The monarch is one of the best known butterfly species. Although native to North and South America, it has expanded its distribution to Australia, many Pacific islands, and a few places in Europe. Monarch larvae, like those of many other Danaini, feed on milkweeds of the family Asclepiadaceae, from which they sequester cardiac glycosides that often render them unpalatable to potential predators. The species is perhaps best known for its annual migrations, from eastern North America to the mountains of central Mexico, and from western North America to the coast of California. The butterflies rest for the cool winter months in large aggregations which may contain tens of millions of individuals.
Left: Monarch larva feeding on milkweed host plant at Poco Sol (Caribbean slope below Monteverde), Costa Rica. Right: Overwintering monarchs covering a tree branch at Alpha Monarch research site, Mexico. Images © Greg and Marybeth Dimijian
North American Ecology (US and Canada)
- Scott, J. A. 1986. The butterflies of North America. Stanford University Press.
Monarch butterflies are found in North America, South America, the Caribbean, Australia, New Zealand, islands in the Pacific Ocean, Mauritius, the Canary Islands of the Atlantic, and Western Europe.
Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) The Monarch butterfly is attaining near worldwide distribution in suitable places, but is primarily a species of the Americas. Autumnal migrants occasionally stray to Europe including England; the species has been spread widely in the Pacific Ocean area and is well-established in Australia. However, the seasonal ranges in North America are vastly different, by millions of square kilometers. Essential overwintering areas for North American populations are limited to a few dozen places in coastal California and the mountains of Mexico. Specifically that means that the summer range includes portions of up to 49 of the 50 US states and southern portions of all Canadian Provinces bordering the US where milkweeds occur. However the winter range includes none of Canada and in the US only a few dozen square kilometers in California, and at least some years a bit of south Florida. Florida winter populations may be non-migratory and if so contribut little to summer populations. Some winters a few may persist in sothern Arizona.
Both male and female monarchs are bright orange with black borders and black veins. The veins on the female are thicker than those of the male. Male monarchs also have a swollen pouch on both of their hind wings.
Monarchs are poisonous to vertebrates. Their poison comes from the milkweed they feed on.
Monarchs also use their appearance to ward off predators. Orange is considered a warning color, which will warn predators that monarchs are poisonous, and not to attack them. From a distance, monarchs can blend into their surroundings. Sometimes, their spots will appear to be the eyes of a larger animal, and will ward off predators.
Range wingspan: 8.6 to 12.4 cm.
Other Physical Features: ectothermic ; bilateral symmetry ; poisonous
Monarch butterflies have a wingspan of 3-4 1/2 inches (75-114 mm). The male is bright orange and has a black scent patch in the middle of the upper hind wing. The female is dull orange or brown with more thickly scaled black veins. The underside of the hind wing is light orange with black veins and a marginal black border with two rows of white spots. The upper front wing has a black apex (tip) with white spots. The sub-apex has light orange spots on a dark background. The underside of the front wing has white spots and yellow ovals in the outer portion.
- Opler, P. A. and A. Bartlett Wright. 1999. Western Butterflies. Peterson Field Guides. Houghton Mifflin Co., Boston, New York.
- Stewart, B., P. Brodkin, and H. Brodkin. 2001. Butterflies of Arizona. A Photographic Guide. West Coast Lady Press, Arcata, CA.
Monarch butterflies prefer warmer climates; they cannot tolerate frost. They like open country. Females spend much of their time around searching for or staying near the main food plant for the caterpillars (see food habits section). Monarch butterflies require thick tree covering during the winter. In California, they live in eucalyptus trees. These eucalyptus trees are not normally found in California, but were put there to replace trees that had been cut down. Biomes: temperate forest and rainforest, temperate grassland, chaparral, tropical rainforest, tropical deciduous forest, tropical scrub forest, tropical savanna and grasslands, mountains
Comments: Habitat is a complex issue for this species. In general breeding areas are virtually all patches of milkweed in North America and some other regions. The critical conservation feature for North American populations is the overwintering habitats, which are certain high altitude Mexican conifer forests or coastal California conifer or Eucalyptus groves as identified in literature. It appears virtually all North American monarchs overwinter in one of these two areas. Lethal cold would preclude successful overwintering in places like the Gulf Coast and much of Florida some years and it appears thse are not major wintering regions as used to be assumed. In addition certain major coastal migratory stopovers may be important conservation sites especially those in along Delaware Bay in New Jersey including Cape May where adults may holdover for several days awaiting suitable conditions for crossing the Bay. There are major, but probably less important, roosting sites farther north such as east of New Haven and probably others farther south perhaps even as far as Cuba. Coastal regions are important flyways and so nectar (wild or in gardens) is an important reseource in such places. In places like Hawaii and some others the species simply breeds year round and is not really migratory.
