Monarch butterfly

The species was originally described by Carl Linnaeus in his Systema Naturae of 1758 and placed in the genus Papilio. In 1874, entomologist Samuel Hubbard Scudder suggested naming the butterfly "The Monarch". He cited its large size and the notion that it "rules a vast domain" as the basis for the name, which he preferred to William D'Urban's "Storm Fritillary" and Philip Henry Gosse's "Archippus." In 2005, investigators reported that the monophyletic genus Danaus s.l. had previously been divided into three subgenera (Danaus s. s., Salatura, and Anosia). However, based on their own studies, which combined mitochondrial DNA and nuclear DNA sequence information with morphological data, they concluded that the earlier classification was unsustainable and should be abandoned. Danaus (Ancient Greek ), a great-grandson of Zeus, was a mythical king in Egypt or Libya, who founded Argos; Plexippus () was one of the 50 sons of Aegyptus, the twin brother of Danaus. In Homeric Greek, his name means 'one who urges on horses', i.e., 'rider' or 'charioteer'. In the tenth edition of Systema Naturae, at the bottom of page 467, == Taxonomy ==

of the monarch in Hawaii called the white monarch Monarchs belong to the subfamily Danainae of the family Nymphalidae. Danainae was formerly considered a separately family Danaidae. Six subspecies and two color morphs of D. plexippus have been identified: • D. p. plexippus – nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America. • D. p. p. "form nivosus", the white monarch commonly found on Oahu, Hawaii, and rarely in other locations. • D. p. p. (as yet unnamed) – a color morph lacking some wing vein markings. • D. p. nigrippus (Richard Haensch, 1909) – South America - as forma: Danais archippus f. nigrippus. Hay-Roe et al. in 2007 identified this taxon as a subspecies • D. p. megalippe (Jacob Hübner, [1826]) – nonmigratory subspecies, and is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River. • D. p. leucogyne (Arthur G. Butler, 1884) − St. Thomas • D. p. portoricensis Austin Hobart Clark, 1941 − Puerto Rico • D. p. tobagi Austin Hobart Clark, 1941 − Tobago The population level of the white morph in Oahu is nearing 10%. On other Hawaiian islands, the white morph occurs at a relatively low frequency. White monarchs (D. p. p. "form nivosus") have been found throughout the world, including Australia, New Zealand, Indonesia, and the United States. However, some taxonomists disagree on these classifications. == Genome ==

The monarch was the first butterfly to have its genome sequenced. More recently, the genetic basis of monarch migration and warning coloration has been described. No genetic differentiation exists between the migratory populations of eastern and western North America. leading to articles about monarch butterflies being genetically modified organisms. == Life cycle ==

Like all Lepidoptera, monarchs undergo complete metamorphosis; their life cycle has four phases: egg, larva, pupa, and adult. Monarchs transition from eggs to adults during warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During their development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites, and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive. Female monarchs lay eggs singly, most often on the underside of a young leaf of a milkweed plant during the spring and summer. Eggs are cream colored or light green, ovate to conical in shape, and about in size. The eggs weigh less than each and have raised ridges that form longitudinally from the point to apex to the base. Although each egg is the mass of the female, she may lay up to her own mass in eggs. Females lay smaller eggs as they age. Larger females lay larger eggs.The number of eggs laid by a female, which may mate several times, can reach 1,180. Eggs take three to eight days to develop and hatch into larvae or caterpillars. Larva (top), a monarch caterpillar (middle), and a queen caterpillar (bottom) all on a human hand The larva (caterpillar) has five stages (instars), molting at the end of each instar. Instars last about 3 to 5 days, depending on factors such as temperature and food availability. The first-instar caterpillar that emerges from the egg is pale green or grayish-white, shiny, almost translucent, with a large, black head. It lacks banding coloration or tentacles. The larva or caterpillar eats its egg case and begins to feed on milkweed in a circular motion, often leaving a characteristic, arc-shaped hole in the leaf. Older first-instar larvae have dark stripes on a greenish background and develop small bumps that later become front tentacles. The first instar is usually between long. In a laboratory setting, the fourth- and fifth-instar caterpillar stages showed aggressive behavior with lower food availability. Attacked caterpillars were found to be attacked when feeding on milkweed leaves, and the caterpillars attacked when foraging for milkweed. This demonstrates the aggressive behavior of monarch caterpillars due to the availability of milkweed. Pupa into a pupa To prepare for the pupal or chrysalis stage, the caterpillar chooses a safe place for pupation, where it spins a silk pad on a downward-facing horizontal surface. At this point, it turns around and securely latches on with its last pair of hind legs and hangs upside down, in the form of the letter J. After "J-hanging" for about 12–16 hours, it soon straightens out its body and goes into peristalsis some seconds before its skin splits behind its head. It then sheds its skin over a few minutes, revealing a green chrysalis. At first, the chrysalis is long, soft, and somewhat amorphous, but over a few hours, it compacts into its distinct shape – an opaque, pale-green chrysalis with small golden dots near the bottom, and a gold-and-black rim around the dorsal side near the top. At first, its exoskeleton is soft and fragile, but it hardens and becomes more durable within about a day. At this point, it is about long and wide, weighing about . At normal summer temperatures, it matures in 8–15 days (usually 11–12 days). During this pupal stage, the adult butterfly forms inside. A day or so before emerging, the exoskeleton first becomes translucent and the chrysalis more bluish. Finally, within 12 hours or so, it becomes transparent, revealing the black and orange colors of the butterfly inside before it ecloses (emerges). In 2009, monarchs were reared on the International Space Station as part of the Monarchs in Space project, launched aboard the Space Shuttle Atlantis mission STS-129, successfully emerging from pupae located in the station's Commercial Generic Bioprocessing Apparatus. Adult The adult emerges from its chrysalis after about two weeks of pupation. The emergent adult hangs upside down for several hours while it pumps fluids and air into its wings, which expand, dry, and stiffen. The butterfly then extends and retracts its wings. Once conditions allow, it flies and feeds on many nectar plants. During the breeding season, adults reach sexual maturity in 4–5 days. However, the migrating generation does not reach maturity until overwintering is complete. Adults are sexually dimorphic. Males are slightly larger than females and have a black spot on a vein on each hindwing. The spots contain androconial scales that produce pheromones that many Lepidoptera use during courtship. Females are often darker than males and have wider veins on their wings. The ends of the abdomens of males and females differ in shape. The adult thorax has six legs, but like all Nymphalidae, the forelegs are small and held against the body. The butterfly uses only its middle and hind legs when walking and clinging. Male Male monarch butterflies can be distinguished from females by the small, black, fuzzy spots on their hind wings, which are scent glands used for courtship, and by their slightly thinner black wing veins. Female Female monarch butterflies lack these spots, have a more orange-brown appearance, and generally have thicker, wider black wing veins than males. Key Differences • Wing Spots: The most reliable way to tell them apart is by looking for a small, black, fuzzy spot on the inner surface of the hind wings. This spot is present on all male monarch butterflies but is absent in females. • Wing Veins: Female monarchs typically have thicker, wider black veins on their wings compared to males. • Color: While both are bright orange, females tend to have a darker, more orange-brown appearance, and their coloring can be described as having a more robust and darker orange. • Size: Males are generally slightly larger than females, although this is not always noticeable and may vary with genetics and nutrition. • Body: Both sexes have a black body with white markings. == Vision ==

