Overwintering Monarch Butterfly Populations

Caitlin Dempsey

Updated:

Each year the Monarch butterfly (Danaus plexippus) migrates south to overwinter across sites located in Mexico and California. These geographic sites offer offer hospitable microclimates that allows the Monarch butterfly to conserve energy until spring.

The majority of the monarch butterflies that breed in the United States and Canada migrate to Mexico for the winter. Butterflies that overwinter in the forests found around the border of Michoacán and the State of Mexico migrate from the Great Lakes region of Canada and the United States. The smaller population of Monarch butterflies found west of the Rockies migrates towards several overwintering sites spread along the coast of California.

Concerned with radically reduced numbers compared to historical populations, researchers have been tracking Monarch butterfly overwintering populations in both Mexico and California. Widespread declines in Monarch butterfly population have been attributed to habitat loss due to agriculture and urbanization, use of a type of insecticide called neonicotinoids, the decline of milkweed (critical for their breeding), and the impacts of climate change.

Overwintering Monarch butterfly populations in Mexico

The latest Monarch butterfly overwintering survey, published on February 7, 2024 by the World Wildlife Fund (WWF) looked at monarch butterfly colonies in Mexico during their overwintering period in 2023-2024. Researchers censused overwintering butterflies at 13 colonies both within the Monarch Butterfly Biosphere Reserve (MBBR) and outside of the MBBR.

The size of the forest area occupied by the Monarch butterfly colonies was measured and benchmarked against records dating back to 1993. This year’s finding of only 0.9 ha area of monarch colonies underscores a troubling trend in the decrease of forest area occupied by monarch butterflies in Mexico, marking a significant reduction from the largest area recorded in 1996-97 of 18.19 ha and the second lowest after the smallest recorded area in 2013-14 of 0.67 ha. Compared to 2022 which measured 2.22 ha, the 2023 area was a 59.3% decrease.

Overwintering population fluctuations in California

A review of research on Monarch butterfly populations in North America was recently published on January 7, 2024 in the journal Insects. This review looked at fluctuations in overwintering Monarch butterfly populations in California. Conducted by an associate professor with the Department of Entomology at Washington State University, the review found that these overwintering population have fluctuated dramatically over the last decade.

Insecticide use, habitat destruction, harsh winter storms, and higher than average fall temperatures have all played a role in monarch butterfly populations in California. Population counts of monarchs overall have dropped from millions in the 1980s to less than 400,000 by the early 2000s. Between 2001 to 2017, overwintering monarch populations in California numbered under 300,000.

The biggest swings since 2001 have occurred since 2017. Harsh winter storms and bad spring weather in 2018 and 2019 resulted in fewer than 30,000 monarchs in California. In 2020, an unusually hot fall resulting in an alarming drop to only 1,899 butterflies that migrated to overwintering sites in California. Monarchs are normally reproductively diapause during migration. Instead, the warmer weather was driving monarchs passing through the San Francisco Bay Area to start laying areas in urban areas instead of continuing on to their overwintering sites.

Overwintering populations did start rebounding the following year: butterfly census counts found 247,246 overwintering monarchs in 2021 and 335,479 in 2022. 2023 monarch butterfly counts conducted by the Xerces Society’s Western Monarch Count, tallied at 233,394 butterflies across 256 overwintering sites in the western United States.

The collective body of research into overwintering Monarch butterfly populations points to an urgent need for conservation measures addressing the primary threats to monarch butterflies, including habitat destruction, climate change, and the use of herbicides affecting milkweed availability.

References

James, D. G. (2024). Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation. Insects15(1), 40.

Rendón-Salinas, E., Fernández-Islas, A., Cruz-Piña, M., Mondragón-Contreras, G., & Martínez-Pacheco, A. (n.d.). Area of Forest Occupied by the Colonies of Monarch Butterflies in Mexico, During the 2023-2024 Overwintering Period. Monarch Butterfly Survey Report 2023-2024 – World Wildlife Foundation. https://files.worldwildlife.org/wwfcmsprod/files/Publication/file/5wx7wlzzjm_Monarch_Butterfly_Survey_Report_Feb_7_2024_.pdf

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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