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Monarch females lay their eggs on milkweed plants (Asclepias), these plants are then the main food for the caterpillars. Adult monarchs feed on nectar from many kinds of flowers.
Comments: Larval foodplants are milkweeds.
Flowering Plants Visited by Danaus plexippus in Illinois
(observations are from Robertson, Graenicher, Betz et al., Reed, Hilty, Clinebell, Smith & Snow, Hapeman, Macior, Broyles & Wyatt, and Willson & Bertin; this is the Monarch butterfly)
Apiaceae: Eryngium yuccifolium sn (Rb), Pastinaca sativa sn (Rb), Sium suave sn (Rb); Asclepiadaceae: Ampelamus albidus [plup sn] (H), Asclepias exaltata [plup sn] (BW), Asclepias incarnata [plpr sn] [plup sn] (Rb, Btz), Asclepias syriaca [plpr sn] (Rb), Asclepias tuberosa [plab] [plup] (Rb, Btz, H), Asclepias verticillata [plpr sn] [plup sn] (Rb, WB); Asteraceae: Aster drummondii sn (Gr), Aster furcatus sn (Gr), Aster laevis sn (Gr), Aster lanceolatus sn (Gr), Aster macrophyllus sn (Gr), Aster novae-angliae sn (Rb, Gr, H), Aster pilosus sn (Rb), Aster puniceus sn (Gr), Aster salicifolius sn (Rb), Bidens aristosa sn (Rb), Bidens cernua sn fq (Rb), Boltonia asterioides sn (Rb), Cirsium altissimum sn (Gr), Cirsium discolor sn (Rb, H), Cirsium hillii sn (Rb), Cirsium vulgare sn (Rb, Gr), Conoclinium coelestinum sn (Rb), Coreopsis tripteris sn (Rb), Echinacea pallida sn (Rb), Echinacea purpurea sn (Rb, H), Eupatoriadelphus purpureus sn (Rb, Gr, H), Eupatorium altissimum sn (H), Eupatorium perfoliatum sn (Rb, Gr), Euthamia graminifolia sn (Gr), Helianthus annuus sn (Rb, H), Helianthus grosseserratus sn fq (Rb), Helianthus mollis sn (H), Helianthus strumosus sn (Gr), Heliopsis helianthoides sn (Gr), Liatris aspera sn (Rb, H), Liatris pycnostachya sn (Rb), Liatris spicata sn (Gr), Oligoneuron rigidum sn (H, Re), Rudbeckia laciniata sn (Gr), Silphium integrifolium sn (H), Silphium laciniatum sn (Rb), Silphium perfoliatum sn (Rb), Silphium terebinthinaceum sn (H), Solidago canadensis sn (Rb, Gr), Solidago nemoralis sn (Rb), Vernonia fasciculata sn (Rb); Boraginaceae: Mertensia virginica sn (Rb); Campanulaceae: Lobelia siphilitica sn (Rb); Caprifoliaceae: Symphoricarpos albus sn (Gr), Symphoricarpos occidentalis sn (Gr); Dipsacaceae: Dipsacus fullonum sn (Rb); Fabaceae: Melilotus alba sn (Rb), Robinia pseudoacacia sn np (Rb), Trifolium pratense sn (Rb); Lamiaceae: Agastache nepetoides sn (Rb), Blephilia hirsuta sn (Rb), Monarda fistulosa sn (Rb, Cl), Physostegia virginiana sn np (Rb), Salvia azurea sn np (H); Liliaceae: Lilium michiganense sn (Rb); Orchidaceae: Platanthera blephariglottis sn (SS), Platanthera ciliaris sn (SS), Platanthera peramoena sn (Hpm); Polemoniaceae: Phlox divaricata laphamii sn (Rb), Phlox glaberrima sn (Rb); Polygonaceae: Persicaria pensylvanica sn (H); Ranunculaceae: Delphinium tricorne sn np (Rb, Mc); Rosaceae: Crataegus crus-galli sn (Rb), Crataegus intricata sn (Rb), Malus coronaria sn (Rb), Prunus americana sn (Rb), Prunus serotina [flwr sn] (Rb); Rubiaceae: Cephalanthus occidentalis sn fq (Rb); Verbenaceae: Verbena stricta sn (Rb)
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: > 300
Comments: This refers to global occurrences for the species as a whole and includes introduced populations in Hawaii, Australia etc., as well as central and South America. In these places and where non-migratory native populations occur, habitat may be more or less used all year. North American populations also have a lot of breeding habitat, most of it patchy and suboptimally managed for Monarch survival, but still producing millions of adults each summer. These face an annual bottleneck in terms of dependence on a few dozen overwintering areas. Western North American populations may also be under pressure from scarcity of good breeding sites, especially in dry years, and incompatible management.