Physiological experiments suggest that monarch butterflies view the world through a tetrachromatic system. Like humans, their retina contain three types of opsin proteins, expressed in distinct photoreceptor cells, each of which absorbs light at a different wavelength. Unlike humans, one of those types of photoreceptor cells corresponds to a wavelength in the ultraviolet range; the other two correspond to blue and green. In addition to these three photoreceptor cells in the main retina, monarch butterfly eyes contain orange filtering pigments that filter the light reaching some green-absorbing opsins, thereby making a fourth photoreceptor cell sensitive to longer-wavelength light. These butterflies are capable of distinguishing colors based on their wavelength only, and not based on intensity; this phenomenon is termed "true color vision". This is important for many butterfly behaviors, including seeking nectar for nourishment, choosing a mate, and finding milkweed on which to lay eggs. One study found that floral color is more easily recognized at a distance by butterflies searching for nectar than floral shape. This may be because flowers have highly contrasting colors to the green background of a vegetative landscape. ==Courtship and mating==

Monarch courtship occurs in two phases. During the aerial phase, a male pursues and often forces a female to the ground. During a ground phase and a post-nuptial flight, the butterflies copulate and remain attached for up to 16 hours. Only 30% of mating attempts end in copulation, suggesting that females can avoid mating, though some have more success than others. ==Distribution and habitat==

Monarch Preserve (2015) The range of the western and eastern populations of D. p. plexippus expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts. the Philippines, and Morocco. On the East Coast of the United States, they have overwintered as far north as Virginia Beach, Virginia. Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and is relatively free of predators. Overwintering, roosting butterflies have been seen on basswoods, elms, sumacs, locusts, oaks, osage-oranges, mulberries, pecans, willows, cottonwoods, and mesquites. While breeding, monarch habitats can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides – anywhere there is access to larval host plants. == Larval host plants ==

'', common milkweed '', green antelopehorn milkweed '', antelope horns milkweed '', tropical milkweed The host plants used by the monarch caterpillar include: • Araujia sericifera – white bladderflower • Asclepias angustifolia – Arizona milkweed • Asclepias albicans – whitestem milkweed • Asclepias asperula – antelope horns milkweed • Asclepias californica – California milkweed • Asclepias eriocarpa – woollypod milkweed • Asclepias fascicularis – Mexican whorled milkweed • Asclepias humistrata – sandhill/pinewoods milkweed • Asclepias lanceolata – fewflower milkweed • Asclepias linaria – pineneedle milkweed • Asclepias meadii – Meade's milkweed • Asclepias nivea – Caribbean milkweed • Asclepias oenotheroides – zizotes milkweed • Asclepias perennis – aquatic milkweed • Asclepias rubra — red milkweed • Asclepias speciosa – showy milkweed • Asclepias subulata – rush milkweed • Asclepias syriaca – common milkweed • Asclepias tuberosa – butterfly weed • Calotropis gigantea – crown flower • Calotropis procera – giant milkweed • Cynanchum angustifolium – Gulf coast swallow-wort • Cynanchum laeve – sand vine milkweed • Gomphocarpus fruticosus – swan plant • Gomphocarpus physocarpus – balloon plant • Sarcostemma clausa – white vine The eastern monarch migration largely depends upon only three of these species: Asclepias syriaca, A. viridis, and A. asperula. Using cardenolide fingerprinting analysis, investigators have found that 85-92% of monarchs overwintering in Mexico had fed on A. syriaca as caterpillars, while 84% of the first-generation monarchs that colonized the Upper Midwest had fed as larvae on A. viridis. However, Asclepias curassavica, or tropical milkweed, is often planted as an ornamental in butterfly gardens. Year-round plantings in the USA are controversial and criticized, as they may cause new overwintering sites along the U.S. Gulf Coast, leading to year-round breeding of monarchs. This is thought to adversely affect migration patterns and to cause a dramatic buildup of the dangerous parasite, Ophryocystis elektroscirrha. New research also has shown that monarch larvae reared on tropical milkweed show reduced migratory development (reproductive diapause), and when migratory adults are exposed to tropical milkweed, it stimulates reproductive tissue growth. == Adult food sources ==

