The number of monarch butterflies in California plummeted to a historic low last year, threatening the state's overall population of the orange and black butterflies, according to environmental experts.

There were an estimated 20,500 western monarchs in 2018, an 86 percent drop from the nearly 148,000 spotted the previous year, according to an annual census conducted by the Xerces Society for Invertebrate Conservation, The New York Times reported Wednesday.

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The western monarch migrates to California each winter, and volunteers for the conservation group count its population in the state each November.

Biologist Emma Pelton, who oversees the annual count, told the Times that the 2018 count is “potentially catastrophic” when combined with the 97 percent drop in the overall monarch population since the 1980s.

“We think this is a huge wake-up call,” Pelton said, adding that the butterflies are impacted by ecological changes and often serve as a warning sign for the overall health of an ecosystem.

Harsh winters and droughts in California from 2011 to 2017 have threatened the butterflies, the Times reported. The state also experienced the deadliest wildfire season in recorded history in 2018, causing widespread smoke damage and poor air quality.

Pesticides, climate change and habitat loss are also affecting the western monarch population, Xerces said in a statement.

If nothing is done, the monarch butterflies could face extinction, Pelton warned.

“We don’t think it is too late to act,” she said. “But everyone needs to step up their effort.”

Pelton encouraged gardeners to plant milkweed, which the butterflies need for breeding and migration. She said she also hopes towns will plant more trees.