Monarch Butterfly

North America's Most Iconic Migrating Insect, in Decline

The monarch butterfly is the only insect on the planet that undertakes a multi-generational migration across an entire continent. The eastern North American population travels up to 4,800 km each autumn from southern Canada and the northern United States to overwintering grounds in central Mexico's Sierra Madre forests; the western population overwinters along the California coast [Brower 1995; Oberhauser et al. 2017]. In 2022 the IUCN listed the migratory subspecies (D. p. plexippus) as Endangered, citing a multi-decade collapse in overwintering numbers [Walker et al. 2022]. The eastern overwintering colony covered an estimated 21 hectares of fir forest in the mid-1990s; the 2023–24 winter survey measured just 2.21 hectares, a 95% decline [WWF Mexico / CONANP 2024].

Biology and identification

Danaus plexippus adults are 9–10 cm across the wings, with the distinctive orange-and-black pattern interrupted by white-spotted black borders [Oberhauser et al. 2017]. Males are distinguished from females by a small black pheromone-scent patch on each hindwing.

The monarch's life cycle is the key to understanding its conservation: an individual butterfly does not complete the round-trip migration. The autumn generation that flies to Mexico lives 8–9 months in a state of reproductive diapause; on the return flight north in spring, it lays eggs in the southern United States and dies. The 2nd, 3rd, and 4th summer generations live only 2–6 weeks each, breeding rapidly northward, before the next overwintering ("Methuselah") generation begins the cycle again [Brower 1995].

Caterpillars are obligate specialists on milkweed (Asclepias spp.). Milkweed produces cardiac glycoside toxins that monarch larvae sequester and use as predator defense throughout adulthood — the famous "warning coloration" pattern protects monarchs from most birds [Brower 1969]. Loss of milkweed = no monarchs.

Habitat and range

The monarch occupies most of North America during the breeding season. Two distinct migratory populations: the eastern population (the much larger of the two) overwinters in oyamel fir (Abies religiosa) forest in central Mexico's Sierra Madre Oriental, principally within the Monarch Butterfly Biosphere Reserve in Michoacán and the State of Mexico — a UNESCO World Heritage Site since 2008 [UNESCO 2008]. The western population overwinters in eucalyptus, Monterey pine, and Monterey cypress groves along ~500 km of the California coast.

A separate non-migratory subspecies, D. p. megalippe, occupies Central and South America and Caribbean islands and is not currently considered at population-level risk.

Conservation status

The IUCN assessed the migratory monarch (D. p. plexippus) as Endangered in July 2022 [Walker et al. 2022], finding that the overwintering population had declined 22–72% per decade depending on monitoring methodology. CITES does not list the species; it is not on the U.S. Endangered Species Act list as of writing, though the U.S. Fish and Wildlife Service determined in December 2024 that ESA listing as Threatened is warranted, with a formal listing process underway [USFWS 2024].

Annual overwintering colony surveys are the principal population metric. Mexico's CONANP and WWF Mexico jointly publish the eastern colony measurement each winter; the 2023–24 measurement of 2.21 hectares is the second-lowest on record (after 2013–14's 0.67 ha) [WWF Mexico / CONANP 2024]. The western population's peak Thanksgiving count fell from millions in the late 1990s to fewer than 2,000 individuals in 2020, partially recovered to ~250,000 by 2023, but remains a fraction of historical levels [Xerces Society 2024].

Threats

Milkweed loss across the U.S. corn belt is the structural driver. Glyphosate-tolerant ("Roundup Ready") corn and soybean varieties, introduced in the late 1990s, enabled the elimination of milkweed and other broadleaf weeds from intensively-farmed agricultural land. Milkweed density on Midwestern agricultural acres fell by an estimated 64% between 1999 and 2010 [Pleasants & Oberhauser 2013]. This is a single, well-quantified mechanism with a clear causal link to monarch reproductive success.

Climate change affects every life-history stage. Warming + increased precipitation variability shifts the timing and extent of milkweed availability in the summer breeding range; severe winter storms in the Sierra Madre fir forest have caused mass-mortality events at overwintering sites; drought + heat stress on the migration corridor reduce nectar availability for adults [Saunders et al. 2018].

Deforestation in the Mexican overwintering range has been significantly reduced since the establishment of the Biosphere Reserve in 1986 and especially since UNESCO inscription in 2008, but illegal logging within and around the reserve continues at lower levels [CONANP 2023].

Western population factors include eucalyptus removal in California (eucalyptus is a non-native species, sometimes targeted for ecological restoration; this trades one conservation goal against another), urban encroachment on overwintering groves, and pesticide exposure.