Comments: At least millions of individuals, probably more than a billion in many years, worldwide.
Can contain poison derived from their larval foodplant that helps avoid predation by birds (Scott, 1986).
Life History and Behavior
- Scott, J. A. 1986. The butterflies of North America. Stanford University Press.
Small caterpillars hatch from eggs laid by female Monarchs. They grow, shedding their skin to get bigger. Eventually each caterpillar stops growing and forms a case around itself called a chrysalis. Inside the chrysalis it changes its body its body in a process called metamorphosis. When it is done it emerges as an adult butterfly.
Development - Life Cycle: metamorphosis
The mating period occurs in the spring, just prior to migration from the overwintering sites. The courtship of Danaus plexippus is fairly simple and less dependent on chemical pheromones in comparison with other species in its genus. Courtship is composed of two distinct stages, the aerial phase and the ground phase. During the aerial phase, the male pursues, nudges, and eventually takes down the female. Copulation occurs during the ground phase and involves the transfer of a spermatophore from the male to female. Along with sperm, the spermatophore is thought to provide the female with energy resources that aid her in carrying out reproduction and remigration.
Breeding interval: Monarch butterflies mate in the spring before they migrate.
Key Reproductive Features: semelparous ; sexual ; fertilization (Internal ); oviparous
In Baja California may be encountered throughout the year (November-April in coastal regions and June-October in the Mountains) (Brown et al., 1992)
Molecular Biology and Genetics
Barcode data: Danaus plexippus
There are 39 barcode sequences available from BOLD and GenBank. Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species. See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Danaus plexippus
Public Records: 19
Specimens with Barcodes: 75
Species With Barcodes: 1
The places where monarchs spend the winter are threatened. The United States and Mexico are trying to help the areas where monarch butterflies migrate.
IUCN Red List of Threatened Species: no special status
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: The Monarch is globally secure, because it is doing well in many places where populations are not native and/or not strongly migratory, which drives the G5 rank. However, this is misleading in North America where the Monarch is no longer secure because of serious threats to their obligate overwintering areas in Mexico (mostly) and and a recent order of magnitude decline in California based population, which apparently reflects threats in the western breeding range. Threats also exist in the eastern spring migration route (especially in Texas). The migratory North American populations have been declining in recent years with 2009 among the worst ever (2010 data not evaluated) for California winter population. Deteriorating spring climate conditions in Texas appear to be a major contributor to recently declining numbers in the eastern US in the 2000s. Western North American populations are probably more threatened than eastern ones because suitable milkweeds are less reliable in generally more arid western regions in dry periods. Rank Calculator v3.0 is also G5. Regardless of the fate of North american populations, the species as a whole is globally secure.
Environmental Specificity: Broad. Generalist or community with all key requirements common.
Comments: Very broad as a breeder everywhere and globally overall, but very narrow for overwintering populations in North America.
Other Considerations: Elimination of Mexican sites would mean virtual extinction of eastern North American populations. Nearly all western individuals had been thought to winter in California, but some of these also go to th mountains of Mexico. This species has become a significant ecotourism resource locally in Mexico.
National NatureServe Conservation Status
Rounded National Status Rank: N4B - Apparently Secure
Rounded National Status Rank: N5B,N2N : N5B: Secure - Breeding, N2N: Imperiled - Nonbreeding
Global Short Term Trend: Increase of 10-25% to decline of 30%
Comments: Difficult to assess globally. Stable to increasing in many places where it is not native and in some Neotropical places where it is. North American populations, which are still a large portion of the global total, are well documented to be declining, but fluctuate from year to year. California based populations may be down more than 90% from 1997-2009.
Global Long Term Trend: Increase of >25% to decline of 50%
Comments: See short term.
