Monarch butterfly

• 1. Introduction

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  In 2009, monarch butterflies emerged from their chrysalises aboard the International Space Station. They had launched on the Space Shuttle Atlantis, mission STS-129, as part of a project called Monarchs in Space. Pupae housed in the station's Commercial Generic Bioprocessing Apparatus produced healthy adults.

  The monarch butterfly, Danaus plexippus, is a milkweed butterfly in the family Nymphalidae. Its black, orange and white wings make it one of the most familiar butterflies in North America. Depending on the region, people also call it the wanderer, the common tiger, the black-veined brown, or simply the milkweed.

  Those names hint at a life far stranger than a spaceflight. How does a fragile insect cross a continent, when no single butterfly completes the whole trip? Why would a butterfly carry a poison that can stop a heart? And why has an insect this common become the subject of lawsuits, petitions and yearly counts measured in hectares of forest?

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• 2. A Name From Myth

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  In 1758, Carl Linnaeus described the monarch in his Systema Naturae and placed it in the genus Papilio. Jan Krzysztof Kluk moved it into a new genus, Danaus, in 1780, using the monarch as the type species. Even so, many works published between 1883 and 1964 called it Anosia plexippus or Anosia archippus.

  The English name arrived in 1874, when entomologist Samuel Hubbard Scudder suggested calling the insect The Monarch. He pointed to its large size and to the idea that it "rules a vast domain." He preferred this to William D'Urban's Storm Fritillary and to Philip Henry Gosse's Archippus.

  The scientific name comes from Greek myth. Zeus's great-grandson Danaus was a legendary king in Egypt or Libya who founded Argos. Plexippus was one of the 50 sons of Aegyptus, who was the twin brother of Danaus. In Homeric Greek, Plexippus means one who urges on horses, a rider or charioteer.

  On page 467 of the tenth edition of Systema Naturae, Linnaeus explained his choice. He had divided his huge genus Papilio, which held every butterfly he knew, into groups we would now call subgenera. The colorful species were the Danai festivi, and the bright white-winged species were the Danai candidi. He wrote: "The names of the Danai candidi have been derived from the daughters of Danaus, those of the Danai festivi from the sons of Aegyptus."

  Robert Michael Pyle offered a different reading. He suggested that Danaus is a masculinized form of Danaë, a great-great-granddaughter of Danaus, to whom Zeus came as a shower of gold. That image seemed to him a better source for the name of this butterfly.

  The Linnaean species, D. plexippus, ranges from North America to northern South America. It has also reached Hawaii, Australia, New Zealand, Spain and the Pacific Islands. Pieter Cramer described a second species, the southern monarch Danaus erippus, in 1775. It lives in tropical and subtropical South America, mainly in Brazil, Uruguay, Paraguay, Argentina, Bolivia, Chile and southern Peru.

  Some researchers think the southern monarch split from the northern one about two million years ago, at the end of the Pliocene. Sea levels were higher then, and the Amazon lowland was a vast brackish swamp that offered little habitat for butterflies. In 1819, Jean-Baptiste Godart described the third species, the Jamaican monarch Danaus cleophile, which ranges from Jamaica to Hispaniola.

  Within D. plexippus, researchers have identified six subspecies. The migratory nominate subspecies covers most of North America. A nonmigratory one, D. p. megalippe, lives from Florida and Georgia southward through the Caribbean and Central America to the Amazon River. Austin Hobart Clark named the Puerto Rican and Tobago subspecies in 1941, though some taxonomists dispute these classifications.

  The monarch was the first butterfly to have its genome sequenced. The draft covers 273 million base pairs and 16,866 protein-coding genes. It gave researchers a window on migratory behavior, the circadian clock and juvenile hormone pathways. It also revealed microRNAs that are expressed differently in summer monarchs and migratory ones.

  Later work found that a migrating monarch and a nonmigrating one carry the same gene for migration. The difference is that only the migrant switches it on. Eastern and western North American migratory populations show no genetic differentiation at all.

  A 2015 publication reported genes from wasp bracoviruses inside the genome of the North American monarch. That finding led to articles describing monarch butterflies as genetically modified organisms.

