When Mia Monroe first began counting monarch butterflies in Marin County nearly three decades ago, she remembers seeing “tens of thousands of monarchs hanging—and when the sunshine hit them, it was like an orange cloud.” But in her time volunteering to count overwintering monarchs in West Marin, she has seen a precipitous decline in populations. 

Each year during the three-week period around Thanksgiving, volunteers traverse coastal California to count the butterflies as part of the Xerces Society’s Western Monarch Thanksgiving Count. The final data is released in January, but the Xerces Society—an environmental nonprofit focused on the conservation of invertebrates—has already reported disturbingly low numbers for 2018. 

Last year, results from 97 sites accounted for 148,000 monarchs: 77 percent of the total monarch overwintering population. This year, those same sites have only 20,456 monarchs, an 86 percent decline from 2017.

During this year’s count, volunteers in West Marin found no butterflies at Stinson Beach or Muir Beach, said Ms. Monroe, who is a Marin community liaison for the National Park Service. Last year, 210 butterflies were reported in Stinson Beach. In 1997, there were 15,500. 

In Bolinas, volunteers sighted roughly 1,600 monarchs this year: a shadow of last year’s 17,000. 

“We don’t know exactly when or how that decline started, but at least since the ’90s, things have not been doing very well,” said Emma Pelton, a conservation biologist at the Xerces Society. “In the 1980s, there were millions of butterflies—we estimate 4.5 million. By the 90s, things started to go up and down. Moving into 2000s and 2010s, now we are at about 3 percent.”

Researchers have attributed the decline to a number of factors: unseasonably warm weather, loss of habitat and an increased use of pesticides.

While insects do go through natural boom and bust cycles, the continuous decline of the monarchs points to a more troubling trend, Ms. Pelton said. 

“Now the problem is that normal ups and downs become really dire,” she said. This year “we think we’re going to have fewer than 30,000 butterflies counted. That, compared to 4.5 million, is a pretty scary situation where we could potentially lose the migration.”

Monarch butterflies, which typically live six to eight months, get their protective toxins from milkweed, where they also lay their eggs. Caterpillars only eat milkweed, but adult monarchs drink nectar from other flowering plants as well, like clover and goldenrod. 

When they die off, said Ms. Pelton, “there are other, larger impacts.” Caterpillars are a major food source for birds, and adults serve as pollinators in the ecosystems they travel through. “You could connect the decline of butterflies to grizzly bears and pollinating fruit,” she added. 

And without a migratory butterfly population, Ms. Pelton went on, there would be no way the monarchs could make it through the winter anywhere except in warmer states like Arizona and California. “We’d lose monarchs in the rest of the West, I think, if we didn’t have migrating monarchs from California,” she said. 

Ms. Monroe advised those concerned about the monarch population to plant nectar plants, consult with a biologist before trimming trees—both because monarchs can roost in trees and because even nearby trees can offer shelter by making an area warmer or less windy—and avoid using pesticides. 

“Some people think the monarchs are like the canary in the coal mine, a clue that something is awry in our world,” Ms. Monroe said. “Those are clues that something is out of balance: corporate agriculture, rising use of herbicide, loss of surface water, suburbanization, loss of habitat in the coastal areas. Those are the things I’m concerned about in general, and monarch butterflies are my little window into how it’s changing the world.”