Degree of Threat: B : Moderately threatened throughout its range, communities provide natural resources that when exploited alter the composition and structure of the community over the long-term, but are apparently recoverable
Comments: The species as a whole is not seriously threatened, but the typical subspecies may be secure mostly where it is not native. The monaach appears to be doing well or even increasing in may places, but not in its core North American range. The native eastern North American populations are in trouble at their overwintering grounds. Overwintering habitats in Mexico are primarily in a few hectares each and have been under pressure from logging, agricultural and urban development. California based populations are declining more dues to problems in the breeding areas. Climate change is emerging as a current, rather than an expected, threat in Mexico, California, and Texas at least. There was some improvement in the eastern, as well as western, USA in 2010 so at least populations can still respond quickly to temporarily good conditions. Regardless of what happens in North America this species is at virtually no risk of global extinction in the forseeable future.
Biological Research Needs: The basics of Monarch biology, migration, etc. are now rather well known, although some of the details regarding western US populations may still need resolution. Main needs now appear to be understanding impacts of and how to prevent or mitigate habitat changes and climate change primarily in overwintering areas and during spring migration particularly in Texas. Accurate information for Central and South America needs to be compiled, because this is globally significant to the species even if unrelated to its status in North America.
Global Protection: Unknown whether any occurrences are appropriately protected and managed
Comments: Too difficult to assess because needs vary so widely with local ecology.
Needs: Winter habitats in California and Mexico need protection from logging, development, etc. This need has been known for decades. In the near term mitigation of climate change impacts could become an urgent need.
Relevance to Humans and Ecosystems
Economic Importance for Humans: Positive
Some milkweed plants are poisonous to cattle. Monarch butterflies help reduce the amount of these plants.
Compared to bees, butterflies are often less efficient at transferring pollen between plants because pollen often does not stick to their bodies and they lack specialized structures for collecting pollen. However, on the occasions in which pollen does stick to the butterflies' bodies, it is often inadvertently transferred to another flower while the butterflies are nectaring, earning them the title of pollinators.
Research has shown that most butterfly species do indeed end up pollinating some of the flowers they visit, and because monarch butterflies have such a long migration route they are potential pollinators to plants across the country! Studies have found that monarch butterflies do carry pollen from swamp milkweed (Asclepias incarnata) and are, therefore, potential pollinators. Monarch butterflies are known pollinators of common milkweed (Asclepias syriaca), butterfly weed (Asclepias tuberosa), and rattlesnake master (Eryngium yuccifolium). Additionally, monarch butterflies feed on the nectar of many other plants, like rabbitbrush (Ericameria nauseosa), desert broom (Baccharis spp.), aster (Aster spp.), sunflower (Helianthus spp.), goldenrod (Solidago spp.), verbena (Verbena spp.), clover (Trifolium spp.), and other milkweed species (Asclepias spp.).
Although adult monarchs feed on the nectar of many flowers, they will breed only where milkweed plants are found. That is because monarch larvae feed exclusively on milkweed plants. Milkweed plants contain compounds called cardenolide alkaloids that are toxic to most vertebrate herbivores if ingested. However, while monarch larvae feed on milkweed plants they also ingest the toxins. They then sequester the compounds in their bodies, wings, and exoskeletons, making both the larvae and adult butterflies toxic to many predators.
- The Monarch Butterfly in North America, US Forest Service
- Diversity and temporal change in the effective pollinators of Asclepias tuberosa, D. L. Venable, Ecology, Vol. 77, No. 4, 1996, p. 1061-1073, Pollinator Conservation Digital Library
- Migratory Pollinators Program: Monarch Butterfly (Danaus plexippus), Arizona-Sonora Desert Museum
- Reproductive biology of Eryngium yuccifolium (Apiaceae), a prairie species, Brenda Molano-Flores, Journal of the Torrey Botanical Society, Vol. 128, No. 1, 2001, p. 1-6, Pollinator Conservation Digital Library
The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all North American butterflies. Since the 19th century, it has been found in New Zealand, and in Australia since 1871, where it is called the wanderer. It is resident in the Canary Islands, the Azores, and Madeira, and is found as an occasional migrant in Western Europe and a rare migrant in the United Kingdom. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in). (The viceroy butterfly is similar in color and pattern, but is markedly smaller, and has an extra black stripe across the hind wing.) Female monarchs have darker veins on their wings, and the males have a spot called the androconium in the center of each hind wing. Males are also slightly larger than female monarchs.
