Monarch Butterfly

The monarch butterfly, Danaus plexippus, is a pollinator well known for its impressive long-distance migration and its recent declines. Linnaeus described the species in 1758 as Papilio plexippus, a name now recorded as a synonym of Danaus plexippus. It belongs to the family Nymphalidae.

Both male and female monarchs are bright orange with black borders and black veins; the female's veins are thicker, while males have a swollen pouch and a black scent gland on each hind wing.

The monarch's bright orange and black coloration serves as a warning to predators, since monarchs obtain toxins called cardenolides by consuming milkweed plants. Most milkweed species are toxic to vertebrate herbivores because of cardenolide alkaloids, and monarch larvae ingest these toxins, called cardiac glycosides, and sequester them in their exoskeletons, making larvae and adults toxic to many potential predators. Larval monarch butterflies feed almost exclusively on milkweed plants.

The monarch's wingspan is 8.6–12.4 cm (3.39–4.88 in). Its weight is 0.0095–0.026 ounces.

The monarch is a new-world species whose distribution extends from at least 50N in Canada to Colombia, Venezuela, and coastal Ecuador and Peru. It is native to the Americas, and introductions linked to colonists and steamships established it in Australia, New Zealand, Hawaii and other Pacific islands. The species is likewise established in Mauritius, the Canary Islands and Western Europe.

The monarch is a predominantly open-country, frost-intolerant species whose breeding range depends greatly on milkweeds. It requires dense tree cover for overwintering, and most present California sites are associated with blue gum eucalyptus.

North American migratory monarchs are divided into eastern and western populations separated generally by the Rocky Mountains, and the two occasionally interbreed. Non-migratory monarchs also remain year-round at the southern end of the breeding range, including parts of Florida, the Gulf Coast and California.

A female monarch lays each egg individually on a milkweed leaf, attaching it with a bit of glue she secretes, and usually lays between 300 and 500 eggs over a two- to five-week period. About 400 eggs per female is the average.

The egg and larval period lasts about two weeks, after which pupation lasts 9 to 15 days before an adult butterfly emerges. Generation length from egg to adult is 25 to 50 days. Monarchs are multigenerational, cycling through four to five generations that play different roles depending on the time of year they emerge; first generations feed and reproduce locally, while later generations migrate north or south.

Summer breeding monarchs live only 2 to 5 weeks. Migratory monarchs that survive the winter in Mexico are 8 to 9 months of age and may be the longest lived of all butterflies.

Mating monarchs remain together from one afternoon until early the next morning, often up to 16 hours, and females immediately begin to lay eggs after their first mating, with both sexes able to mate several times.

Monarchs utilize about 30 milkweed species as larval hosts, with common milkweed (Asclepias syriaca) predominant; estimates are that 90% of the monarchs that overwinter in Mexico each year fed on that species as larvae.

The protozoan parasite Ophryocystis elektroscirrha is the most-studied monarch parasite; infections occur when larvae consume spores on milkweed and, while not typically fatal, negatively impact monarchs' survival, size, and life span.

Each autumn the eastern population of monarchs undertakes a southward journey of more than 2,000 miles (3,000 km) to reach overwintering sites in central Mexico. The fall migration begins in mid-August in the north and in September at mid-latitudes, advancing at a pace of 25 to 30 miles per day when conditions are favourable. The movement appears to last about 2 to 2.5 months, and most monarchs originate from locations more than 1,500 linear miles from the overwintering sites. Only monarchs that emerge in late summer or early fall make the annual migration south, huddling together on oyamel fir trees to wait out the winter.

Western monarchs follow a shorter route, migrating annually 300 to 1,000 miles (about 500 to 1,600 km) and overwintering in hundreds of groves along the California coast and into northern Baja California, Mexico.

Migratory monarchs are non-reproductive and generally remain so until mid-February, a state of suspended reproduction termed "reproductive diapause".

Monarchs use the sun to stay on course and also have a magnetic compass to help them navigate on cloudy days, plus a special gene for highly efficient muscles that gives them an advantage for long-distance flight. Similar migratory behaviour has been observed in Costa Rican and Australian populations.

The eastern population is the largest in both individuals and range, encompassing upwards of 70% of the total North American monarch range.

The winter destination of the eastern monarch population remained unknown until the overwintering colony was finally located in January 1975, after which the Urquharts joined an expedition in January 1976 to view the final southern destination of the eastern population. The colony stood on the slope of a volcanic mountain in the northern part of Michoacan, Mexico, at a height of slightly over 3,000 m, with a predominant tree averaging 26 m in height. It occupied an area of roughly 20 acres, its oyamel trunks and branches completely covered with monarchs so closely packed that it was impossible to see even a small portion of the underlying bark. In January 1976 the Urquharts climbed into the Sierra Madre, reached the fir forests of remote Michoacan, and found butterflies covering thousands of trees so thickly that branches broke beneath their weight.

Fred and Norah Urquhart started tracking monarchs with tags in 1950; their tags asked finders to report locations to the University of Toronto, an early exercise in classic citizen science. Because monarchs weigh less than a gram, they were too delicate for typical animal trackers, and the solution was a small sticker with instructions asking finders to return tagged monarchs to the University of Toronto. Over 300,000 monarchs had been alar tagged over the course of the investigation, and by the report the project had travelled more than 100,000 miles in field expeditions during a 25-year period.

The 56,259-hectare biosphere lies within rugged forested mountains about 100 km northwest of Mexico City, where every autumn millions, perhaps a billion, butterflies cluster and bend the branches under their collective weight. The Bruggers' multi-year tracking led them to mountainous oyamel fir trees where roughly 60 million semidormant butterflies clustered — the winter home of the monarchs.