What is being done

• Monarch Butterfly Biosphere Reserve — protects ~56,000 hectares in central Mexico encompassing nearly all known eastern overwintering colonies [UNESCO 2008]. CONANP and Mexican federal authorities patrol the reserve; community land tenure agreements with surrounding ejido communities provide alternative livelihoods including butterfly ecotourism.
• Monarch Joint Venture — a U.S. partnership of ~110 federal, state, NGO, and academic organizations coordinating habitat restoration across the U.S. breeding range [Monarch Joint Venture 2024]. Programs include milkweed seeding on agricultural easements, roadside right-of-way restoration, and tribal-led restoration on Indigenous lands.
• U.S. Fish and Wildlife Service announced in December 2024 that listing the monarch as Threatened under the Endangered Species Act is warranted. The formal listing process will proceed via Federal Register notice with a 90-day public comment period [USFWS 2024].
• Xerces Society for Invertebrate Conservation — leads the western Thanksgiving overwintering count, milkweed seed distribution, and pesticide-reduction advocacy.
• WWF Mexico — coordinates the eastern overwintering survey jointly with CONANP and runs ejido-community conservation finance programs.

How readers can help

• Plant native milkweed. Asclepias species native to your area — A. tuberosa, A. incarnata, A. syriaca in the eastern United States; A. fascicularis, A. eriocarpa in California; etc. Avoid tropical milkweed (A. curassavica), which encourages monarchs to break diapause and exposes them to a debilitating protozoan parasite (Ophryocystis elektroscirrha).
• Reduce or eliminate pesticide use in your yard and community. Neonicotinoid insecticides affect monarchs and most other beneficial insects.
• Support the Monarch Joint Venture, Xerces Society, and WWF Mexico. Direct donations fund habitat restoration, monitoring, and Mexican overwintering-site protection.
• Engage on USFWS's ESA listing process. The 90-day public comment period on the Threatened listing is the most-direct civic action; comments from members of the public are counted and influence final-rule design.
• Visit responsibly in Mexico. Ecotourism revenue at El Rosario and Sierra Chincua sanctuaries supports the surrounding ejido communities whose cooperation is essential to keeping the overwintering forest intact. Choose CONANP-permitted guides.
• Plant native nectar plants for migration corridors. Monarchs need nectar across the entire migration route; planting native fall-blooming plants (asters, goldenrods) supports the autumn southward flight.

Last verified: 2026-05-23 Conservation status: Endangered (IUCN Red List 2022, migratory subspecies); ESA listing process underway (USFWS Dec 2024).

References

• Brower, L. P. (1969). Ecological chemistry. Scientific American 220(2): 22–29.
• Brower, L. P. (1995). Understanding and misunderstanding the migration of the monarch butterfly (Nymphalidae) in North America: 1857–1995. Journal of the Lepidopterists' Society 49(4): 304–385.
• CONANP (2023). Programa de Manejo de la Reserva de la Biosfera Mariposa Monarca. Comisión Nacional de Áreas Naturales Protegidas, México.
• Monarch Joint Venture (2024). Annual report and milkweed restoration programs. https://monarchjointventure.org/
• Oberhauser, K. S., Nail, K. R., & Altizer, S. (Eds.) (2017). Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly. Cornell University Press.
• Pleasants, J. M., & Oberhauser, K. S. (2013). Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity 6: 135–144.
• Saunders, S. P., Ries, L., Oberhauser, K. S., Thogmartin, W. E., & Zipkin, E. F. (2018). Local and cross-seasonal associations of climate and land use with abundance of monarch butterflies. Ecography 41: 278–290.
• UNESCO World Heritage Centre (2008). Monarch Butterfly Biosphere Reserve inscription. https://whc.unesco.org/en/list/1290/
• U.S. Fish and Wildlife Service (2024). 12-Month Finding: Monarch Butterfly Threatened. Federal Register notice, December 2024.
• Walker, A. J., Lemoine, N. P., Lopez-Hoffman, L., Sáenz-Romero, C., Wilcove, D. S., & Oberhauser, K. S. (2022). Danaus plexippus plexippus. IUCN Red List of Threatened Species. e.T194052138A219151590.
• WWF Mexico / CONANP (2024). Monarch Butterfly Overwintering Forest Area 2023–24. https://www.worldwildlife.org/
• Xerces Society (2024). Western Monarch Thanksgiving Count summary. https://xerces.org/western-monarch-count