The monarch is famous for its southward migration and northward return in summer from Canada to Mexico and Baja California which spans the life of three to four generations of the butterfly. The migration route was fully determined by Canadian entomologists Fred and Norah Urquhart after a 38-year search, aided by naturalists Kenneth C. Brugger and Catalina Trail who solved the final piece of the puzzle by identifying the butterflies' overwintering sites in Mexico. The discovery has been called "entomological discovery of the 20th century". An IMAX film, Flight of the Butterflies, tells the story of the long search by the Urquharts, Brugger and Trail to unlock the secret of the butterflies' migration.
The common name “monarch” was first published in 1874 by Samuel H. Scudder because “it is one of the largest of our butterflies, and rules a vast domain”; however, the name may be in honour of King William III of England.
The monarch was one of the many species originally named by Linnaeus in his Systema Naturae of 1758. It was first placed in the genus Papilio. In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus; Danaus.
The monarch is closely related to two very similar species which formed the Danaus (Danaus) subgenus before 2005. The first is the Jamaican monarch (D. cleophile) from Jamaica and Hispaniola. The second is the southern monarch (D. erippus), of South America south of the Amazon river. The southern monarch is almost indistinguishable from the monarch as an adult, though the pupae are somewhat different, and is often considered a subspecies of the monarch proper. But analysis of morphological, mtDNA 12S rRNA, cytochrome c oxidase subunit I, nuclear DNA 18S rRNA and EF1 subunit α sequence data by Smith et al. (2005) indicates it is better considered a distinct species. The separation of the monarch and southern monarch is comparatively recent. In all likelihood, the ancestors of the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. At the time, sea levels were higher and the entire Amazonas lowland was a vast expanse of brackish swamp that offered hardly any butterfly habitat.
Following the review of Smith et al. (2005), two subspecies of the monarch are recognized:
- D. p. plexippus, the nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America.
- D. p. megalippe, named in 1826 by Jacob Hübner, is a nonmigratory subspecies, and is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River. Three local forms were at first considered to be other subspecies, but are actually colour varieties of D. p. megalippe:
- D. p. m. forma leucogyne, named by Arthur G. Butler in 1884
- D. p. m. forma portoricensis, named in 1941 by A.H. Clark
- D. p. m. forma tobagi, also named in 1941 by A.H. Clark
- D. p. nigrippus, named in 1909 by Richard Haensch as forma: Danais [sic] archippus f. nigrippus, and is found in South America south of Nicaragua
Origin of name 
In Homeric Greek plexippos (πληξιππος) means "one who urges on horses", i.e. "rider or charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467, Linnaeus wrote that the names of the Danai festivi, the division of the genus to which Papilio plexippus belonged, were derived from the sons of Aegyptus.
The monarch’s wingspan ranges from 8.9–10.2 cm (3½–4 in). The upper side of the wings is tawny-orange, the veins and margins are black, and in the margins are two series of small white spots. The fore wings also have a few orange spots near the tip. The underside is similar, but the tip of the fore wing and hind wing are yellow-brown instead of tawny-orange and the white spots are larger.
The male has a black patch of androconial scales on either hind wing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs), and the black veins on its wing are narrower than the female’s. The male is also slightly larger.
A color variation has been observed in Australia, New Zealand, Indonesia and the United States as early as the late 19th century. Named nivosus by lepidopterists, it is grayish-white in all areas of the wings that are normally orange. Generally, it is only about 1% or less of all monarchs, but has maintained populations as high as 10% on Oahu in Hawaii, possibly due to selective predation.
Like all insects, the monarch has six legs, but uses the four hindlegs as it carries its two front legs against its body.
The eggs are creamy white and later turn pale yellow. They are elongated and subconical, with about 23 longitudinal ridges and many fine traverse lines. A single egg weighs about 0.46 mg (0.0071 gr), and measures about 1.2 mm (47 mils) high and 0.9 mm (35 mils) wide.
The caterpillar is banded with yellow, black, and white stripes. The head is also striped with yellow and black. Two pairs of black filaments are seen, one pair on each end of the body. The caterpillar will reach a length of 5 cm (2 in).
The chrysalis is blue-green with a band of black and gold on the end of the abdomen. Other gold spots occur on the thorax, the wing bases, and the eyes.
Range and distribution 
In North America, the monarch ranges from southern Canada to northern South America. It rarely strays to western Europe (rarely as far as Greece) from being transported by US ships or by flying there if weather and wind conditions are right. It has also been found in Bermuda, Hawaii, the Solomons, New Caledonia, New Zealand, Australia, New Guinea, Ceylon, India, the Azores, and the Canary Islands.