'') (August 2019) '') (2006) Although monarch larvae eat only milkweed, adult monarchs and other pollinators feed on the nectar of many plants, including: • Apocynum cannabinum — Indian hemp • Asclepias spp. — milkweeds • Asteraceae family (many genera, including Eurybia — asters) • Caryopteris x clandonensis — bluebeard • Cephalanthus occidentalis — buttonbush • Cirsium spp. — thistles • Coreopsis spp. — calliopsis, tickseed • Dasiphora fruticosa — shrubby cinquefoil • Erigeron glaucus — Seaside fleabane • Heptacodium miconioides — seven-son flower • Sambucus spp. — elderberries • Tithonia rotundifolia — Mexican sunflower • Trifolium pinetorum – woods clover • Verbena spp. — verbenas • Verbesina virginica – frostweed Publications have reported that Mexican sunflower (Tithonia rotundifolia) is especially attractive to adult monarchs. During the spring migration, important nectar sources include early-blooming Asclepias spp., Coreopsis spp., Viburnum spp., and Phlox spp. Monarchs obtain moisture and minerals from damp soil and wet gravel, a behavior known as mud-puddling. The monarch has also been noticed puddling at an oil stain on the pavement. == Flight and migration ==

'') in Piedra Herrada, Mexico, within the Monarch Butterfly Biosphere Reserve (February 2023) on Fire Island, New York (September 2021) In North America, monarchs migrate both north and south on an annual basis, making long-distance journeys that are fraught with risks. The population east of the Rocky Mountains attempts to migrate to the sanctuaries of the Monarch Butterfly Biosphere Reserve in the Mexican state of Michoacán northwest of Mexico City, the Joya Redonda Monarch Butterfly Sanctuary, located southeast of Mexico City in the foothills of the volcanoes Popocatépetl and Ixtaccíhuatl in the State of México's municipality of Atlautla, and to parts of Florida. The western population tries to reach overwintering destinations in various coastal sites in central and southern California. The populations east of the Rocky Mountains, which mostly overwinter in central Mexico, may return the following spring as far north as Texas and Oklahoma before producing offspring to carry the journey northward. The second, third, and fourth generations return to their northern locations in the United States and Canada later in the spring and far into the summer. In 2025, Project Monarch deployed over 400 solar-powered ultralight transmitters on the thoraxes of that year's southward-migrating monarchs. A mid-November 2025 web page showed the routes that the monarchs had taken up to that time. Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico, though they have a much lower migratory success rate than do wild monarchs (see section on captive-rearing below). Monarch overwintering sites have been discovered recently in Arizona. Monarchs from the eastern US generally migrate longer distances than monarchs from the western US. Since the 1800s, monarchs have spread globally, and there are now many non-migratory populations. Climate change and habitat loss have significantly altered the monarch butterfly’s migration behavior. Rising temperatures and the widespread loss of milkweed (Asclepias spp.), the primary host plant for monarch larvae, have disrupted breeding and migratory patterns across North America, contributing to population declines over the past two decades. == Interactions with predators ==

