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Monarch butterfly
Monarch butterfly

The monarch butterfly or simply monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae.[4] Other common names depending on region include milkweed, common tiger, wanderer, and black veined brown.[5] It may be the most familiar North American butterfly, and is considered an iconic pollinator species.[6] Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm (3 1⁄2–4 in)[7] The viceroy butterfly is similar in color and pattern, but is markedly smaller and has an extra black stripe across each hindwing.

The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the northern and central United States and southern Canada to Florida and Mexico.[4] During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California but has been found in overwintering Mexican sites as well.[8][9] Monarchs have been bred on the International Space Station.[10]

Taxonomy


The name "monarch" is believed to be given in honor of King William III of England, whose secondary title Prince of Orange makes a reference to the butterfly's main color.[11] The monarch was originally described by Carl Linnaeus in his Systema Naturae of 1758of 1758]]and placed in the genus Papilio[12] Jan Krzysztof Kluk used the monarch as the type species for a new genus Danaus

Danaus (Ancient Greek Δαναός), a great-grandson of Zeus, was a mythical king in Egypt or Libya, who founded Argos; Plexippus (Πλήξιππος) was one of the 50 sons of Aegyptus, the twin brother of Danaus. In Homeric Greek, his name means "one who urges on horses", i.e. "rider" or "charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467,[13] 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. Linnaeus divided his large genus Papilio, containing all known butterfly species, into what we would now call subgenera. The Danai festivi formed one of the "subgenera", containing colorful species, as opposed to the Danai candidi, containing species with bright white wings. Linnaeus wrote: "Danaorum Candidorum nomina a filiabus Danai Aegypti, Festivorum a filiis mutuatus sunt." (English: "The names of the Danai candidi have been derived from the daughters of Danaus, those of the Danai festivi from the sons of Aegyptus.")

Robert Michael Pyle suggested Danaus is a masculinized version of Danaë (Greek Δανάη), Danaus's great-great-granddaughter, to whom Zeus came as a shower of gold, which seemed to him a more appropriate source for the name of this butterfly.[14]

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. Its range actually extends worldwide and can be found in Hawaii, Australia, New Zealand, Spain and the Pacific Islands.
  • D. erippus, the southern monarch, was described by Pieter 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, and the entire Amazonas lowland was a vast expanse of brackish swamp that offered limited butterfly habitat.[15]
  • D. cleophile, the Jamaican monarch, described by Jean Baptiste Godart in 1819, ranges from Jamaica to Hispaniola.[16]

Six subspecies and two color morphs of D. plexippus have been identified:[5]

  • D. p. plexippus – nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America. D. p. p. form nivosus, the white monarch commonly found on Oahu, Hawaii, and rarely in other locations.[17] D. p. p. (as yet unnamed) – a color morph lacking some wing vein markings.[18]
  • D. p. nigrippus (Richard Haensch, 1909) – as forma: Danais [ sic] archippus f. nigrippus. Hay-Roe et al. in 2007 identified this taxon as a subspecies:[19]
  • D. p. megalippe (Jacob Hübner, [1826]) – nonmigratory subspecies, and is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River.
  • D. p. leucogyne (Arthur G. Butler, 1884) − St. Thomas
  • D. p. portoricensis Austin Hobart Clark, 1941 − Puerto Rico
  • D. p. tobagi Austin Hobart Clark, 1941 − Tobago

The percentage of the white morph in Oahu is nearing 10%.

Some taxonomists disagree on these classifications.[15][19]

Monarchs belong in the subfamily Danainae of the family Nymphalidae; Danainae was formerly considered a separately family Danaidae.[20]

A 2015 paper identified genes from wasp bracoviruses in the genome of the North American monarch[21] leading to articles about monarch butterflies being genetically modified organisms.[22][23]

Description


Commonly and easily mistaken for the similar viceroy butterfly, the monarch's wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in).[7] The uppersides of the wings are tawny orange, the veins and margins are black, and there are two series of small white spots in the margins. Monarch forewings also have a few orange spots near their tips. Wing undersides are similar, but the tips of forewings and hindwings are yellow brown instead of tawny orange and the white spots are larger.[24] The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants.[25] Wings size and shape differ between migratory and non-migratory monarchs. Monarchs from eastern North America have larger and more angular forewings than those in the western population.[10]

Monarch flight has been described as "slow and sailing".[26] Monarch flight speed has been estimated by a number of researchers. One scientist examined all prior estimates and concluded their flight speed is approximately 9 km/hr or 5.5 mph.[27] For comparison, the average human jogs at a rate of 6–8 mph.

