The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the most familiar North American butterfly. 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.)
The eastern North American monarch population is notable for its southward late summer/autumn migration from the United States and southern Canada to Mexico, covering thousands of miles. The western North American population of monarchs west of the Rocky Mountains most often migrate to sites in California but have been found in overwintering Mexico sites. 
- 1 Range
- 2 Habitat
- 3 Status
- 4 Reproduction
- 5 Life cycle
- 6 Taxonomy
- 7 Sexual dimorphism
- 8 Food plants
- 9 Adult food sources
- 10 See also
- 11 Origin of name
- 12 Description
- 13 Migration
- 14 Defense against predators
- 15 Human Interactions
- 16 Threats
- 17 Genome
- 18 Conservation
- 19 See also
- 20 References
- 21 External links
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, Cook Islands, Hawaii, the Solomons, New Caledonia, New Zealand, Australia, New Guinea, Sri Lanka, India, Nepal, the Azores, the Canary Islands and Honolulu.
Overwintering populations of D. plexippus are found in Mexico, California, along the Gulf coast, year-round in Florida, and in Arizona where the habitat provides the specific conditions necessary for their survival. The overwintering habitat typically provides access to streams or damp soil, cool temperatures, necessary leves of humidity, periods of sunlight (for body temperatures that allow flight), appropriate vegetation on which to roost, relatively free of predators. Forest sanctuaries but can be found in agricultural fields and pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides. The eastern North American overwinters in Mexican conifer groves.
The monarch butterfly is not currently listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) or protected specifically under U.S. domestic laws.
Reproduction is influenced by size, symmetry, and wing condition of females. Damaged wings decrease mating in females. Males who are fit are more likely to mate. Females and males typically mate more than once. Females that mate several times lay more eggs. Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.  Courtship occurs in two phases. During the aerial phase, the male pursues and often forces the female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes.  Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating though some have more success than others.   During copulation, the male transfers the spermatophore to the female. Along with sperm, the spermatophore provides the female with nutrition to aid her in egg-laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more female's eggs.
The overwintering population returning north from Mexico, travels north to find early milkweed growth in southern Texas.
The monarch undergoes complete metamorphosis consisting of four 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), into larvae, or caterpillars. The caterpillar consumes its egg case then begins to feed on milkweed. and sequester substances called cardenolides, a type of cardiac glycoside. During the larval stage, monarchs store energy in the form of fat and nutrients to carry them through the nonfeeding pupal stage. The larval stage lasts around two weeks.
- In the pupa or chrysalis stage, the caterpillar spins a silk pad onto a horizontal substrate. It then hangs from the pad by the last pair of prolegs upside down resembling the letter 'J'. It sheds its skin leaving itself encased in an articulated green exoskeleton. During this pupal stage, the adult butterfly forms inside. The exoskeleton becomes transparent before it ecloses (emerges), and its adult colors can be seen.
- The mature butterfly emerges after about two pupal weeks, and hangs until its wings are dry. Fluids are pumped into wings and they expand and stiffen. The monarch expands and contracts its wings, and when conditions allow it then flies to feed on a variety of nectar plants. In about three days the adult reaches reproductive maturity.
Adult monarchs live two to eight weeks during the breeding season.
The name 'monarch' may be in honor of King William III of England. The monarch was originally described by Linnaeus in his Systema Naturae of 1758 and it was placed in the genus Papilio. In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus; Danaus. There are three species of Monarch butterflies:
- D. plexippus, described by Linnaeus in 1758, is the species known most commonly as the monarch butterfly of North America. It’s range actually extends worldwide and can be found in Hawaii, Australia, New Zealand, Spain and on Oceanic Islands.
- D. erippus, the southern monarch was described by Cramer in 1775, This species is found in tropical and subtropical latitudes of South America, mainly in Brazil, Uruguay, Paraguay, Argentina, Bolivia, Chile and southern Peru. The south American monarch and the North American monarch may have been one species at one time. Some researchers believe the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. Sea levels were higher then and the entire Amazonas lowland was a vast expanse of brackish swamp that offered limited butterfly habitat.
- D. cleophile Jamaican monarch described by Godart in 1819, the Lepidoptera Specialist Group in 1996, and the IUCN Red List of Threatened Species, Version 2014.1, in 2014. It ranges from Jamaica to Hispaniola.
Three subspecies and two color morphs of D. plexippus have been identified:
- D. p. plexippus, the nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America.