In both caterpillar and butterfly forms, monarchs are aposematic, warding off predators with a bright display of contrasting colors to warn potential predators of their undesirable taste and poisonous characteristics. One monarch researcher emphasizes that predation on eggs, larvae, or adults is natural since monarchs are part of the food chain, thus people should not take steps to kill predators of monarchs. Larvae feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in the Asclepias species vary. Not all monarchs are unpalatable, but exhibit Batesian or automimics. Cardiac glycoside levels are higher in the abdomen and wings. Some predators can differentiate between these parts and consume the most palatable ones. Butterfly weed (A. tuberosa) lacks significant amounts of cardiac glycosides (cardenolides) but instead contains other types of toxic glycosides, including pregnanes. This difference may reduce the toxicity of monarchs whose larvae feed on that milkweed species and affect the butterfly's breeding choices, as a naturalist and others have reported that egg-laying monarchs do not favor the plant. Some other milkweeds have similar characteristics. Types of predators takes a monarch butterfly caterpillar from within a narrow leaf milkweed umbel. Part is shown at one fourth speed. While monarchs have a wide range of natural predators, none of these is suspected of causing harm to the overall population, or are the cause of the long-term declines in winter colony sizes. Several species of birds have acquired methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides (cardenolides). The black-backed oriole can eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The black-headed grosbeak, though, has developed an insensitivity to secondary plant poisons that allows it to ingest monarchs without vomiting. As a result, these orioles and grosbeaks periodically have high levels of cardenolides in their bodies, and they are forced to go on periods of reduced monarch consumption. This cycle effectively reduces potential predation of monarchs by 50% indicating that monarch aposematism has a legitimate purpose. Known bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, and pinyon jays. The monarch's white morph appeared in Oahu after the 1965–1966 introduction of two bulbul bird species, Pycnonotus cafer and Pycnonotus jocosus. These are now the most common avian insectivores in Hawaii, and probably the only ones that eat insects as large as monarchs. Although Hawaiian monarchs have low cardiac glycoside levels, the birds may also be tolerant of that toxin. The two species hunt the larvae and some pupae from the branches and undersides of leaves in milkweed bushes. The bulbuls also eat resting and ovipositing adults, but rarely flying ones. Because of its color, the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e., the birds have learned the orange monarchs can be eaten), because of camouflage (the white morph matches the white pubescence of milkweed or the patches of light shining through foliage), or because the white morph does not fit the bird's search image of a typical monarch, so is thus avoided. Some mice, particularly the black-eared mouse (Peromyscus melanotis), are, like all rodents, able to tolerate large doses of cardenolides and can eat monarchs. Overwintering adults become less toxic over time making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice and black-eared mice can eat up to 40 monarchs per night. The Chinese mantis (Tenodera sinensis) will consume the larvae once the gut is removed thus avoiding cardenolides. Predatory wasps commonly consume larvae. Many Hemipteran bugs including predatory stink bugs in the subfamily Asopinae and assassin bugs in family Reduviidae prey on monarch caterpillars or eggs, as do many species of spiders, grasshoppers, crickets, ants, flies, beetles and other arthropods. Larvae can sometimes avoid predation by dropping from the plant or by jerking their bodies. Parasitoids, including tachinid flies and braconid wasps develop inside the monarch larvae eventually killing them and emerging from the larvae or pupa. Non-insect parasites and infectious diseases (pathogens) also kill monarchs. Aposematism , one of the cardiac glycosides Monarchs are toxic and foul-tasting because of the presence of cardenolides in their bodies, which the caterpillars ingest as they feed on milkweed. By ingesting a large number of plants in the genus Asclepias, primarily milkweed, monarch caterpillars can sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis. It has been found that monarchs can sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content. Three mutations that evolved in the monarch's Na+/ K+-ATPase were found to be sufficient together to confer resistance to dietary cardiac glycosides. were completely resistant to dietary ouabain, a cardiac glycoside found in Apocynaceae, and even sequestered some through metamorphosis, like the monarch. One species, Asclepias curassavica, appears to reduce the symptoms of Ophryocystis elektroscirrha (OE) infection. The two possible explanations for this include that it promotes overall monarch health to boost the monarch's immune system or that chemicals from the plant have a direct negative effect on the OE parasites. After the caterpillar becomes a butterfly, the toxins shift to different body parts. Since many birds attack the butterfly's wings, having three times the cardiac glycosides in the wings leaves predators with a foul taste and may prevent them from ever ingesting the butterfly's body. Mimicry Monarchs share the defense of noxious taste with the similar-appearing viceroy butterfly in what is perhaps one of the most well-known examples of mimicry. Though long purported to be an example of Batesian mimicry, the viceroy is more unpalatable than the monarch, making this a case of Müllerian mimicry. == Human interaction ==

The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, Vermont, and West Virginia. Legislation was introduced to make it the national insect of the United States, but this failed in 1989 and again in 1991. Homeowners are increasingly establishing butterfly gardens; monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these monarch waystations. A 2012 IMAX film, Flight of the Butterflies, describes the story of the Urquharts, Brugger, and Trail to document the then-unknown monarch migration to Mexican overwintering areas. (Washington, D.C., October 2012) Sanctuaries and reserves have been created at overwintering locations in Mexico and California to limit habitat destruction. These sites can generate significant tourism revenue. However, with less tourism, monarch butterflies may exhibit higher survival rates, as butterflies in tourist isolated areas have shown increases in protein content, immune response and oxidative defense. Organizations and individuals participate in tagging programs. Tagging information is used to study migration patterns. The 2012 novel by Barbara Kingsolver, Flight Behavior, deals with the fictional appearance of a large population in the Appalachians. Captive rearing Humans interact with monarchs when rearing them in captivity, which has become increasingly popular. However, risks occur in this controversial activity. On one hand, captive rearing has many positive aspects. Monarchs are bred in schools and used for butterfly releases at hospices, memorial events, and weddings. Memorial services for the September 11 attacks include the release of captive-bred monarchs. Monarchs are used in schools and nature centers for educational purposes. Adult monarchs will extend their proboscises to reach people's fingertips. With assistance, they will also extend their proboscises into a dish containing artificial food. On the other hand, this practice becomes problematic when monarchs are "mass-reared". Stories in the Huffington Post in 2015 and Discover magazine in 2016 have summarized the controversy around this issue. The frequent media reports of monarch declines have encouraged many homeowners to attempt to rear as many monarchs as possible in their homes and then release them to the wild to "boost the monarch population". Some individuals, such as one in Linn County, Iowa, have reared thousands of monarchs at the same time. Some monarch scientists do not condone the practice of rearing "large" numbers of monarchs in captivity for release into the wild because of the risks of genetic issues and disease spread. One of the biggest concerns of mass rearing is the potential for spreading the monarch parasite Ophryocystis elektroscirrha into the wild. This parasite can rapidly build up in captive monarchs, especially if they are housed together. The parasite spores can quickly contaminate all housing equipment so all subsequent monarchs reared in the same containers become infected. One researcher stated that rearing more than 100 monarchs constitutes "mass rearing" and should not be done. In addition to the disease risks, researchers believe these captive-reared monarchs are not as fit as wild ones, owing to the unnatural conditions in which they are raised. Homeowners often raise monarchs in plastic or glass containers in their kitchens, basements, porches, etc., and under artificial lighting and controlled temperatures. Such conditions would not mimic what the monarchs are used to in the wild and may result in adults unsuited for the realities of their wild existence. In support of this, a recent study by a citizen scientist found that captive-reared monarchs have a lower migration success rate than wild monarchs. In that study, monarchs that were reared to adulthood in artificial conditions showed a reduction in navigational ability. This happened even with monarchs brought into captivity from the wild for a few days. A few captive-reared monarchs did show proper navigation. This study revealed the fragility of monarch development; if the conditions are not suitable, their ability to properly migrate could be impaired. The same study also examined the genetics of a collection of reared monarchs purchased from a butterfly breeder and found they were dramatically different from wild monarchs, so much so that the lead author described them as "franken-monarchs". An unpublished study in 2019 compared the behavior of captive-reared versus wild monarch larvae. The study showed that reared larvae exhibited more defensive behavior than wild larvae. The reason for this is unknown, but it could relate to reared larvae being frequently handled and/or disturbed. == Threats ==