During the 2024-2025 overwintering season eight colonies of monarch butterflies — three in Michoacán and five in the State of Mexico — were recorded occupying a total of 4.42 acres of forest, an increase of 99% over the 2023-2024 winter. Three colonies were located inside the Monarch Butterfly Biosphere Reserve, occupying 3.01 acres of forest, and five were outside it, occupying 1.42 acres. The largest colony, 2.46 acres, was recorded in the ejido El Rosario in Michoacán, and the smallest, 0.005 acres, in San Andrés in the Los Azufres sanctuary.

The highest recorded overwintering area was 44.95 acres in the 1996-1997 winter, while the lowest historical occupation was 1.65 acres in the 2013-2014 winter. The second lowest overwintering area of the last 30 years was in the 2023-2024 winter with 2.22 acres.

CONANP and WWF have been monitoring the monarch overwintering sites in Mexico since the 2004-2005 winter.

Approximately 12,260 monarchs were recorded across 249 sites during the peak overwintering period of late November through early December 2025 — the third-lowest tally since the count began in 1997 and another sign of a historically low population.

In the 1980s over 4.5 million western monarchs flocked to overwintering grounds in coastal California, and in the mid-1990s an estimated 380 million eastern monarchs made the long-distance journey to Mexico — one of the longest insect migrations in the world.

The eastern migratory population has declined by about 80% over the decade preceding a 2016 modelling study, and the population of monarch butterflies has declined by more than 80% within the last two decades, with one possible cause the loss of 1.3 billion or more stems of milkweed that monarchs require for reproduction. Western monarch populations have declined steadily since the 1980s, with an abrupt drop of 86% between 2018 and 2019; counts in 2020 indicated fewer than 2,000 migratory monarchs in California, but the 2021 count exceeded 200,000 butterflies. Population declines, mostly attributed to climate factors, host plant declines in summer breeding areas, and habitat loss in overwintering sites, have been observed in both the eastern and western migratory populations.

The Service estimated the western population's chance of extinction by 2080 at greater than 99%, while the probability of extinction for eastern monarchs ranged from 56 to 74%. An approximately five-fold increase of the monarch population size relative to the winter of 2014-15 would be necessary to halve the risk of quasi-extinction across all thresholds considered.

The 27th annual Western Monarch Count tallied 233,394 butterflies at 256 overwintering sites, slightly lower than the previous year's total of over 330,000 butterflies. The largest count at a single location was 33,080 monarchs at a Santa Barbara County site owned by The Nature Conservancy, with 16,038 at the Pismo State Beach Monarch Butterfly Grove.

The January 2025 Western Monarch Count recorded just over 9,000 monarchs overwintering in coastal California — the second lowest number ever recorded, far below the millions of the 1980s that scientists consider a stable population.

The three lowest counts in the programme's history have all occurred in recent years (1,901 monarchs in 2020; 9,119 in 2024; 12,260 in 2025), reflecting the increasing frequency of record-low years. Despite the low overall numbers, Natural Bridges State Beach in Santa Cruz recorded the season's highest count with 2,500 monarchs in early December.

The Service determined that the monarch meets the definition of a threatened species due to ongoing loss and degradation of breeding, migratory, and overwintering habitat — including conversion of grasslands and shrublands to agriculture, herbicide use, logging in Mexico and urban development in California — as well as exposure to insecticides and the effects of climate change. Since 1997 the use of Roundup Ready soybeans engineered to resist glyphosate has resulted in the loss of at least 100 million acres of monarch habitat in row crops of corn and soybeans.

Because a near doubling of average abundance would require roughly a doubling of stems, researchers calculated that 1.6 billion additional stems of milkweed were needed in the Midwest to reach the 6-hectare overwintering goal.

Forest degradation in the core zone of the Monarch Butterfly Biosphere Reserve decreased by 10%: between April 2023 and February 2024 the damage amounted to 9.14 acres, down from 10.13 acres the previous year. The main causes of forest degradation were illegal logging (6.17 acres), natural causes primarily drought (2.47 acres), and removal of trees to prevent disease spread (0.45 acres).

The migratory monarch subspecies was listed as Endangered under criterion A2ab in the IUCN's 2022 Red List publication, following an assessment dated December 30 of 2021. On December 11 of 2023 the IUCN changed the listing of the migratory monarch from endangered to vulnerable, following a petition that challenged the population models used in its assessment. In contrast to the Endangered status of the migratory subspecies, the IUCN classified the full species Danaus plexippus as Least Concern in its 2022 assessment.

The change stemmed from IUCN criteria based on population reduction over the past ten years or three generations; the ten-year decline did not match the longer-term trend, though both methods of evaluation were considered scientifically valid. The Monarch Joint Venture notes that the IUCN Red List provides international recommendations and no legal implications in the United States, where the authority to classify the migratory monarch lies with the U.S. Fish and Wildlife Service.

On December 15 of 2020 the USFWS concluded that listing the monarch under the Endangered Species Act is "warranted but precluded", making monarch butterflies a candidate species. On December 17 of 2020 the Service published a 12-month finding that listing the monarch as an endangered or threatened species is warranted but precluded by higher priority actions, and the species remained a candidate in the 2022 and 2023 notices of review. On December 10 of 2024 the Service proposed listing the monarch as threatened with species-specific protections and flexibilities under section 4(d) of the Endangered Species Act and opened a public comment period on the proposal.

The Service proposed to designate approximately 4,395 acres (1,778 hectares) of critical habitat in Alameda, Marin, Monterey, San Luis Obispo, Santa Barbara, Santa Cruz, and Ventura Counties, California.