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• 3. From Egg To Wings

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  A female monarch lays 300 to 500 eggs over a period of two to five weeks. She lays them one at a time, usually on the underside of a young milkweed leaf in spring or summer, and fixes each with a dab of glue. A female that mates several times can reach 1,180 eggs.

  Each egg measures about 1.2 by 0.9 millimeters and weighs less than half a milligram. It is cream colored or light green, with raised ridges running from tip to base. Females lay smaller eggs as they age, and larger females lay larger ones. Eggs hatch in three to eight days. Commonly fewer than 10% of monarch eggs and caterpillars survive.

  The newborn caterpillar starts by eating its own egg case. Then it grows through five stages, called instars, each lasting about 3 to 5 days. The first instar is pale green or grayish-white, shiny and nearly translucent, with a large black head. It feeds on milkweed in a circular motion and leaves a telltale arc-shaped hole in the leaf.

  By the second instar, the caterpillar shows bands of white, yellow and black, with a yellow triangle on its head. Pairs of black tentacles begin to grow, a larger pair on the thorax and a smaller pair on the abdomen. Third-instar larvae cut along leaf edges while they feed. Fourth-instar larvae gain white spots on their prolegs, and the fifth instar shows the most complex banding.

  Milkweed fights back with sticky latex. Older caterpillars chew through a latex vein to relieve the pressure, then feed beyond it. Fifth-instar larvae often notch the leaf stalk, which lets the leaf fall into a vertical position.

  A full-grown caterpillar is 4.5 centimeters long, and large ones reach 5. It weighs about 1.5 grams. Then it stops eating and wanders, sometimes up to 10 meters from its milkweed, to find a safe place to pupate.

  Pupation begins with a silk pad spun on a downward-facing surface. The caterpillar latches on with its last pair of hind legs and hangs in the shape of the letter J. After about 12 to 16 hours, it straightens out and its skin splits behind the head. Within a few minutes it sheds that skin and reveals a soft green chrysalis.

  Over a few hours the chrysalis compacts into its final shape. It is opaque and pale green, with small golden dots near the bottom and a gold-and-black rim near the top. The shell hardens within about a day. It is then about 2.5 centimeters long and weighs about 1.2 grams.

  In normal summer temperatures, the adult forms inside over 8 to 15 days, usually 11 or 12. A day or so before emerging, the shell turns translucent and bluish. Within roughly 12 hours it becomes transparent, and the black and orange of the butterfly show through. From egg to adult, the whole process takes as little as 25 days in warm summer weather, and up to seven weeks in a cool spring.

  The adult emerges after about two weeks of pupation and hangs upside down for several hours. During that time it pumps fluids and air into its wings, which expand, dry and stiffen. Its wingspan ranges from 8.9 to 10.2 centimeters. The upper sides are tawny orange, with black veins and margins and two rows of small white spots along the edges.

  During the breeding season, adults reach sexual maturity in four to five days and live two to five weeks. The generation that migrates is different. It does not reach maturity until overwintering is complete.

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• 4. Colors And Courtship

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  On each hindwing of a male monarch sits a small black spot on a vein. The spot holds androconial scales that produce pheromones used in courtship. Males are slightly larger and heavier than females, with thinner black veins. Females look darker and more orange-brown, with wider veins.

  Females also have thicker wings, which are thought to give greater tensile strength and reduce damage during migration. Their lower wing loading means they need less energy to fly. Both sexes have similar thoracic dimensions. Like all Nymphalidae, monarchs keep their small forelegs pressed against the body and walk and cling with only the middle and hind pairs.

  Physiological experiments suggest that monarchs see the world through four kinds of color receptors. Like humans, they have three opsin proteins, but one of theirs responds to ultraviolet light, while the other two respond to blue and green. Orange filtering pigments cover some green-absorbing opsins, creating a fourth receptor sensitive to longer wavelengths. This lets monarchs tell yellow from orange.

  One study proposes that ultraviolet opsin in the upper rim of the eye lets monarchs detect polarized skylight. That would help them orient by the sun during their long flight. Monarchs also have what is called true color vision. They judge color by wavelength alone, not by brightness.