In February 2015, the United States Fish and Wildlife Service (USFWS) reported a study that showed that nearly a billion monarchs had vanished from the butterfly's overwintering sites since 1990. The agency attributed the monarch's decline partly to a loss of milkweed caused by herbicides that farmers and homeowners had used. A 2017 report included mention of the new ethanol-in-gasoline standards as reducing the amount of acreage left fallow in the U.S. midwest: "Federal policies such as the Ethanol Fuel Standards (Renewable Fuel Standard), crop insurance, and waning Farm Bill support for the Conservation Reserve Program (CRP) reduce support for integrated agro-ecological landscapes capable of sustaining both food production and monarch habitat, principally because these policies promote row crops over mixed, herbaceous perennial vegetation." In 2018, a study correlated monarch butterfly decline to the fact that 95% of corn and soybean crops grown in the United States used genetically modified seeds resistant to the herbicide glyphosate. This meant that instead of spreading the herbicide only before seed planting, farmers could spread the herbicide a second time by air when weeds had begun to challenge the crops. Air application of the herbicide meant that the unplowed margins between the field and road that previously supported milkweed and a range of nectar flowers were now greatly diminished. By 2024, the USFWS calculated that the eastern butterflies had declined by approximately 80 percent since the 1980s. The western population was more imperiled, declining by 95 percent. According to the USFWS, the species faces a host of threats, including the loss and degradation of its breeding, migratory, and overwintering habitats, exposure to insecticides, and the growing impacts of climate change. The society's annual winter counts have shown a significant decline in the California population. The three lowest counts in the program’s history have all occurred in recent years: 1,901 individual monarchs in 2020, 9,119 in 2024, and 12,260 in 2025. One Pacific Grove site had no monarch butterflies in 2020. A primary explanation for this was the destruction of the butterfly's milkweed habitats. Eastern and midwestern monarch populations The number of monarchs overwintering in Mexico has shown a long-term downward trend. Since 1995, coverage numbers have been as high as during the winter of 1996–1997, but on average about . Coverage declined to its lowest point to date () during the winter of 2013–2014, but rebounded to in 2015–2016. The average population of monarchs in 2016 was estimated at 200million. Historically, on average there are 300million monarchs. The 2016 increase was attributed to favorable breeding conditions in the summer of 2015. However, coverage declined by 27% to during the winter of 2016–2017. Some believe this was because of a storm that had occurred during March 2016 in the monarchs' previous overwintering season. However, this seems unlikely since most current research shows that the overwintering colony sizes do not predict the size of the next summer breeding population. A 2016 study attributed the previous decade's 90% decline in overwintering numbers of the eastern monarch population primarily to the loss of breeding habitat and milkweed. The publication's authors stated that an 11%–57% probability existed that this population will become "quasi-extinct" over the next 20 years (i.e. unable to sustain a stable population). Other threats identified in the study include climate change, insecticides, and disease. Scientists found that monarchs occupied of Mexican forest during the winter of 2025-2026, a 64% increase over the previous winter. Chip Taylor, the director of Monarch Watch at the University of Kansas, stated in 2013 that the Midwest milkweed habitat "is virtually gone" with 120–150million acres lost. However, he predicted in 2024 that in the immediate future and perhaps into the next two decades, the eastern monarch butterfly population will be relatively stable because it is not presently on a continuous downward trend as it was from 2000-2006. Habitat loss due to herbicide use and genetically modified crops Declines in milkweed abundance and monarch populations between 1999 and 2010 are correlated with the adoption of herbicide-tolerant genetically modified (GM) corn and soybeans, which now constitute 89% and 94% of these crops, respectively, in the U.S. In 2015, the Natural Resources Defense Council filed a suit against the United States Environmental Protection Agency (EPA). The Council argued that the agency ignored warnings about the dangers of glyphosate usage for monarchs. However, a 2018 study has suggested that the decline in milkweed predates the arrival of GM crops. Losses during migration Eastern and midwestern monarchs are experiencing problems reaching Mexico. Many monarch researchers have cited recent evidence from long-term citizen science data that shows that the number of breeding (adult) monarchs has not declined in the last two decades. The lack of long-term declines in the numbers of breeding and migratory monarchs, yet the clear declines in overwintering numbers, suggests a growing disconnect exists between these life stages. One researcher has suggested that mortality from car strikes constitutes an increasing threat to migrating monarchs. A study of road mortality in northern Mexico, published in 2019, showed very high mortality from just two "hotspots" each year, amounting to 200,000 monarchs killed. Importance of overwintering habitat The area of Mexican forest to which eastern and midwestern monarchs migrate reached its lowest level in two decades in 2013. The decline was expected to increase during the 2013–2014 season. Mexican environmental authorities continue to monitor illegal logging of the oyamel fir trees; however, organized criminals have repeatedly crushed such efforts in the name of very short-term financial gain. The oyamel is a major species of evergreen on which the overwintering butterflies spend a significant time during their winter diapause, or suspended development. A 2014 study acknowledged that while "the protection of overwintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America", their research indicates that habitat loss on breeding grounds in the United States is the main cause of recent and projected population declines. Western monarch populations have rebounded slightly since 2014 with the Western Monarch Thanksgiving Count tallying 335,479 monarchs in 2022. The population still has much to go for a full recovery. Parasites '' on monarch chrysalis Parasites include the tachinid flies Sturmia convergens, Compsilura concinnata, Madremyia saundersii, Pteromalid wasps, specifically Pteromalus cassotis, parasitize monarch pupae. These wasps lay their eggs in the pupae while the chrysalis is still soft. Up to 400 adults emerge from the chrysalis after 14–20 days, It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the bodies of infected butterflies, with the greatest number on the abdomen. These spores are passed, from female to caterpillar, when spores rub off during egg laying and are then ingested by caterpillars. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. This is not the case in laboratory rearing, where after a few generations, all individuals can be infected. Infection with O. elektroscirrha creates an effect known as culling, whereby migrating monarchs that are infected are less likely to complete the migration. This results in overwintering populations with lower parasite loads. Owners of commercial butterfly-breeding operations claim that they take steps to control this parasite in their practices, although this claim is doubted by many scientists who study monarchs. Confusion of host plants The black swallow-wort (Cynanchum louiseae) and pale swallow-wort (Cynanchum rossicum) plants are problematic for monarchs in North America. Monarchs lay their eggs on these relatives of native vining milkweed (Cynanchum laeve) because they produce stimuli similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant from Europe. Climate Climate variations during the fall and summer affect butterfly reproduction. Rainfall and freezing temperatures affect milkweed growth. Omar Vidal, director general of WWF-Mexico, said, "The monarch's lifecycle depends on the climatic conditions in the places where they breed. Eggs, larvae, and pupae develop more quickly in milder conditions. Temperatures above can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate." If a monarch's body temperatures is below , a monarch cannot fly. To warm up, they sit in the sun or rapidly shiver their wings to warm themselves. Climate change may dramatically affect the monarch migration. A study from 2015 examined the impact of warming temperatures on the breeding range of the monarch and showed that in the next 50 years, the monarch host plant will expand its range further north into Canada and that the monarchs will follow this. While this will expand the breeding locations of the monarch, it will also have the effect of increasing the distance that monarchs must travel to reach their overwintering destination in Mexico, which could result in greater mortality during the migration. Milkweeds grown at increased temperatures have been shown to contain higher cardenolide concentrations, making the leaves too toxic for the monarch caterpillars. However, these increased concentrations are likely in response to increased insect herbivory, which is also caused by the increased temperatures. Whether increased temperatures make milkweed too toxic for monarch caterpillars when other factors are not present is unknown. Additionally, milkweed grown at carbon dioxide levels of 760 parts per million was found to produce a different mix of the toxic cardenolides, one of which was less effective against monarch parasites. == Conservation status ==

