invertebrates and their habitats.

“We may not know all the monarch winter sites; some may remain on private land,” she said.

In 2000, as a volunteer with Xerces, Monroe took over coordination of the annual Thanksgiving monarch count, and she continues in that role today.

“I go to sites, locate monarchs, count them, do field reconnaissance, lead volunteer training to conduct the counts,” she explained.

Xerces, along with several other organizations, first proposed in 2014 to the U.S. Fish and Wildlife Service that monarchs be added to the list of threatened and endangered species. In December, after studying monarch populations for several years, the department decided not to include monarchs on the list, saying there were higher priorities for listing. However, the department said the monarch could be added to the list in the future.

Monarchs under pressure

Why the monarchs have stopped returning is still an open question for scientists. But all agree on one thing: the hardy fliers are facing an unprecedented number of challenges, all at the same time.

According to Stu Weiss, chief scientist with Creekside Science, a Northern California consulting firm specializing in conservation, habitat restoration and endangered species studies, the complexity of monarch migration leaves them vulnerable.

“A combination of factors are pushing things over the edge,” he said. “Not just climate extremes, but toxic pesticides, loss of habitat, smoke from wildfires and especially the loss of native milkweed.” The milkweed plant is the monarch caterpillar’s only food.

And in recent decades, as tree groves have been logged or removed for construction and agriculture, fewer protected sites remain for overwintering monarchs.

Monarchs are very particular about the sites they choose to overwinter in, Weiss said. Their lives depend on it.

“The tree groves have to provide several essential elements. They have to be near a source of drinking water and very close to the sea, so they don’t freeze,” Weiss said. “The butterflies need wind shelter, so dense groves in sheltered ravines are usually preferred. And they need a well-lit spot — some direct sun and dappled light. What they really like to find is a medium-to-large gap in the middle of a grove.”

How the monarchs manage to find their way across the country, back to the same groves each year, remains a mystery. The feat is even more amazing, Weiss explained, because the butterflies that return aren’t the same ones that left the year before.

Migrating monarchs typically live only three to five weeks, although the monarchs that overwinter live much longer, between six to eight months. As the migrating monarchs move northward in spring and summer, they produce up to three or four new generations of caterpillars and butterflies. Only the last generation, triggered by some signal in the environment, makes the marathon flight all the way back to the coast to where their great-grandparents departed months before.

“Once they arrive at the coast,” Weiss said, “they find the safest groves by sampling, actually crowdsourcing. And they’re very attracted to each other, probably visually, so when they settle on a site, they concentrate in great numbers.”

As part of the Sonoma Coast monarch project, Weiss is conducting studies to identify unknown or potential overwintering sites along the coast, including on private land. He uses hemispherical (fisheye-type) photography and LIDAR, an airborne laser light technique capable of imaging the ground in very fine detail. With that data, he can use computer modeling to help narrow down the best possible locations.

“The Gold Ridge (Resource Conservation District) can then use this information for targeted outreach efforts to landowners, to ensure they are aware of the potential habitat value of these stands and potentially to survey the more promising stands with responsive landowners,” Johnson said. Money for the local monarch project comes from the state’s Wildlife Conservation Board.

Although the situation looks dire, Johnson, Weiss and Monroe remain positive. By employing new tools and knowledge, they hope to turn the tide, Weiss said. There is some cause for optimism: he noted that the population of monarchs east of the Rocky Mountains dipped catastrophically in 2011 and 2012, but counts have since increased, although number have declined overall since at least the 1980s.

The Gold Ridge district’s project is sharply focused on the critical key factors identified by researchers: protecting and restoring overwintering sites, providing nectar resources along the migratory flyway and increasing the availability of early-season native milkweed. Sonoma County residents are being encouraged to plant native milkweed inland, so the butterflies have nearby places to lay their first-generation of eggs. That first set of caterpillars and butterflies, scientists say, is key to the entire year’s population.

The Gold Ridge district also is seeking landowners to help protect and identify overwintering sites.

Whether their efforts ultimately succeed will also depend, Weiss said, on how we address the much larger issues working against the monarchs, including climate change and the use of pesticides. Increasing milkweed and improving overwintering sites is essential, but won’t be enough by themselves.

Monroe remains firmly optimistic. The butterflies are resilient, she said. And conservation efforts here have been successful before, rescuing the brown pelican, river otter and gray whale from catastrophically low numbers.

“People do point out monarch butterflies are not like whales or birds, and that’s true,” Monroe said.

“But there is one common thread to their recovery,” she added. “People took action.”

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at

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