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Monarch Population in California Spirals to Another Record Low

By Emma Pelton on 30. November 2020
Emma Pelton
[Editor's note: This blog was revised on Friday, 12/11/20, to update the population estimates with new count data.]

Early count numbers from Xerces’ Western Monarch Thanksgiving Count suggest that the western migratory population is headed for an all-time low. With approximately 95% of the data in, only 1,800 monarchs have been reported. We anticipate a final total of less than 2,000 monarchs overwintering in California this year. This is a significant decline from the low numbers of the last two years where the total hovered just under 30,000 monarchs. These numbers are a tiny fraction of the millions of monarchs that likely visited overwintering sites in the 1980s and the hundreds of thousands of monarchs that graced California’s coast as recently as the mid-2010s. In fact, this represents an overall decline of more than 99.9% in the migratory population.


"Small clusters of monarch at an overwintering site"
Small clusters of monarchs at an overwintering site in Ventura County. (Photo: Lara Drizd, US Fish and Wildlife Service.)


The poor showing at the overwintering sites was somewhat predicted by an anemic breeding season where monarchs were scarce in much of the West outside of California and Utah. (Note that Utah likely contains eastern and western monarch migrants). 

Monarchs are not the only butterflies having a poor year in California. Art Shapiro, a professor at the University of California at Davis who has been monitoring butterflies across central California for over 40 years, commented by email that numbers were low across most butterfly species at his low elevation transects in 2020. 


"Thanksgiving count regional coordinator Charis van der Heide and colleagues practicing socially distanced monitoring"
Thanksgiving count regional coordinator Charis van der Heide and colleagues practicing socially distanced monitoring. (Photo: Charis van der Heide.)


Why is this such a bad year? 

Scott Black, executive director of the Xerces Society, noted that monarch populations may be suffering from “death by a thousand cuts.” Destruction, degradation and neglect of overwintering sites, climate change, and overuse of insecticides are likely contributing factors. 

Protecting monarch overwintering sites is paramount. Many are still subject to development on private lands and many sites on state lands are in urgent need of restoration and management.


"Sparse clusters of monarchs in fall 2020 at Pismo State Beach, usually the largest overwintering site in California."
Sparse clusters of monarchs in fall 2020 at Pismo State Beach, usually the largest overwintering site in California. (Photo: Sarah Sindel, California State Parks.)


study by the Xerces Society and the University of Nevada, Reno found that milkweed plants in California (monarchs only larval food) contained pesticides at all sites sampled, which included conventional and organic agriculture, wildlife refuges, and urban areas. Many of the insecticides found in milkweeds can harm or kill larval monarchs. 

Other issues may also be impacting monarchs. The impact of severe wildfires is largely unstudied, but may have influenced migration or late season breeding. Temperatures may play a role as they were warmer in California during much of the late summer and early fall, but November temperatures have been average. Below average rainfall could possibly limit the availability of nectar during migration and at overwintering sites. An unusually high number of reports of late breeding and captive rearing near the coast—primarily on tropical milkweed (Asclepias curassavica)—may also suggest there are higher numbers of non-migrating monarchs than usual which can negatively impact the population. 

The most striking thing about the dwindling number of monarchs is that this is closely following what models predicted. A 2017 study by Cheryl Schultz, a professor at Washington State University, and coauthors, predicted if the population dropped below 30,000 monarchs—which it did in 2018 and 20019—there was a high probability that it would spiral down further. Unfortunately, we seem to be witnessing that prediction come true.

The western monarch’s bad news comes on the heels of a recent court decision that the State of California does not have the legal authority to protect insects under the California Endangered Species Act, limiting the options for state protection of monarchs. Federal protection for monarchs under the Endangered Species Act is currently under consideration by the U.S. Fish and Wildlife Service and a decision is expected in mid-December. Protection under state or federal law is desperately needed if we hope to avoid total population collapse.

We must hold out hope that we can still recover monarchs in the west. But we also must step up to truly protect the monarch butterfly, its overwintering sites, and breeding areas if that hope is to become reality.   


Cluster at Pismo State Beach in 2016. Large, dense clusters of monarchs were not an uncommon sight even just a few years ago. (Photo: Brian Baer, California State Parks.) 


Western monarchs need everyone’s help today more than ever. Check out our Western Monarch Call to Action and our This Is How You Can Help handout to learn more about how to support western monarchs during this crucial time. These actions will benefit monarchs and also support our many other native bees and butterflies. 

Our thanks go to our many volunteers who make the count possible. 


Further Reading

Find general information about monarch conservation

Read the Western Monarch Call to Action

Learn about the Western Monarch Thanksgiving Count

Download detailed guidance on managing monarch overwintering sites




As the Xerces Society's western monarch lead, Emma works on the western population of monarch butterflies, including adaptive management of overwintering habitat in California and breeding habitat throughout the western U.S. Emma completed a master's degree in agroecology and entomology at the University of Wisconsin–Madison where her research focused on landscape ecology and an invasive fly that affects fruit crops.

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