Adults exhibit sexual dimorphism. Males are slightly larger than females[10][24] and have a black patch or spot of androconial scales on each hindwing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs). The male's black wing veins are lighter and narrower than those of females.[28]

One variation, the "white monarch", observed in Australia, New Zealand, Indonesia and the United States, is called nivosus by lepidopterists. It is grayish white in all areas of its 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.[17]

The monarch has six legs like all insects, but uses only its middle legs and hindlegs as the forelegs are vestigial, as in all other Nymphalidae, and held against its body.[29]

A study in 2015 examined a preserved collection of male and female monarch specimens from eastern North America to evaluate the sex-based differences in fine-scale wing and body structure.[30] The study found significant differences in overall wing size and in the physical dimensions of wings.

Distribution and habitat


The range of the western and eastern populations of D. plexippus plexippus expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts.[10] However, no genetic differences between the western and eastern monarch populations exist;[32] reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species' range.[10]

In the Americas, the monarch ranges from southern Canada through northern South America.[4] It has also been found in Bermuda, Cook Islands,[33] Hawaii,[34][35] Cuba,[36] and other Caribbean islands[10] the Solomons, New Caledonia, New Zealand,[37] Papua New Guinea,[38] Australia, the Azores, the Canary Islands, Gibraltar,[39] the Philippines, and North Africa.[40] It appears in the UK in some years as an accidental migrant.[41]

Overwintering populations of D. plexippus plexippus are found in Mexico, California, along the Gulf Coast, year round in Florida, and in Arizona where the habitat has the specific conditions necessary for their survival.[42][43] On the US East Coast, they have overwintered as far north as Lago Mar, Virginia Beach, Virginia.[44] Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and is relatively free of predators. Overwintering, roosting butterflies have been seen on basswoods, elms, sumacs, locusts, oaks, osage-oranges, mulberries, pecans, willows, cottonwoods, and mesquites.[45] While breeding, monarch habitats 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.[46]

Life cycle


The monarch butterfly undergoes four stages of complete metamorphosis:

The eggs are derived from materials ingested as a larva and from the spermatophores received from males during mating.[47] Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months.[48] The eggs are cream colored or light green, ovate to conical in shape, and about 1.2×0.9 mm in size. The eggs weigh less than 0.5 mg each and have raised ridges that form longitudinally from the point to apex to the base. Though each egg is 1⁄1000 the mass of the female, she may lay up to her own mass in eggs. Females lay smaller eggs as they age. Larger females lay larger eggs.[47] The number of eggs laid by a female, who may mate several times, ranges from 290 to 1180.[49] Females lay their eggs on the underside of the milkweed leaves; the offspring's consumption of the milkweed benefits health and helps defend them against predators.[50][51] Eggs take 3 to 8 days to develop and hatch into larva or caterpillars.[10] Monarchs will lay eggs along the southern migration route.[52]

The caterpillar goes through five major, distinct stages of growth and after each one, it molts. Each caterpillar, or instar, that molts is larger than the previous as it eats and stores energy in the form of fat and nutrients to carry it through the nonfeeding pupal stage. Each instar usually lasts about 3 to 5 days, depending on various factors such as temperature and food availability. [4]

The first instar caterpillar that emerges out of the egg is pale green and translucent. It lacks banding coloration or tentacles. The larvae or caterpillar eats its egg case and begins to feed on milkweed. It is during this stage of growth that the caterpillar begins to sequester cardenolides. The circular motion a caterpillar uses while eating milkweed prevents the flow of latex that could entrap it. The first instar is usually between 2 and 6 mm long.

The second instar larva develops a characteristic pattern of white, yellow and black transverse bands. It is no longer translucent but is covered in short setae. Pairs of black tentacles begin to grow. One pair grows on the thorax and another pair on the abdomen. Like the first instar, second instar larvae usually eat holes in the middle of the leaf, rather than at the edges. The second instar is usually between 6 mm and 1 cm long.