- D. p. nigrippus named in 1909 by Richard Haensch as forma: Danais [sic] archippus f. nigrippus.Hay-Roe et al. in 2007 identified this taxon as a subspecies:
- 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 other subspecies, but are considered color varieties of D. p. megalippe These are:
- D. p. m. leucogyne, named by Arthur G. Butler in 1884
- D. p. m. portoricensis, named in 1941 by A.H. Clark
- D. p. m. tobagi, also named in 1941 by A.H. Clark
The levels of cardiac glycosides in white monarchs in Hawaii are low because their host plant Calotropis gigantea (crown flower) contains a low level of these toxins. The percentage the white morph in Oahu, is nearing 10%. On other Hawaiian islands, the white morph occurs at a relatively low frequency. White Monarchs (nivosus) have been found throughout the world, including Australia, New Zealand, Indonesia and the United States.
Larvae 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
- Foeniculum vulgare - California fennel
- Gomphocarpus physocarpus (syn Asclepias physocarpa) – balloon milkweed
- Gomphocarpus fruticosus (syn Asclepias fruticosa) – swan plant
- Cynanchum acutum (native plant)
Asclepias curassavica has been planted as an ornamental and naturalized. Its distribution is probably wordwide. Year-round plantings may be the cause of new overwintering sites along the Gulf coast and in Spain.
Adult food sources
Although larvae eat only milkweed, adult monarchs feed on the following nectar plants:
- 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
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. One variation has been observed in Australia, New Zealand, Indonesia and the United States termed nivosus by lepidopterists. It is grayish-white in all areas of the wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist 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 reaches 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.
The eastern population migrates hundreds to thousands of miles to overwintering sites in Mexico. Southward migrations start in August and end at the first frost. There is a northward migration in the spring. The eastern population migrates both north and south on an annual basis. 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. Tagging studies indicate some monarchs migrate south to Florida in the fall. The western population overwinters in various coastal sites in central 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.
Migratory Theory Mechanisms
The North American Western and Eastern populations (D. plexippus) migrate to established overwintering spots each autumn. In one study monarchs released during the fall migration from Albuquerque, New Mexico were found overwintering in California and in Mexico. The same study tested the ability of commercially bred monarchs (to the 9th generation) to migrate to overwintering areas. Flight navigational patterns may be inherherited, 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. These populations may use the earth's magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the light spectrum. In the presence of violet or blue light, it can function as a chemical compass. Studies demonstrate that eastern and western populations do not use an internal ‘map’ to navigate to overwintering locations but instead are guided by a ‘compass’ that compels them to migrate in a southwest direction. This southwest directional migration is affected by large geographical features like the Rocky Mountains and The Gulf of Mexico.
On June 24, 2014, scientists from the University of Massachusetts Medical School and Worcester Polytechnic Institute published the results of their study that confirms that monarch butterflies, like many birds & sea turtles, are aided by a geomagnetic compass They also reported that it was proteins called CRY in the butterflies’ antennae that activate this inclination compass when light of a particular wavelength — the ultra-violet/blue end of the spectrum — fell on it.
New methods of studying the migration include the use of VHF transmitters and commercial aircraft.
The 'Columbus Theory' has been suggested to account for the migration. According to this theory, the eastern D.plexippus population did not always migrate, but began migrating soon after European colonization. At that time, settlers harvested or cleared large tracts of lumber. This led to the spread of milkweed, an aggressive, opportunistic plant that flourished in the areas devoid of the tree canopy. The spread of milkweed allowed the spread of monarch populations. Some suggest that this resulted in what is termed an "overspill". This mechanism, the clearing of land by colonists, eventually led to the establishment of D.plexippus in temperate regions in the world. No evidence exists to support observations of long-distance migrations before about 1865. Populations found in other regions do not migrate over such long distances (in Australia, for example) This observation suggests that the migratory behavior of the eastern population of the monarch butterfly developed after other populations had become established in other regions. Other researchers remain doubtful that such behavior could have devloped in such a short time.
Monarch butterflies can and have crossed the Atlantic. They are becoming more common in Bermuda and Spain, due to increased use of milkweed as an ornamental plant. Monarch butterflies in Bermuda and Spain do not migrate. Butterflies sometimes appear in Great Britain. In Australia, monarchs make limited migrations in cooler areas, On the islands of Hawaii, no migrations have been noted. The Southern Monarch, D. erippus migrates along the eastern edge of the Andean mountains in the autumn in Bolivia and Peru.
One study examined wing colors of migrating monarchs using computer image analysis, and found migrants had darker orange (reddish-colored) wings than breeding monarchs.
Defense against predators
In both caterpillar and butterfly form, monarchs possess a bright display of contrasting colors to warn potential predators of its undesirable taste and poisonous characteristics.
Large larvae are able to avoid wasp predation by dropping from the plant or by jerking their bodies.
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. By ingesting a large amount of plants in the genus Asclepias, primarily milkweed, monarch caterpillars are able to 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 are able to sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content.