On July 20, 2022, the International Union for Conservation of Nature added the migratory monarch butterfly (the subspecies common in North America) to its red list as an endangered species. However, a petition in 2023 resulted in its status being changed to "vulnerable". On December 17, 2020, the USFWS published in the Federal Register a notice in which it stated that adding the butterfly to the list of threatened and endangered species was "warranted-but-precluded" because budgetary limitations required it to devote its resources to species with higher priorities for listing. The notice stated that the USFWS had 422 12-month petition findings for domestic species yet to be initiated and completed at the beginning of Fiscal Year 2020 (October 1, 2019). On June 27, 2023, the USFWS published in the Federal Register a Notice of Proposed Rulemaking for a rule that would give the butterfly a listing priority number (LPN) of 8 for adding its species to that list. LPNs range from 1 to 12 (the lower the LPN, the higher the listing priority). On December 12, 2024, the USFWS published in the Federal Register a proposed rule that would list the monarch butterfly (Danaus plexippus) as a threatened species and would designate the butterfly's critical habitat per the provisions of the Endangered Species Act. The USFWS estimated in the proposed rule that the probability of extinction in the foreseeable future (60 years) is 56-74 percent for the eastern monarch migratory population and 99 percent for the western migratory population. The proposed rule designated seven areas near California's Pacific coast as "critical habitat units" for monarch butterflies. The USFWS accepted comments on the proposed rule until March 12, 2025. On March 19, 2025, the USFWS reopened the comment period on the proposed rule until May 19, 2025. In December 2023, the Government of Canada listed the monarch as an endangered species under the federal Species at Risk Act. The listing protects the butterfly on Canadian federal lands by making it illegal to kill, hurt, catch or remove a monarch egg, caterpillar, chrysalis or adult when on land that the Canadian federal government owns and/or administers. These include Canadian national parks, national wildlife areas, military bases and First Nations reserves. It is also illegal to possess, collect, buy, sell, or trade an individual monarch or any part or derivative of an individual when on that land. The Canadian government must prepare a recovery strategy and one or more action plans for the species to outline the work that can be done to conserve the species. The strategy must identify the critical habitat necessary for the monarch to survive and recover in Canada. In Ontario, Canada, the monarch butterfly is listed as a species of special concern. Two recent studies have been conducted examining long-term trends in monarch abundance in Canada, using either butterfly atlas records or citizen science butterfly surveys, and neither shows evidence of a population decline in Canada. ==Conservation efforts==