Monarchs are especially noted for their yearly migration over long distances. In North America they make massive southward migrations starting in August until the first frost. There is a northward migration in the spring. The monarch is the only butterfly that migrates both north and south as the birds do regularly, but no individual makes the entire round trip. Female monarchs lay eggs for the next generation during these migrations.
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in the Mexican states of Michoacán and México. The western population overwinters in various sites in central coastal and southern California, United States, notably in Pacific Grove, Santa Cruz, and Grover Beach.
The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a nonreproductive phase known as diapause, which may last seven months or more. During diapause, butterflies fly to one of many overwintering sites. The overwintering generation generally does not reproduce until it leaves the overwintering site sometime in February and March.
The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of the position of the sun in the sky and a time-compensated Sun compass that depends upon a circadian clock based in their antennae. New research has also shown these butterflies can use the earth's magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the spectrum. In presence of violet or blue light, it can function as a chemical compass, which tells the animal if it is aligned with the earth's magnetic field, but it cannot tell the difference between magnetic north or south. The complete magnetic sense is present in a single antenna.
Monarch butterflies are one of the few insects which can cross the Atlantic. They are becoming more common in Bermuda due to increased usage of milkweed as an ornamental plant in flower gardens. Monarch butterflies born in Bermuda remain year round due to the island's mild climate. A few monarchs turn up in the far southwest of Great Britain in years when the wind conditions are right, and have been seen as far east as Long Bennington in Lincolnshire. In Australia, monarchs make limited migrations in cooler areas, but the blue tiger butterfly is better known in Australia for its lengthy migration. Monarchs can also be found in New Zealand. On the islands of Hawaii, no migrations have been noted.
Monarch butterflies are poisonous or distasteful to birds and mammals because of the presence of the cardiac glycosides contained in milkweed eaten by the larvae. The bright colors of larvae and adults are thought to function as warning colors. During hibernation, monarch butterflies sometimes suffer losses because hungry birds pick through them looking for the butterflies with the least amount of poison, but in the process kill those they reject.
One study examined wing colors of migrating monarchs using computer image analysis, and found migrants had darker orange (reddish-colored) wings than breeding monarchs.
Research also has overturned a prevailing theory that the migration patterns of the eastern and western populations are due to genetic reasons and that their genetic material was different. The American populations have been found to be distinct from the populations in New Zealand and Hawaii, but not from each other.
Adult food sources 
Adult monarchs have been seen on a number of different nectar plants. A list of nectar resources exploited by monarch butterflies is:
- Apocynum cannabinum - Indian hemp
- Asclepias californica - California milkweed
- Asclepias incarnata - swamp milkweed
- Asclepias syriaca - common milkweed
- Asclepias tuberosa - butterfly weed
- Aster sp. - asters
- Cirsium sp. - thistles
- Daucus carota - wild carrot
- Dipsacus sylvestris - teasel
- Erigeron canadensis - horseweed
- Eupatorium maculatum - spotted joe-pye weed
- Eupatorium perfoliatum - common boneset
- Hesperis matronalis - dame's rocket
- Medicago sativa - alfalfa
- Solidago sp. - goldenrod
- Syringa vulgaris - lilac
- Trifolium pratense - red clover
- Vernonia altissima - tall ironweed
The mating period for the overwinter population occurs in the spring, just prior to migration from the overwintering sites. The courtship is fairly simple and less dependent on chemical pheromones in comparison with other species in its genus. Courtship is composed of two distinct stages, the aerial phase and the ground phase. During the aerial phase, the male pursues, nudges, and eventually takes down the female. Copulation occurs during the ground phase, where the male and female remain attached for about 30 to 60 minutes. A spermatophore is transferred from the male to the female. Along with sperm, the spermatophore is thought to provide the female with energy resources to aid her in carrying out reproduction and remigration. The overwinter population returns only as far north as they need to go to find the early milkweed growth; in the case of the eastern butterflies, that is commonly southern Texas. The life cycle of a monarch includes a change of form called complete metamorphosis. The monarch goes through four radically different stages:
- The eggs are laid by the females during spring and summer breeding months onto the leaves of milkweed plants.
- The eggs hatch (after four days), revealing worm-like larvae, the caterpillars. The caterpillars consume their egg cases, then feed on milkweed, and sequester substances called cardenolides, a type of cardiac glycoside. During the caterpillar stage, monarchs store energy in the form of fat and nutrients to carry them through the nonfeeding pupal stage. The caterpillar stage lasts around two weeks.