The third instar larva has more distinct bands and the two pairs of tentacles become longer. Legs on the thorax differentiate into a smaller pair near the head and larger pairs further back. These third-stage caterpillars begin to eat along the leaf edges. The third instar is usually between 1 and 1.5 cm long.

The fourth instar has a different banding pattern. It develops white spots on the prolegs near the back of the caterpillar. It is usually between 1.5 and 2.5 cm long.

The fifth instar larva has a more complex banding pattern and white dots on the prolegs, with front legs that are small and very close to the head. A caterpillar at this stage has enormous appetite, being able to consume a large milkweed leaf in a day. Its length ranges from 2.5 to 4.5 cm.[4]

At this stage of development, it is relatively large compared to the earlier instars.

To prepare for the pupa or chrysalis stage, the caterpillar chooses a safe place for pupation, where it spins a silk pad on a downward-facing horizontal surface. At this point, it securely latches on with its last pair of hindlegs and hangs upside down, in the form of the letter J. After "J-hanging" for about 12-16 hours, it then sheds its skin, revealing a green chrysalis. At first, the chrysalis is long, soft, and somewhat amorphous, but over a few hours it compacts and hardens into its distinct shape - an opaque, pale-green chrysalis with small gold dots. At this point, it is about 2.5 cm (1") long and 10-12 mm (3/8" - 7/16") wide, weighing about a gram. At normal summer temperatures, it matures in 8–15 days. During this pupal stage, the adult butterfly forms inside. The exoskeleton first becomes darker and more bluish, and finally transparent before the butterfly ecloses (emerges), and its adult colors can finally be seen.

An adult butterfly emerges after about two weeks as a chrysalis, and hangs upside down for a few hours until its wings are dry.

Healthy males are more likely to mate than unhealthy ones.

Courtship occurs in two phases.

Larval host plants


The host plants used by the monarch caterpillar include:

Asclepias curassavica or tropical milkweed, is often planted as an ornamental in butterfly gardens. Year-round plantings in the USA are controversial and criticised, as they may be the cause of new overwintering sites along the U.S. Gulf Coast, leading to year-round breeding of monarchs.[67] This is thought to adversely affect migration patterns, and to cause a dramatic buildup of the dangerous parasite, Ophryocystis elektroscirrha.[68]

Adult food sources


Although larvae eat only milkweed, adult monarchs feed on the nectar of many plants including:

Monarchs obtain moisture and minerals from damp soil and wet gravel, a behavior known as mud-puddling. The monarch has also been noticed puddling at an oil stain on pavement.[43]

Migration


In North America, monarchs migrate both north and south on an annual basis.[4] The population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in Mexico and parts of Florida. 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.[69] Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico,[70] though they have a much lower migratory success rate than wild monarchs do.[71] Recent discoveries have located monarch overwintering sites in Arizona.[72]

Vision


Physiological experiments suggest that monarch butterflies view the world through a tetrachromatic system.[73] Like humans, their retina contain three types of opsin proteins, expressed in distinct photoreceptor cells, each of which absorbs light at a different wavelength. Unlike humans, one of those types of photoreceptor cells corresponds to a wavelength in the ultraviolet range; the other two correspond to blue and green.[74] In addition to these three photoreceptors cells in the main retina, monarch butterfly eyes contain orange filtering pigments that filter the light reaching some but not all green-absorbing opsins, thereby making a fourth photoreceptor cell sensitive to longer wavelength light.[73] The combination of filtered and unfiltered green opsins permits the butterflies to distinguish yellow from orange colors.[73] The ultraviolet opsin protein has also been detected in the dorsal rim region of monarch eyes. One study suggests that this allows the butterflies the ability to detect ultraviolet polarized skylight in order to orient themselves with the sun for their long migratory flight.[75]

These butterflies are capable of distinguishing colors based on their wavelength only, and not based on intensity; this phenomenon is termed “true color vision.”

Beyond the perception of color, the ability to remember certain colors is essential in the life of monarch butterflies.