Additional studies have shown that different species of milkweed have differing effects on growth, virulence, and transmission of parasites. One specific species (Asclepias curassavica) appears to reduce the proportion of monarchs infected by parasites. There are two possible explanations for the positive role of A. curassavica on the monarch caterpillar. The first is that A. curassavica promotes overall monarch health to boost the monarch’s immune system. A second theory is that A. curassavica has a direct negative effect on the parasites.
After the caterpillar becomes a butterfly, the toxin shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste, and may prevent them from ever ingesting the body of the butterfly. In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.
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 actually reportedly more unpalatable than the monarch, making this a case of Müllerian mimicry.
The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, Vermont, and West Virginia. It was nominated in 1990 as the national insect of the United States of America. but the legislation did not pass.
Monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these Monarch Waystations. Monarchs are raised as a hobby and for educational purposes. Butterfly farmers raise Monarchs and ship them to individuals and organizations to be released at a wedding or funeral, for example. The release of captive bred Monarchs remains controversial.
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.
Larva feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepius sp.vary. Not all monarchs are unpalatable, but exibit Batesian or automimics. Cardiac glycosides levels are higher in the abdomen and wings. Some predators can differentiate betwwen these parts and consume the most palatable ones. Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, pinyon jaysBlack-headed Grosbeak and orioles.
Some mice are able to withstand large doses of the toxin. 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.
In North America, eggs and first instar larvae of the monarch are eaten by larvae and adults of the introduced Asian lady beetle (Harmonia axyridis). The Chinese mantid ("Tenodera sinensis") will consume the larvae once the gut is removed thus avoiding cardenolides.
Several birds have also adapted various methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides. The oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The grosbeak, on the other hand, has adapted the ability an insensitivity to secondary plant poisons which allows it to ingest monarchs without vomiting. As a result, orioles and grosbeaks will periodically have high levels of cardenolides in their bodies, and they will be forced to go on periods of reduced monarch consumption. This cycle of predation effectively reduces the potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.
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 Lespesia archippivora. Lesperia-parasitized butterfly larvae complete the formation of their crysalid but die before they emerge as an adult. Before pupation is complete, one white maggot comes out of the chrysalid. The maggot forms a brown pupa on the ground then emerges as an adult.
The bacterium Micrococcus flacidifex danai also infects larvae. Just before pupation, the larvae migrate to a horizontal surface and 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.
The yearly decrease in the monarch butterfly population has been linked to the decrease in the milkweed plant (Asclepias)—a primary food for monarchs—from herbicide use in the butterfly’s reproductive and feeding areas. The destruction of common milkweed has effectively eliminated the food source from most of the habitat monarchs used to use. Common milkweed is susceptible to the use of herbicides. Varietals do exist, however, (see Human Interactions) that can be successfully planted in gardens and other areas to help mitigate habitat loss in the wild.
Genetically Modified Crops
Conservationists attribute the disappearance of mikweed speices to monolithic agricultural practices in the Midwest, where genetically modified seeds are bred to resist herbicides that eliminate milkweed nearby. Growers eliminate milkweed that previously grew between the rows of food crops. Corn and soybeans are resistant to the effect of the herbicide glyphosate. The increased use of these crop strains is correlated with the decline in Monarch populations between 1999 and 2010. Chip Taylor, director of Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120–150 million acres lost.
Loss of Overwintering Habitat
The area of forest occupied by overwintering monarch butterflies in Mexico reached its lowest level in two decades in 2013. 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 same survey in 2012-2013 showed the decline is continuing. There were only seven colonies occupying 0.67 hectares (1.66 acres), the third consecutive record low since record-keeping began in 1995-1996. It represents a 44% decrease from the previous year, a 76% decrease from 2011-2012 and a 92% decrease compared to the 1996-1997 count.
Mexican environmental authorites continue to monitor illegal logging of the [Oyamel trees]. The Oyamel is a major spieces of evergreen on which the overwintering butterflies spend a significant time during their winter diapause.
Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth and the survival of migrating adult butterflies.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 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate.” 
The 273-million base pair draft sequence includes a set of 16,866 protein-coding genes. The genome provides researchers insights into migratory behavior, the circadian clock, juvenile hormone pathways and microRNAs that are differentially expressed between summer and migratory monarchs.
Conservationists are lobbying transportation departments and utilities to reduce their use of herbicides and specifically encourage milkweed to grow along roadways and power lines. The goal is to reduce roadside mowing and application of herbicides during the butterfly breeding season. Environmental conservationists are lobbying large-scale agriculture companies to leave small areas of cropland unsprayed to allow the butterflies to breed.
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