- In the pupa or chrysalis stage, the caterpillar spins a silk pad on a twig or leaf, and hangs from this pad by its last pair of prolegs. It hangs upside down in the shape of a 'J', and then molts, leaving itself encased in an articulated green exoskeleton. At this point, hormonal changes occur, leading to the development of a butterfly (metamorphosis). The chrysalis darkens (the exoskeleton becomes transparent) a day before it emerges, and its orange and black wings can be seen.
- The mature butterfly emerges after about two pupal weeks, and hangs from the split chrysalis for several hours until its wings are dry (often in the morning). Meanwhile, fluids are pumped into the crinkled wings until they become full and stiff. Some of this orangey fluid (called meconium) drips from the wings. Finally (usually in the afternoon), the monarch spreads its wings, quivers them to be sure they are stiff, and then flies away to feed on a variety of flowers, including milkweed flowers, red clover, and goldenrod.
Monarchs can live two to eight weeks in a garden having their host Asclepias plants and sufficient flowers for nectar. This is especially true if the flower garden happens to be surrounded by native forests that seem to be lacking in flowers.
Pictorial lifecycle 
Monarch butterfly laying eggs on Asclepias curassavica 'Silky Gold'
Monarch eggs on milkweed
An early instar monarch caterpillar
Monarch butterfly in Santa Barbara California
Adult monarch butterfly feeding on a Zinnia
Host plants 
The host plants used by the monarch caterpillar include:
- Asclepias amplexicaulis - clasping milkweed
- Asclepias asperula - antelope horns
- Asclepias californica - California milkweed
- Asclepias cordifolia - heart-leaf milkweed
- Asclepias curassavica - scarlet milkweed
- Asclepias curtissii - Curtiss' milkweed
- Asclepias eriocarpa - woollypod milkweed
- Asclepias erosa - desert milkweed
- Asclepias exaltata - poke milkweed
- Asclepias fascicularis - narrow-leaf milkweed
- Asclepias humistrata - sandhill milkweed
- Asclepias incarnata - swamp milkweed
- Asclepias linaria - pine-needle milkweed
- Asclepias meadii - Mead's milkweed
- Asclepias nivea - Caribbean milkweed
- Asclepias purpurascens - purple milkweed
- Asclepias speciosa - showy milkweed
- Asclepias subulata - rush milkweed
- Asclepias subverticillata - horsetail milkweed
- Asclepias syriaca - common milkweed
- Asclepias tuberosa - butterfly weed
- Asclepias verticillata - whorled milkweed
- Calotropis gigantea - crown flower
- Calotropis procera - apple of Sodom
- Cynanchum laeva - sand vine
- Sarcostemma clausa - white vine
- Gomphocarpus physocarpus (syn Asclepias physocarpa) - balloon milkweed
- Gomphocarpus fruticosus (syn Asclepias fruticosa) - swan plant
- Cynanchum acutum (native plant)
Defense against predators 
Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed. Both forms advertise their unpalatability with bright colors and areas of high contrast on the skin or wings in a phenomenon known as aposematism.
Monarchs also contain cardiac glycosides in their bodies from the Asclepias plants the caterpillars eat. Overwintering monarchs in Mexico are often preyed upon by black-headed grosbeaks, which are immune to that toxin. Other birds, such as orioles and jays, have learned to eat only the thoracic muscles and abdominal contents because these contain less poison than the rest of the body. Some mice are also able to withstand large doses of the poison. Over time, overwintering adults become less poisonous, thus making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice.
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. Although long purported to be an example of Batesian mimicry, the viceroy is actually reported to be more unpalatable than the monarch, making this a case of Müllerian mimicry.
Relationship with humans 
The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, and the state butterfly of Vermont and West Virginia. It was nominated in 1990 as the national insect of the United States of America, along with the honeybee (Apis mellifera), but the legislation did not pass.
Many people like to attract monarchs by growing a butterfly garden with a specific milkweed species. Others enjoy raising them for pleasure or for educational purposes. For migrating flocks, sanctuaries have been created at favorite wintering locations, and these migrations can generate significant tourism revenue.
Many schools also enjoy growing and attending to monarch butterflies, starting with the caterpillar form. When the butterflies reach adulthood, they are then released into the wild.
Some organizations, such as the Cape May Bird Observatory, have monarch identification tagging programs. Plastic stickers are placed on the wing of the insect with identification information. Tracking information is used to study their migration patterns, including how far and where they fly.