Defense against predators


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.[77]

Monarchs are foul tasting and poisonous due to the presence of cardenolides in their bodies, which the caterpillars ingest as they feed on milkweed.[56] 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.[78] 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.[79]

Additional studies have shown that different species of milkweed have different effects on growth, virulence, and transmission of parasites.[80] 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.[80]

After the caterpillar becomes a butterfly, the toxins shift to different parts of the body.

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.[82]

Human interaction


The monarch is the state insect of Alabama,[83] Idaho,[84] Illinois,[85] Minnesota,[86] Texas,[87] Vermont,[88] and West Virginia.[89] Legislation was introduced to make it the national insect of the United States,[90] but this failed in 1989[91] and again in 1991.[92]

A growing number of homeowners are establishing butterfly gardens; monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these monarch waystations. [93]

An IMAX film, Flight of the Butterflies, describes the story of the Urquharts, Brugger and Trail to document the then unknown monarch migration to Mexican overwintering areas.[94]

Sanctuaries and reserves have been created at overwintering locations in Mexico and California to limit habitat destruction. These sites can generate significant tourism revenue.[95]

Organizations and individuals participate in tagging programs.

The 2012 novel by Barbara Kingsolver, Flight Behavior, deals with the appearance of a large population in the Appalachians.[97]

One of the most direct ways humans are interacting with monarchs is by rearing them in captivity, which has become increasingly popular, although there are risks to this activity, and this has become a controversial topic.

On the other hand this practice becomes problematic when monarchs are "mass-reared".

In addition to the disease risks, researchers believe these captive-reared monarchs are not as fit as wild ones, owing to the unnatural conditions they are raised in.

Genome


The monarch was the first butterfly to have its genome sequenced.[10] 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.[32][110][111] More recently, the genetic basis of monarch migration and warning coloration has been described.[112]

There is no genetic differentiation between the migratory populations of eastern and western North America.[10] Recent research has identified the specific areas in the genome of the monarch that regulate migration.

Conservation status


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.[113] On 14 August 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.[10] The U.S. Fish and Wildlife Service initiated a status review of the monarch butterfly under the Endangered Species Act with a due date for information submission of 3 March 2015.

The number of monarchs overwintering in Mexico has shown a long-term downward trend.

A study in 2016 claimed that the long-term trend in the size of the overwintering sites is cause for concern.

In Ontario, Canada, the monarch butterfly is listed as a Species of Special Concern.[117] In fall 2016, the Committee on the Status of Endangered Wildlife in Canada proposed that the monarch be listed as endangered in Canada, as opposed to its current listing as a "species of concern" in that country. This move, once enacted, would protect critical monarch habitat in Canada, such as major fall accumulation areas in southern Ontario, but it would also have implications for citizen scientists who work with monarchs, and for classroom activities. If the monarch were federally protected in Canada, these activities could be limited, or require federal permits.[118]

Threats


There is increasing concern related to the ongoing decline of monarchs at their overwintering sites; based on a 2014 twenty-year comparison, the overwintering numbers west of the Rocky Mountains have dropped more than 50 percent since 1997 and the overwintering numbers east of the Rockies have declined by more than 90 percent since 1995.[10]

In February 2015, the U.S.

A number of conservationists attribute the disappearance of milkweed species 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.[121][122] 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.[123][124] To help fight this problem, Monarch Watch encourages the planting of "Monarch Waystations".[103] The Natural Resources Defense Council filed a suit in 2015 against the EPA, in which it is argued that the agency ignored warnings about the dangers of glyphosate usage for monarchs.[125]

While herbicide-use has been proposed as one factor causing the decline in overwintering numbers of eastern monarchs, it is not the only possibility.

Larvae feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepias species vary. Not all monarchs are unpalatable, but exhibit Batesian or automimics. Cardiac glycosides levels are higher in the abdomen and wings. Some predators can differentiate between these parts and consume the most palatable ones.[130] Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, pinyon jays,[130] black-headed grosbeak, and orioles.[34]

Several species of birds have acquired methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides.

Some mice are able to withstand large doses of the toxin.

