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Not evaluated (IUCN 3.1)
Not evaluated (IUCN 3.1)
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 multi-generational 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.
In North America, the monarch ranges from southern Canada to northern South America. It has also been found in Bermuda, Cook Islands, Hawaii, the Solomons, New Caledonia, New Zealand, Papua New Guinea, Australia, New Guinea, Sri Lanka, India, Nepal, the Azores, the Canary Islands Philippines, North Africa and Honolulu. It appears in the UK in some years as an acccidental.
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, plenty of sunlight (for body temperatures that allow flight), appropriate vegetation on which to roost, and is relatively free of predators. Overwintering, roosting butterflies have been seen on sumacs, locusts, basswood elm, oak, osage orange, mulberry, pecan, willow, cottonwood, and mesquite. While breeding, its habitat can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides - anywhere where there is access to larval host plants. Habitat restoration is a primary goal in monarch conservation efforts. Habitat requirements change during migration. During the fall migration, butterflies must have access to nectar-producing plants. During the spring migration, butterflies must have access to larval food plants and nectar plants.
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. On August 14, 2014, the Center for Biological Diversity and the Center for Food Safety filed a legal petition requesting Endangered Species Act protection for the monarch and its habitat.
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 monarch undergoes complete metamorphosis consisting of four stages:
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:
Three subspecies and two color morphs of D. plexippus have been identified:
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.
The host plants used by the monarch caterpillar include:
Asclepias curassavica has been planted as an ornamental and naturalized. Its distribution is probably worldwide. Year-round plantings may be the cause of new overwintering sites along the Gulf coast and in Spain.
Although larvae eat only milkweed, adult monarchs feed on the following nectar plants:
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 both north and south on an annual basis. The population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in Mexico. The western population overwinters in various coastal sites in central and southern California. 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.
In both caterpillar and butterfly form, monarchs are aposematic—warding off predators with a bright display of contrasting colors to warn potential predators of their 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 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: 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 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.
Organizations and individuals participate in tagging programs. Tagging information is used to study migration patterns.
Larva feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepias 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 jays,Black-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. Wasps commonly consume larvae.
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 butterfly entering the larval stage. The spores are found on the body of infected butterflies. These spores are passed, from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels variy in populations. This is not the case in laboratory rearing, where after a few generations, all individuals can be infected. overwintering populations with lower parasite loads.
Populations counts show a decline in Monarch populations at their overwintering sites. Though most researchers accept these data it has currently come into question.
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.
Conservationists attribute the disappearance of mikweed speices to 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. To help fight this problem, Monarch Watch encourages the planting of “Monarch Waystations.”  Garden-friendly milkweed not only helps fight the declining Monarch population, but also encourages other pollinators as well.
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 authorities continue to monitor illegal logging of the oyamel trees. The Oyamel is a major species 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. More recently, the genetic basis of monarch migration and warning coloration has been described.
The Center for Biological Diversity, The Center for Food Safety, The Xerces Society and Lincoln Brower have filed a petition to the Interior Department (USA) to protect the monarch by having it declared as an endangered species.
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. Butterfly gardening is thought to increase the populations of butterflies.
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