  Color guides their daily choices. When hunting for nectar, a monarch notices flower color first, and one study found color is easier to recognize at a distance than flower shape. When choosing where to lay eggs, the order reverses, and leaf shape helps confirm a plant is milkweed. Monarchs learn to link color with a sugary reward, and no differences between the sexes were found in this ability.

  Only 30% of mating attempts end in copulation, which suggests females can often avoid mating. Courtship has two phases. In the aerial phase, a male chases a female and often forces her to the ground. In the ground phase and a post-nuptial flight, the pair stays attached for up to 16 hours.

  The male passes a spermatophore to the female. Along with sperm, it provides nutrition that helps her produce eggs. Larger spermatophores increase her fecundity, and males that produce them also fertilize more of her eggs. Females that mate several times lay more eggs.

  Monarch mating depends less on pheromones than it does in other species of the genus. Male search and capture strategies may shape success, and human-caused habitat change can affect mating at overwintering sites. Mating for the overwintering populations takes place in spring, before the butterflies disperse.

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• 5. The Long Migration

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  Between 1,200 and 2,800 miles or more separate the northeastern United States and southeastern Canada from the mountain forests of central Mexico. Monarchs fly that distance every autumn, then hibernate there from the beginning of November to mid-March. The trip is multigenerational, so each individual makes only part of the full journey.

  Populations east of the Rocky Mountains aim for the Monarch Butterfly Biosphere Reserve in the Mexican state of Michoacán, northwest of Mexico City. Others head for the Joya Redonda Monarch Butterfly Sanctuary, 50 miles southeast of Mexico City. It lies in the foothills of the volcanoes Popocatépetl and Ixtaccíhuatl, in the municipality of Atlautla. Some eastern monarchs end up in parts of Florida instead.

  In spring, the monarchs that wintered in central Mexico may return as far north as Texas and Oklahoma before producing offspring. Those offspring carry the journey northward. The second, third and fourth generations reach their northern homes in the United States and Canada later in spring and far into summer. Adults fly at around 9 kilometers per hour.

  Western monarchs follow a shorter route. The population west of the Rocky Mountains tries to reach coastal sites in central and southern California, though some have turned up at Mexican sites. Overwintering sites have also been found recently in Arizona. Eastern monarchs generally migrate longer distances than western ones.

  The butterflies change shape for the trip. At the start of migration, wings look redder and more elongated than those of later migrants. Eastern monarchs have larger, more angular forewings than western ones. Wing size and shape also differ between migratory and nonmigratory monarchs.

  In 2025, Project Monarch fitted more than 400 solar-powered ultralight transmitters to the thoraxes of that year's southbound monarchs. A web page in mid-November 2025 showed the routes they had taken up to that point.

  Winter sites share certain features. They have access to streams and plenty of sunlight, which warms the butterflies enough to fly. They offer suitable roosting vegetation and few predators. Roosting monarchs have been seen on basswoods, elms, sumacs, locusts, oaks, osage-oranges, mulberries, pecans, willows, cottonwoods and mesquites. On the East Coast, they have overwintered as far north as Virginia Beach, Virginia, and they stay year-round in Florida.

  Since the 1800s, monarchs have spread around the world, and many populations no longer migrate. They now appear in the Azores, the Canary Islands, Madeira, Portugal, Gibraltar, the Philippines and Morocco. In some years the United Kingdom sees an accidental migrant. During the breeding season, they live in farm fields, pastures, prairie remnants, suburban yards, roadsides and anywhere else with larval host plants.

  Adults need fuel along the way. In autumn they depend on late-flowering plants of the sunflower family, including asters and goldenrods. Publications report that Mexican sunflower is especially attractive. In spring they turn to early-blooming milkweeds, Coreopsis, Viburnum and Phlox. They also sip moisture and minerals from damp soil and wet gravel in a habit called mud-puddling, and one monarch was noticed puddling at an oil stain on the pavement.

