The Number of Monarch Butterflies in Western North America Is Plummeting

Millions of monarch butterflies once inhabited the West Coast, but the number may now be just 300,000, highlighting the need for Endangered Species Act protections.

Up until the 1980s, around 10 million monarch butterflies spent their winters in coastal California. Their arrival in places like Pacific Grove, aka Butterfly Town USA, was admired by locals and visitors eager to witness these beautiful, delicate insects and their distinctive orange wings.

While Butterfly Town USA still has its renowned monarch butterfly sanctuary, this and other places across western North America have seen numbers of the pollinating insects plummet. 

“Today there are barely 300,000,” said Cheryl Schultz, an associate professor of Washington State University, Vancouver.

She and her colleagues arrived at that estimate after combining data from hundreds of volunteers who have participated in the Xerces Society for Invertebrate Conservation’s Western Monarch Thanksgiving Count since 1997, with earlier monarch counts conducted by amateur and professional butterfly enthusiasts in the 1980s and 1990s. The findings are published in the journal Biological Conservation.

Western monarchs form a distinct population segment from eastern monarchs, much like the relationship between different runs of salmon in the Pacific Northwest. Western monarchs intermix with eastern monarchs from time to time, but are not a different species or subspecies.

Like eastern monarchs, which overwinter in Mexico, western monarchs have an impressive and lengthy migration. They spend the winter months in forested groves along coastal California before fanning out in the spring to lay their eggs on milkweed and consume nectar from flowers in Arizona, California, Nevada, Oregon, Washington, Idaho, and Utah. They then return to their coastal overwintering sites in the fall.

In a second paper, Sarina Jepsen, endangered species program director at Xerces, and her team list the top 25 highest priority overwintering sites for conservation.

“Among the sites with the most severe population drops are Lighthouse Field and Natural Bridges, which are both in Santa Cruz,” Jepsen said. “However, Santa Cruz is in the core of the overwintering range, and as the population has gotten smaller, many of the peripheral sites in the San Diego area and north of San Francisco no longer host monarchs in the winter.”

Schultz’s colleague Elizabeth Crone, a professor of biology at Tufts University, said some conservation biologists mistakenly assume monarch butterflies are in decline primarily because the insects cannot find enough places to lay their eggs. Instead, research indicates multiple factors could be causing the decline. Drought severity may be one, along with loss of milkweed-rich grasslands throughout the West, and particularly in California’s Central Valley.

“We also know that the use of many insecticides and herbicides have increased dramatically in the past couple of decades in some parts of the West that monarchs use,” Jepsen said. “We have documented the loss of dozens of monarch overwintering sites along coastal California, as housing developments replace forested habitat, and dozens more sites have been degraded.”

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Some environmentalists suspect that increased use of glyphosate and neonicotinoid chemicals in crop fields hurt butterflies, along with other pesticides and herbicides. The use of chemicals, however, can actually come back to hurt farmers, since butterflies are pollinators of many plants.

“Monarchs, in particular, are toxic to birds, which allows other, non-toxic butterflies to survive because they look like monarchs and therefore don’t get eaten,” Crone explained.

She added that artists and engineers have observed monarchs for inspiration. Navigation systems, for example, have even been modeled upon butterfly flight.

The insect’s beauty, however, is what tends to captivate us the most.

“So many people spent their childhoods watching monarch caterpillars transform into a chrysalis, then emerge as butterflies,” Jepsen said. “The story of their migration is amazing.”

Given the new data, all of the researchers now believe that western monarch butterflies should be listed as a threatened species under the Endangered Species Act. Crone explained that the listing would help to motivate agencies and landowners to participate in conservation practices.

“Listing would also substantially raise awareness of the plight of monarchs and similar species,” she continued. “For example, many other butterfly species also seem to be less abundant now than 30 years ago.”

The researchers said homeowners can take part in the butterfly conservation effort too. They are encouraged to plant native, locally sourced milkweed plants. Xerces has a milkweed seed finder to help locate vendors. Homeowners should also include other native plants and monarch-attracting wildflowers in their gardens.

“Some common garden flowers, like petunias and geraniums, don’t produce nectar,” Crone explained. “Others, like pansies, provide nectar and are used by native insects.”

The scientists also urge homeowners to minimize “homogenous” land surfaces, like pavement and grass-only lawns, and to support organic and other wildlife-friendly gardening and farming practices.

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At present, Schultz and her team are monitoring where and when monarchs breed across the West. They are working across five states: California, Nevada, Idaho, Oregon, and Washington. They are also involved in a joint five-year project evaluating the importance of species interactions for population viability of rare butterflies, and how these interactions might be impacted by climate change.

The scientists are in talks with scientists in Mexico about collaborating on the larger eastern monarch population that overwinters in Mexico.

The researchers encourage the public to participate in projects in North America, in addition to the Western Monarch Thanksgiving Count, aimed at collecting data about butterfly populations. These projects include the Western Monarch Milkweed Mapper, Project Monarch Health, Journey North, and the Monarch Larva Monitoring Project.

Crone resists the notion that monitoring the decline of butterfly populations is depressing and instead emphasizes researchers’ work at helping species rebound from the threat of extinction.

“We have been successful in helping other species, like the Fender’s blue butterfly, toward recovery in the past,” she said. “If we start now, we are optimistic that monarchs can be restored and maintained in the West, and throughout the US”

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