Although the numbers of breeding monarchs in eastern North America have not decreased, reports of declining numbers of overwintering butterflies have inspired efforts to conserve the species. In May 2015, the Pollinator Health Task Force issued a "National Strategy to Promote the Health of Honey Bees and Other Pollinators". The strategy laid out federal actions to achieve three goals, two of which were: Many of the priority projects that the National Strategy identified focused on the I-35 corridor, which extends for from Texas to Minnesota. The area through which that highway travels provides spring and summer breeding habitats in the United States' key monarch migration corridor. In May 2015, the U.S. Department of Agriculture (USDA) and the U.S. Department of the Interior (USDI) issued a 52-page document entitled "Pollinator-Friendly Best Management Practices for Federal Lands". The document consolidated general information about the practices and procedures to use when considering pollinator needs in project development and management of Federal lands that are managed for native diversity and multiple uses. The document also contained a series of actions to be considered when determining those lands best suited for restoration and rehabilitation of monarch habitat. These included an assurance that native wildflowers are available, diverse, and abundant to provide nectar for monarchs and an assurance that milkweed species that female monarchs prefer for egg laying are available or will be planted. The document identified those milkweed species for each of seven regions within the United States. On December 4, 2015, President Obama signed into law the Fixing America's Surface Transportation (FAST) Act (Pub. L. 114-94). The FAST Act placed a new emphasis on efforts to support pollinators. To accomplish this, the FAST Act amended Title 23 (Highways) of the United States Code. The amendment directed the United States Secretary of Transportation when carrying out programs under that title in conjunction with willing states, to: • encourage integrated vegetation management practices on roadsides and other transportation rights-of-way, including reduced mowing; and • encourage the development of habitat and forage for Monarch butterflies, other native pollinators, and honey bees through plantings of native forbs and grasses, including noninvasive, native milkweed species that can serve as migratory way stations for butterflies and facilitate migrations of other pollinators. The FAST Act also stated that activities to establish and improve pollinator habitat, forage, and migratory way stations may be eligible for Federal funding if related to transportation projects funded under Title 23. In June 2016, the Pollinator Health Task Force issued a "Pollinator Partnership Action Plan". That Plan provided examples of past, ongoing, and possible future collaborations between the federal government and non-federal institutions to support pollinator health under each of the National Strategy's goals. The USDA's Farm Service Agency helps increase U.S. populations of the monarch butterfly and other pollinators through its Conservation Reserve Program's State Acres for Wildlife Enhancement (SAFE) Initiative. The SAFE Initiative provides an annual rental payment to farmers who agree to remove environmentally sensitive land from agricultural production and plant species that will improve environmental health and quality. Among other things, the initiative encourages landowners to establish wetlands, grasses, and trees to create habitats for species that the USFWS has designated to be threatened or endangered. As part of its targeted monarch butterfly effort, the USDA's Natural Resources Conservation Service (NRCS) works with agricultural producers in the midwest and southern Great Plains to combat the decline of monarch butterflies by planting milkweed and other nectar-rich plants on private lands. The NRCS also provides region-specific guides and plant lists that support populations of monarch butterflies and other pollinators in the Greater Appalachian Mountains Region, the Midwest Region, the Northern and Southern Great Plains, the Southern High Plains and the Western Coastal Plain. In January 2024, staff of the National Capital Planning Commission (the federal government's planning agency for the National Capital Region) introduced a "Pollinator Best Practices Resource Guide". The Guide summarizes existing federal guidance from the Council on Environmental Quality, the USDA, and the GSA regarding best practices for creating pollinator habitats in both meadow and non-meadow/designed landscapes. When discussing milkweeds, the Guide states that although more than 100 species of such plants are considered native to North America, Asclepias syriaca, or common milkweed, stands out and "is clearly an important species that is critical to the survival of monarch butterflies". Other actions Agriculture companies and other organizations are being asked to set aside unsprayed areas to allow monarchs to breed. In addition, national and local initiatives are underway to help establish and maintain pollinator habitats along corridors containing power lines and roadways. The Federal Highway Administration, state governments, and local jurisdictions are encouraging highway departments and others to limit their use of herbicides, to reduce mowing, to help milkweed to grow, and to encourage monarchs to reproduce within their right-of-ways. State and local governments A Scientific Collecting Permit (SCP) is required to handle wild monarchs in California including for educational purposes. It is unlawful to collect, remove from the wild and/or captively rear monarchs in that state without an SCP. As of June 2025, at least 14 states (California, Connecticut, Illinois, Kentucky, Maryland, Massachusetts, Minnesota, Missouri, New Jersey, New Mexico, New York, North Carolina, Ohio, Vermont and Washington) had enacted legislation to protect, develop and restore habitat suitable for pollinators. For example, the New Jersey General Assembly considered legislation in 2014 that was intended to increase the monarch butterfly's migrating population within the state. New Jersey's government subsequently approved legislation in May 2017 that established an "Integrated Roadside Vegetation Management Program". Among other things, the legislation