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• 6. Poison And Enemies

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  The World Wildlife Fund has published lists of milkweed species that thrive in each of six U.S. regions: Western, Northeast, Southeast, South-central, Arizona and California. Dozens of plants can host monarch caterpillars, mostly milkweeds. Yet the eastern migration depends largely on just three, which are Asclepias syriaca, A. viridis and A. asperula.

  Chemical fingerprinting of the poisons that caterpillars absorb has shown how heavily they lean on a few species. Between 85% and 92% of monarchs overwintering in Mexico had fed on A. syriaca, known as common milkweed, as caterpillars. Among the first-generation monarchs that colonized the Upper Midwest, 84% had fed on A. viridis.

  Tropical milkweed, Asclepias curassavica, is a favorite ornamental in butterfly gardens, but year-round plantings draw criticism. They may create new overwintering sites along the Gulf Coast and lead to year-round breeding. That is thought to harm migration patterns and to build up the dangerous parasite Ophryocystis elektroscirrha. Larvae reared on tropical milkweed show reduced migratory development, and migratory adults exposed to it grow reproductive tissue.

  Black swallow-wort and pale swallow-wort are invasive relatives from Europe. Monarchs lay eggs on them because they give off stimuli similar to milkweed. When the eggs hatch, the caterpillars are poisoned.

  The poison that makes monarchs dangerous comes from milkweed itself. Caterpillars swallow cardenolides and store them. These are steroids that stop hearts in ways similar to digitalis. Monarchs store them most effectively from plants with intermediate levels rather than high or low ones. That diet leaves both caterpillar and butterfly foul-tasting and toxic.

  Three mutations in a monarch enzyme, the Na+/K+-ATPase, are enough to confer resistance to the poison. Researchers tested this by editing the same gene in the fruit fly Drosophila melanogaster with CRISPR-Cas9. The resulting fruit flies were completely resistant to ouabain, a cardiac glycoside. Like monarchs, they even kept some of it through metamorphosis.

  In the adult, the toxins move around. The wings hold three times as much cardiac glycoside as the body, which leaves a taste that may stop birds before they reach the abdomen. For predators that tear off the wings and eat the abdomen, the most potent glycosides are kept there.

  Butterfly weed, Asclepias tuberosa, lacks significant amounts of cardenolides and contains other toxic glycosides called pregnanes. Egg-laying monarchs do not favor the plant. Not all monarchs are unpalatable, and overwintering adults become less toxic over time, which makes them easier prey.

  The viceroy butterfly shares the monarch's colors. It is markedly smaller and has an extra black stripe across each hindwing. For years it was cited as a case of Batesian mimicry. In fact the viceroy is more unpalatable than the monarch, so the two practice Müllerian mimicry, in which both are toxic.

  Some birds beat the poison. The black-backed oriole tastes cardenolides and rejects the worst of them. The black-headed grosbeak has become insensitive to the toxins, and carries resistance mutations in the sodium pump that match those found in some rodents. Both birds periodically build up high levels of cardenolides and must pause their monarch eating. That cycle cuts predation by about 50%.

  Other known bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays and pinyon jays. The black-eared mouse tolerates large doses of the toxins and can eat up to 40 monarchs per night. In Mexico, about 14% of overwintering monarchs are eaten by birds and mice.

  On Oahu, white monarchs make up nearly 10% of the population. The white form appeared there after two bulbul species, Pycnonotus cafer and Pycnonotus jocosus, were introduced in 1965-1966. These birds hunt larvae and pupae on milkweed bushes and eat resting adults, but rarely flying ones. The white form survives better. Possible reasons include that birds have learned orange monarchs are edible, that white blends with the milkweed's pale hairs, or that white does not match the birds' search image.

  Insects also hunt monarchs. The introduced Asian lady beetle feeds on eggs and first-instar larvae. The Chinese mantis eats larvae once the gut is removed, which avoids the toxins. Wasps, stink bugs, assassin bugs, spiders, grasshoppers, crickets and ants take their share. Larvae sometimes escape by dropping from the plant or jerking their bodies.

  Parasitoids kill from the inside. The wasp Pteromalus cassotis lays eggs in a chrysalis while it is still soft. After 14 to 20 days, up to 400 adult wasps emerge and the monarch dies. Tachinid flies and braconid wasps also develop inside larvae or pupae. The tachinid Lespesia archippivora leaves its host larva suspended and dead before pupation.

