There is sobering news emanating from monarch butterfly overwintering sites in coastal California. Early data from the 2020 Western Monarch Thanksgiving Count, including numbers from some of the largest sites, suggest the western migratory population of monarchs is heading toward an all-time low. With about 30% of survey results in, a mere 1,224 butterflies have been reported. Extrapolating to the remaining survey sites, there may be fewer than 10,000 monarchs overwintering in California this year. This is a sizeable decline from the low numbers counted in 2018 and 2019, which hovered at ~30,000 monarchs. But even a few tens of thousands of monarchs is a shadow of the estimated 4 million that overwintered in California in the 1980s, or the hundreds of thousands that clustered at coastal sites as recently as 2016.
The low Thanksgiving Count was not unexpected. Reports from around the west reiterated a poor breeding season and overall scarcity of monarchs. Washington State, which reported hundreds of observations as recently as 2017, reported only 2 observations in 2020. Likewise, here in western Montana, volunteers monitored for monarchs throughout the summer, finding only a few adult butterflies and their progeny.
So why was 2020 such a poor year for monarchs? Scott Black, executive director of the Xerces Society for Invertebrate Conservation, thinks the western population may be suffering from “death by a thousand cuts.”
- Many monarch overwintering sites are in poor ecological condition, in need of urgent restoration and management. Still other sites are on private lands vulnerable to development.
- A study conducted by the Xerces Society and University of Nevada-Reno found pesticides were ubiquitous in California milkweed plants used by the first breeding generation of monarchs, suggesting that pesticide exposure could be a contributing factor to western monarch declines.
- Severe wildfires in the western U.S. may have influenced migration or late-season breeding, though impacts are largely unstudied.
- Excessively high temperatures may have impacted monarch survival. Exposure to temperatures >84.2° F can be detrimental to monarch caterpillar development, and temperatures ≥91.4° F can be lethal to all monarch life stages. I suspect that a week of 95-100°F temperatures caused the death of a cohort of monarch caterpillars I monitored in NW Montana.
- Drought conditions can limit the availability of nectar resources needed by monarchs during migration and at overwintering sites.
- Reports of unusually high numbers of captive-reared and non-migrating monarchs near the coast suggest a disproportionate number of “domesticated” vs. wild monarchs, which can result in negative impacts to wild monarch populations through disease transmission, reduced genetic diversity, and confounding the ability of monitoring programs to understand natural population dynamics.
This troubling news follows a recent Sacramento Superior Court decision that insects are not eligible for listing under the California Endangered Species Act, thereby limiting options for state protection of at-risk pollinators such as monarchs and bumble bees. Federal protection for North American monarchs (both western and eastern populations) is currently under consideration by the U.S. Fish and Wildlife Service and a decision is anticipated later this month. Protection at both state and federal levels is imperative if western monarchs are to avoid a 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.
~ Emma Pelton, Endangered Species Conservation Biologist, Western Monarch Lead, Xerces Society
FOR 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