Although monarchs feed on milkweed, variations in the quantity of cardiac glycosides exist between species, individuals, and even parts of the host plant. The levels of toxins in adult monarchs reflect the levels in their host plants. This means some monarchs are not foul-tasting, but are Batesian or automimics. Some species of predators have learned to measure the toxins by taste and reject butterflies with high cardiac glycosides contents, eating only the ones with low contents.
In the butterfly, the cardiac glycosides are concentrated in the abdomen and wings. Some species of predators differentiate these parts and consume only the most palatable ones. Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays and pinyon jays.
On Oahu, a white morph of the monarch has emerged. This is because of the introduction, in 1965 and 1966, of two bulbul species, Pycnonotus cafer and Pycnonotus jocosus. They are now the most common insectivore birds, and probably the only ones preying on insects as large as the monarch. Monarchs in Hawaii are known to have low cardiac glycoside levels, but the birds may also be tolerant of the chemical. 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 colour, 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.
Parasites include the tachinid flies Sturmia convergens and Lesperia archippivora. Lesperia-parasitized larvae complete their moult and suspend, but die before pupation. At that time, one white maggot comes out of the larvae, suspended by a silken thread. The maggot then forms a brown pupa on the ground.
The bacterium Micrococcus flacidifex danai also infects the larvae and causes “black death”. As usual, just before pupation, the larvae migrate to a horizontal surface. They die a few hours later, attached only by one pair of prolegs, with the thorax and abdomen hanging limp. The body turns black shortly after. The bacterium Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory-reared insects.
The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the body 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 probably occur at low frequencies in nature. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.
Confusion of host plants 
The black swallow-wort is problematic for monarchs in North America. Monarchs lay their eggs on these relatives of native milkweeds because they produce stimuli similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant from Europe.
Habitat destruction 
Recent deforestation of the monarch's overwintering grounds has led to a drastic reduction in the butterfly's population. Efforts to classify it as a protected species and to restore its habitat are under way. Monarch butterflies that once covered 50 acres of fir tree forest during their summer layover in central Mexico now occupy fewer than 3 acres, according to the latest census. Mexico's National Commission of Natural Protected Areas said that populations have crashed in the two decades and are down 59 percent from December 2011 levels, when the insects filled 7 acres of fir trees in central Mexico. Changes in agricultural land use diminish the area of milkweed. Drought and fluctuations in temperature are partly to blame for the decline of the monarchs.
Genetically modified crops 
Herbicide tolerant genetically modified crops (GMCs), specifically corn and soybeans, now enable Midwest farmers to use herbicides which have killed millions of acres of milkweed that used to grow between the rows of food crops. Destruction of the milkweed eliminates the monarchs' main food source. Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120-150 million acres lost.
The monarch formed the subject of a controversial paper in Nature that suggested pollen from genetically modified maize could blow onto the butterfly's favored food plants, Asclepias spp. (milkweed), increasing larval mortality. The percentage of forest occupied by monarch butterflies in Mexico, used as an indicator of the number of butterflies that arrive to that country each winter, reached its lowest level in two decades. According to a survey carried out during the 2012-2013 winter season by the WWF-Telcel Alliance, and Mexico’s National Commission of Protected Areas (CONAP), the nine hibernating colonies occupy a total area of 2.94 acres of forest—representing a 59% decrease from the 2011-2012 survey of 7.14 acres
The latest decrease in monarch butterflies is likely due to a decrease in the milkweed plant (Asclepias)—a primary food for monarchs—from herbicide use in the butterfly’s reproductive and feeding grounds in the US, as well as extreme climate variations during the fall and summer affecting butterfly reproduction.
“Extreme climate fluctuations in the U.S. and Canada affect the survival and reproduction of butterflies,” Omar Vidal, director general of WWF-Mexico, said. “The monarch’s lifecycle depends on the climatic conditions in the places where they develop. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate.”
A 273-million basepair draft sequence of the monarch butterfly genome was published in 2011, including a set of 16,866 protein-coding genes. Comparison to the sequence of the silk moth Bombyx mori reveals the Lepidoptera as a relatively fast-evolving order. The monarch genome provides a number of insights into the butterfly's migratory behavior, including the molecular underpinnings of the circadian clock and juvenile hormone pathway, as well as a suite of microRNAs that are differentially expressed between summer and migratory monarchs.
See also 
- Lepidoptera migration
- List of butterflies of Great Britain
- Peninsula Point Light, Michigan
- Monarch Butterfly Biosphere Reserve
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