  Ophryocystis elektroscirrha infects the tissues beneath the skin and forms spores during the pupal stage. Spores cover the butterfly's body, most thickly on the abdomen, and rub off onto eggs, where caterpillars swallow them. Severely infected monarchs are weak, may fail to expand their wings or emerge, and live shorter lives. Infected migrants are less likely to finish the journey, so overwintering populations carry lighter parasite loads.

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• 7. Shrinking Numbers

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  In February 2015, the United States Fish and Wildlife Service reported that nearly a billion monarchs had vanished from the butterfly's overwintering sites since 1990. The agency blamed the decline partly on milkweed lost to herbicides used by farmers and homeowners. By 2024, it calculated that eastern butterflies had fallen by approximately 80 percent since the 1980s. The western population had dropped by 95 percent.

  In Mexico, winter colonies covered 18 hectares of forest in 1996-1997, but the average since 1995 is only about 6. The lowest coverage to date, 0.67 hectares, came in 2013-2014. A rebound to 4.01 hectares in 2015-2016 was followed by a 27% fall to 2.91 hectares in 2016-2017. The 2016 population was estimated at 200 million, against a historical average of 300 million.

  The winter of 2024-2025 saw 1.79 hectares occupied, almost double the 0.90 hectares of the year before. Scientists found 2.93 hectares occupied in 2025-2026, a 64% increase.

  The western story is harsher. According to the Xerces Society for Invertebrate Conservation, California's monarch population fell 86% in 2018, from millions to tens of thousands. The society's winter counts reached 1,901 in 2020, then 335,479 in 2022. The lowest counts in the program's history have come in recent years, including 9,119 in 2024 and 12,260 in 2025. One Pacific Grove site had no monarchs at all in 2020.

  One culprit is the herbicide glyphosate. A 2018 study linked the monarch decline to the fact that 95% of corn and soybean crops in the United States used seeds engineered to resist it. Farmers could now spray a second time, by air, once weeds began to challenge the crop. The unplowed margins between fields and roads, which had supported milkweed and nectar flowers, shrank dramatically.

  In 2015, the Natural Resources Defense Council sued the Environmental Protection Agency, arguing that it had ignored warnings about glyphosate and monarchs. Yet another 2018 study suggested that milkweed was declining before genetically modified crops arrived. Chip Taylor, director of Monarch Watch at the University of Kansas, said in 2013 that Midwest milkweed habitat "is virtually gone," with 120-150 million acres lost.

  A 2017 report added federal policy to the list. The Renewable Fuel Standard, crop insurance and waning Farm Bill support for the Conservation Reserve Program all favor row crops over mixed perennial vegetation. That leaves less fallow land to support both food production and monarch habitat. In 2024, Taylor predicted that the eastern population would stay relatively stable for the near future, since it is not on a continuous downward trend as it was from 2000 to 2006.

  The numbers do not all point the same way. Long-term citizen science data show no decline in breeding adult monarchs over the last two decades. Yet winter counts have clearly fallen, which suggests a growing disconnect between the stages of life. One researcher points to cars. A 2019 study in northern Mexico found that just two hotspots each year account for 200,000 dead monarchs.

  A 2014 study concluded that protecting wintering habitat has probably helped, but that habitat loss on breeding grounds in the United States is the main driver of decline. Winter habitat still faces illegal logging of oyamel fir trees, where monarchs spend their winter diapause, a period of suspended development. Mexican authorities monitor the logging, but organized criminals have repeatedly crushed such efforts for short-term gain.

  Weather matters too. Omar Vidal, director general of WWF-Mexico, said: "The monarch's lifecycle depends on the climatic conditions in the places where they breed." Temperatures above 35 degrees Celsius can kill larvae, and eggs dry out in hot, arid conditions. Below 30 degrees, a monarch cannot fly, so it basks in the sun or shivers its wings to warm up.