required the state's transportation department to develop and adopt a plan for "cost effective maintenance and planting along roadsides, with an emphasis on adaptable vegetation with long life cycles, native vegetation, and wildflowers". In January 2018, the New Jersey government approved an "Adopt a Monarch Butterfly Waystation Act" and a "Milkweed for Monarchs Act". The latter act required the state's environmental protection department to establish criteria for the planting of milkweed in stormwater management basins on state-owned lands and to take related actions. In June 2019, Tennessee's transportation department announced a partnership with the state's environment and conservation and agriculture departments to promote pollinator health and awareness in the state's parks. The partners expected to install pollinator meadows and interpretive signage in nine such parks. Virginia's wildlife resources department encourages plantings of native species that are highly beneficial to pollinators. These include forbs such as Asclepias spp. (milkweeds), Pycnanthemum spp. (mountain mints), those in the Asteraceae family (which includes Coreopsis spp.), Helianthus spp. (sunflowers), Rudbeckia spp. (coneflowers), Solidago spp. (goldenrods), and Eutrochium spp. (Joe-Pye weeds), as well as shrubs such as Prunus americana (American plum), Cercis spp. (redbuds), Vaccinium spp. (blueberries), and Samubucus spp. (elderberries). In April 2025, the Chicago Park District's general superintendent launched "Project Monarch", a citywide initiative to conserve and celebrate monarch butterflies and their habitats. In addition, many local jurisdictions throughout the United States have made commitments to increase monarch habitat by joining "Monarch City USA" or by taking the National Wildlife Federation's "Mayors' Monarch Pledge" and planting native milkweeds and other nectar plants within their boundaries. Many cities, towns, counties, and communities have joined “Bee City USA” (an initiative of the Xerces Society for Invertebrate Conservation) and made commitments to conserve native bees, butterflies and other pollinators by increasing the abundance of native plants, providing nesting sites, and reducing the use of pesticides. Molesting or interfering in any way with monarch butterflies during their annual migratory visit to the city of Pacific Grove, California, carries a fine of $1,000 except under defined circumstances. National Cooperative Highway Research Program report In 2020, the National Cooperative Highway Research Program (NCHRP) of the Transportation Research Board issued a 208-page report that described a project that had examined the potential for roadway corridors to provide habitat for monarch butterflies. A part of the project developed tools for roadside managers to optimize potential habitats for monarch butterflies in their road rights-of-way. Such efforts are controversial because the risk of butterfly mortality near roads is high. Several studies have shown that motor vehicles kill millions of monarchs and other butterflies annually. The NCHRP report acknowledged that, among other hazards, roads present a danger of traffic collisions for monarchs, stating that these effects appear to be more concentrated in particular funnel areas during migration. Nevertheless, the report concluded: Butterfly gardening and monarch waystations in Prince George's County, Maryland (June 2017) The practice of butterfly gardening and creating "monarch waystations" is commonly thought to increase the populations of butterflies. Efforts to restore falling monarch populations by establishing butterfly gardens and monarch waystations require particular attention to the butterfly's food preferences and population cycles, as well to the conditions needed to propagate and maintain milkweed. For example, in the Washington, D.C., area and elsewhere in the northeastern and midwestern United States, common milkweed (Asclepias syriaca) is among the most important food plants for monarch caterpillars. Within its range it can be found in a broad array of habitats from croplands to pastures, roadsides, ditches and old fields. The plant typically grows to a height of 3-5 feet (0.9-1.5 m), but can reach 8 feet (2.4 m) in ditches and gardens. However, monarchs prefer to lay eggs on A. syriaca when its foliage is soft and fresh. Because monarch reproduction peaks in those areas during the late summer when milkweed foliage is old and tough, A. syriaca needs to be mowed or cut back in June through August to ensure that it will be regrowing rapidly when monarch reproduction reaches its peak. Similar conditions exist for showy milkweed (A. speciosa) in Michigan and for green antelopehorn milkweed (A. viridis), where it grows in the Southern Great Plains and the Western United States. Further, the seeds of A. syriaca and some other milkweeds need periods of cold treatment (cold stratification) before they will germinate. To protect seeds from washing away during heavy rains and from seed-eating birds, one can cover the seeds with a light fabric or with an layer of straw mulch. However, mulch acts as an insulator. Thicker layers of mulch can prevent seeds from germinating if they prevent soil temperatures from rising enough when winter ends. Further, few seedlings can push through a thick layer of mulch. Although monarch caterpillars will feed on butterfly weed (A. tuberosa) in butterfly gardens, it is typically not a heavily used host plant for the species. The plant has rough leaves and a layer of trichomes, which may inhibit oviposition or decrease a female's ability to sense leaf chemicals. The plant's low levels of cardenolides may also deter monarchs from laying eggs on the plant. While A. tuberosa colorful flowers provide nectar for many adult butterflies, the plant may be less suitable for use in butterfly gardens and monarch waystations than are other milkweed species. However, A. incarnata is an early successional plant that usually grows at the margins of wetlands and in seasonally flooded areas. The plant is slow to spread via seeds, does not spread by runners, and tends to disappear as vegetative densities increase and habitats dry out. Although A. incarnata plants can survive up to 20 years, most live only two to five years in gardens. The species is not shade-tolerant and is not a good vegetative competitor. == See also ==