Monarch butterfly

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 late summer/autumn migration from the United States and southern Canada to Mexico and coastal California, and northward return in spring, which occurs over the lifespans 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 the "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.

Taxonomy
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 that 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: Smith et al. did not take up Danaus plexippus nigrippus in their studies. According to Hay-Roe et al. (2007), this taxon is still recognised as a subspecies:
 * 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 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
Danaus (Greek Δαναός), a great-grandson of Zeus, was a mythical king in Egypt or Libya, who founded Argos; Plexippus was one of the 50 sons of Aegyptus, the twin brother of Danaus.

In Homeric Greek 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.

Description
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, and measures about 1.2 mm high and 0.9 mm 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.

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.

Migration
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 North American 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. Painted Ladies also make a return migration from Africa to Northern Europe

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 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. 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 that can cross the Atlantic. They are becoming more common in Bermuda, due to increased use 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.

Habitat
The monarch can be found in a wide range of habitats, such as fields, meadows, prairie remnants, urban and suburban parks, gardens, trees, and roadsides. It overwinters in conifer groves.

Adult food sources
Although larvae eat only milkweed, adult monarchs are more flexible in their diets. Adult monarchs, well known for their completely liquid diet, 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

Males also take in moisture and minerals from damp soil and wet gravel, a behavior known as mud-puddling. The monarch has also been noticed puddling at an oil stain on pavement. Adult butterflies are also attracted to the liquids of foods we consume; they will drink mushy slices of bananas, oranges, and watermelon.

Reproduction


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. Only 30% of mating attempts end in copulation, suggesting that females have methods to avoid unwanted matings. Differences in female ability to resist mating affect pairing patterns. 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:


 * 1) The eggs are laid by the females during spring and summer breeding months onto the leaves of milkweed plants.
 * 2) 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.
 * 3) 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.
 * 4) 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 lack flowers.

Sexual selection
Mate pairing in Danaus plexippus does not depend on parasitism levels, but is affected by size, fluctuating asymmetry, and wing condition of females. Size affected mating occurs early in the mating season, where smaller females mate with smaller males and larger females mate with larger males. However, by the end of the mating season, larger females contain less spermatophores than smaller females. Mating females are more asymmetric than non-mating females, and forewing asymmetry plays a role in determining mate pairing. Studies suggest that large and symmetrical females are more attractive to males, but they are better at resisting male mating attempts. Larger males tend to mate with larger females because they are more likely to overcome the resistance of larger females. In addition, studies suggest that damaged wings decrease mating in females. Studies have failed to reveal correlations between male size or wing condition with male mating success. Successful males,however, mate on a greater proportion of days and are also more likely to be successful, ending in copulation. In addition, both females and males mate multiply. Spermatophore nutrients after mating are digested and used in female tissue. Because female Lepidoptera benefit from spermatophore-derived nutrients, there is a high frequency of multiple mating among female monarchs. In addition, females allowed to mate several times therefore laid more eggs than females who only mate once.

Sperm competition
Male monarchs produce spermatophores, a sperm sac embedded in a gelatinous body, from accessory gland secretions. Since male monarchs are highly polyandrous and females store sperm, there is need for sperm competition, selecting for males that gain sperm precedence. The spermatophore size of males increases with increasing time between matings, and larger spermatophores delay female re-mating. Therefore, by waiting to re-mate, males increase their sexual competitiveness, ultimately increasing the number of ova fertilized by their sperm.

In addition, the production of large spermatophores could also benefit the females because spermatophore constituents may be used by females to affect the amount of offspring produced or the quality of offspring produced. By increasing female reproductive success, male reproductive success also increases. Studies have shown that the contents of Lepidopteran spermatophores are incorporated into the eggs and somatic tissue of females. Therefore, an increase in spermatophore size also increases the fecundity of female monarchs. Sperm from males that produce larger spermatophores could also fertilize more female's eggs without increasing her lifetime reproductive success.

Sperm precedence patterns were also observed, where second-male sperm precedence was more common than first or no-male sperm precedence. The advantage of second-male sperm suggests that the incoming sperm pushes back previously existing sperm, resulting in different layers of sperm from different males. However, second-male sperm precedence is rarely complete, suggesting that the sperm from the first male remains in the fertilization set, acting as a barrier to block sperm from other males.

Host plants
The host plants used by the monarch caterpillar include:

Americas:
 * 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

African:
 * Gomphocarpus physocarpus (syn Asclepias physocarpa) – balloon milkweed
 * Gomphocarpus fruticosus (syn Asclepias fruticosa) – swan plant

European:
 * Cynanchum acutum (native plant)

Defense against predators


In both caterpillar and butterfly form, monarchs use a bright display of contrasting colors to warn potential predators of its undesirable taste and poisonous characteristics. This aposematic behavior is common among many insects, amphibians, and mammals alike. Additionally, monarchs are physically similar to the viceroy butterfly, exhibiting a classic case of mimicry.

Aposematism

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.

Monarch toxins are pharmacologically similar to digitalis and produce extremely similar results in experimental settings. In the wild, the toxins cause many birds to experience intense discomfort and vomiting. Many birds find monarchs unappetizing and quickly begin recognizing their distinct colors and avoiding them as food sources.

Mimicry

Monarchs share the defense of noxious taste with the similar-appearing viceroy butterfly in what is perhaps one of the most well-known examples of mimicry. Though long purported to be an example of Batesian mimicry, the viceroy is actually reportedly 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.

The main villain on The Venture Bros. (a cartoon on the Adult Swim block of Cartoon Network) takes his name, costume and overall lifestyle from monarch butterflies.

Predators
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.

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.

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.

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).

Another predator of the monarchs is the Chinese mantid (Tenodera sinensis). Chinese mantids have been shown to have adapted to handle monarch caterpillars to reduce poisonous effects. Chinese mantids are capable of identifying monarch larvae and approach them in a different manner than other caterpillars. Upon encountering a monarch caterpillar, Chinese mantis will chew open the integument to let the gut fall out. Once the gut is removed, they will proceed to consume the rest of the body. This is effective as monarch caterpillars have much higher levels of cardenolide content in their guts than in the rest of their bodies. By practicing this method of ingestion, Chinese mantids are able to avoid the gut which contains high levels of milkweed particles. This practice has also been documented in some other predators as well.

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
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 that 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.”

Genome
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.