  A 2015 study examined how warming would affect the breeding range. Over the next 50 years, host plants are expected to spread further north into Canada, and monarchs would follow. That would lengthen the trip to Mexico and could raise deaths on the way. Milkweed grown in warmer conditions has shown higher cardenolide concentrations. Milkweed grown at carbon dioxide levels of 760 parts per million produced a different mix of toxins, and one of them was less effective against monarch parasites.

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• 8. Law And Rescue

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  On the 20th of June 2014, President Barack Obama issued a memorandum creating a Pollinator Health Task Force. It stated: "The number of migrating Monarch butterflies sank to the lowest recorded population level in 2013-14, and there is an imminent risk of failed migration." In May 2015, the task force issued a national strategy with a goal of 225 million eastern monarchs in the Mexican overwintering grounds by 2020. It also aimed to restore or enhance 7 million acres of pollinator habitat over five years.

  Many priority projects focused on the I-35 corridor, which runs 1,500 miles from Texas to Minnesota. On the 4th of December 2015, the FAST Act became law and amended Title 23 of the United States Code. It directed the Secretary of Transportation to encourage reduced mowing and plantings of native milkweed along roadsides, as migratory way stations for butterflies. In February 2016, the Secretary of the Interior issued a strategy for pollinator-friendly landscaping at Department of the Interior sites.

  On the 14th of August 2014, the Center for Biological Diversity and the Center for Food Safety petitioned the Interior Secretary to protect the migratory subspecies under the Endangered Species Act. The Fish and Wildlife Service began a review on the 31st of December 2014. On the 17th of December 2020, it ruled that listing was "warranted-but-precluded," because limited budgets forced it to prioritize species with higher listing priority.

  On the 12th of December 2024, the service proposed listing the monarch as threatened and designating seven areas near California's Pacific coast as critical habitat. It estimated the probability of extinction in the foreseeable future of 60 years at 56-74 percent for the eastern migratory population and 99 percent for the western one. Comments were accepted until the 12th of March 2025, then the period reopened on the 19th of March 2025 and ran to the 19th of May 2025.

  The International Union for Conservation of Nature added the migratory monarch to its red list as endangered on the 20th of July 2022. A petition in 2023 changed its status to vulnerable. In December 2023, Canada listed the monarch as endangered under the Species at Risk Act, which makes it illegal to kill, hurt or collect monarchs on federal lands. Nova Scotia lists it as endangered, and Ontario lists it as a species of special concern. Two long-term studies of monarchs in Canada, however, show no population decline.

  The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, Vermont and West Virginia. Legislation to make it the national insect of the United States failed in 1989 and again in 1991. A 2012 IMAX film, Flight of the Butterflies, told how the Urquharts, Brugger and Trail documented the then-unknown migration to Mexico. Barbara Kingsolver's 2012 novel Flight Behavior imagines a huge population appearing in the Appalachians.

  Homeowners build butterfly gardens and monarch waystations, and organizations run tagging programs that trace migration patterns. Sanctuaries in Mexico and California protect overwintering sites and can bring in significant tourism revenue. Yet butterflies in sites isolated from tourists have shown higher protein levels, stronger immune response and better oxidative defense.

  Captive rearing has become popular. Schools and nature centers use monarchs for teaching, and adults will extend their proboscises to a fingertip or a dish of artificial food. Captive-bred monarchs are released at hospices, memorials and weddings, including services remembering the September 11 attacks.

  In 2015 the Huffington Post and in 2016 Discover magazine summarized the controversy over mass rearing. One person in Linn County, Iowa, reared thousands of monarchs at once. One researcher says rearing more than 100 counts as mass rearing and should not be done. The parasite spreads quickly among captive monarchs, and its spores contaminate containers so that every later monarch raised in them becomes infected. Owners of commercial breeding operations say they control the parasite, but many scientists doubt it.

  A 2019 study tested reared and wild monarchs on a tethered flight apparatus that measured navigation. Monarchs raised in artificial conditions showed reduced navigational ability, even those brought in from the wild for only a few days. Monarchs bought from a breeder were so genetically different from wild ones that the lead author called them "franken-monarchs." A few captive-reared monarchs in that study did navigate properly, so the conditions that decide the outcome are still being worked out.

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