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).[132] The Chinese mantis (Tenodera sinensis) will consume the larvae once the gut is removed thus avoiding cardenolides.[133] Wasps commonly consume larvae.[134]

One monarch researcher emphasizes that predation on eggs, larvae or adults is natural, since monarchs are part of the food chain, thus people should not take steps to kill predators of monarchs.[135]

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 color, the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e. the birds have learned the orange monarchs can be eaten), because of camouflage (the white morph matches the white pubescence of milkweed or the patches of light shining through foliage), or because the white morph does not fit the bird's search image of a typical monarch, so is thus avoided.[136]

Parasites include the tachinid flies [137] and Lespesia archippivora. Lesperia-parasitized butterfly larvae suspend, but die prior to pupation. The fly's maggot lowers itself to the ground, forms a brown puparium and then emerges as an adult.[138]

Monarch chrysalises are parasitized by pteromalid wasps, specifically Pteromalus cassotis.[139] These wasps lay their eggs in the larvae while the chrysalis is still soft. Up to 400 adults emerge from the chrysalis after 14–20 days,[139] killing the monarch.

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.[138]

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 parasite levels vary in populations. This is not the case in laboratory rearing, where after a few generations, all individuals can be infected.[140] Infection with this parasite creates an effect known as culling whereby migrating monarchs that are infected are less likely to complete the migration. This results in overwintering populations with lower parasite loads.[141] Owners of commercial butterfly breeding operations claim that they take steps to control this parasite in their practices,[142] although this claim is doubted by most scientists who study monarchs.

The black swallow-wort (Cynanchum louiseae) and pale swallow-wort (Cynanchum rossicum) plants are problematic for monarchs in North America. Monarchs lay their eggs on these relatives of native vining milkweed (Cynanchum laeve) because they produce stimuli similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant from Europe.[143]

The area of forest occupied has been declining and reached its lowest level in two decades in 2013.

A 2014 study acknowledged that while "the protection of overwintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America", their research indicates that habitat loss on breeding grounds in the United States is the main cause of both recent and projected population declines.[145]

Climate variations during the fall and summer affect butterfly reproduction.

There is concern that climate change will dramatically affect the monarch migration. A study from 2015 examined the impact of warming temperatures on the breeding range of the monarch, and showed that in the next 50 years the monarch host plant will expand its range further north into Canada, and that the monarchs will follow this.[148] While this will expand the breeding locations of the monarch, this will also have the effect of increasing the distance that monarchs must travel to reach their overwintering destination in Mexico, and this could result in greater mortality during the migration.[149]

Increasing carbon dioxide levels may be making milkweed—the only food monarch caterpillars will eat—too toxic for the monarchs to tolerate.

Conservation efforts


There has been a major push to conserve the monarch butterfly, which has been largely fueled by reports of the declining numbers of overwintering monarchs.

On 20 June 2014, President Barack Obama issued a presidential memorandum entitled "Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators". The memorandum established a Pollinator Health Task Force, to be co-chaired by the Secretary of Agriculture and the Administrator of the Environmental Protection Agency, and stated:

In May 2015, the Pollinator Health Task Force issued a "National Strategy to Promote the Health of Honey Bees and Other Pollinators".

Many of the priority projects that the national strategy identifies will focus on the I-35 corridor extending for 1,500 miles (2,400 km) from Texas to Minnesota that provides spring and summer breeding habitats in the monarch's key migration corridor.[152]

There have been a number of national and local efforts underway to establish pollinator habitat along highways and roadways, although this effort is controversial.

While there are few scientific studies on the subject, the practice of butterfly gardening and creating "Monarch Waystations" is commonly thought to increase the populations of butterflies.[154] Efforts to increase monarch populations by establishing butterfly gardens and waystations require particular attention to the butterfly's food preferences and population cycles, as well to the conditions needed to propagate milkweed. For example, in the Washington, D.C. area and elsewhere in the northeastern United States, monarchs prefer to reproduce on common milkweed (Asclepias syriaca), especially when its foliage is soft and fresh. As monarch reproduction in that area peaks in late summer when milkweed foliage is old and tough, A. syriaca* needs to be cut back in June - August to assure that it will be regrowing rapidly when monarch reproduction reaches its peak. In addition, milkweed seed may need a period of cold treatment before it will germinate.[61][63][155]

See also


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