The count, done at 256 sites across the western United States from Nov. 11 through Dec. 3, 2023, shows about 233,400 western monarch butterflies stayed at sites along the California coast during the winter—about 102,000 fewer than the year before.
The 2023 population is only 5 percent of its size in the 1980s, according to the study published Jan. 30.
The dwindling monarch butterfly population is representative of declines of other at-risk butterflies, bees, moths, and beetles in North America, according to Isis Howard, an endangered species conservation biologist with the Xerces Society.
The big orange, yellow, and black butterflies are also a special symbol for people across the continent, she added.
“Monarchs have such a unique two-way migration, where they’re migrating hundreds to thousands of miles across many states, entering habitats and environments and communities that span urban, suburban, and rural areas,” Ms. Howard said. “It’s very rare to find someone who has not had an experience with monarchs, because they are so well-known and iconic.”
The 2023 tally is lower than 2022’s but similar to the 2021 count, according to the society.
The Xerces Society, an international nonprofit that protects and conserves invertebrates and their habitats, said it was difficult to predict how conditions during any single year influence the count, but the new count following the state’s severe winter storms in early 2023 revealed “the highest seasonal decrease on record.”
“This meant the population entered the spring breeding season with fewer butterflies,” Ms. Howard said.
Several factors influence how many butterflies show up at the winter sites each year, including temperature, rainfall, and the availability of milkweed and nectar resources, according to the group.
California’s central coast continues to host the state’s majority of monarchs during the winter, with about 178,000 butterflies reported in the latest count in San Luis Obispo and Santa Barbara counties combined. This was 76 percent of the winter count.
One butterfly spotted at the Pacific Grove Monarch Sanctuary in Monterey County had been tagged by a group in northern Utah, meaning it had traveled over 700 miles, according to Xerces.
Despite preservation efforts, the western monarch faces significant challenges, the group reported. The primary threats include the loss or destruction of its winter sites and breeding habitat, pesticides, and extreme weather conditions.
Recent changes include trees being removed at winter sites in Marin and Santa Barbara counties, leading to reduced numbers at both sites. These are locations within the coastal zone, meaning they are legally protected as sensitive habitat areas, according to the study.
The U.S. Fish and Wildlife Service’s proposed listing of monarchs under the federal Endangered Species Act is expected in the fall and could provide protection to the important places that the monarchs rely on each winter, the society said.
