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Monarch butterfly

Milkweed butterfly in the family Nymphalidae

Top 10 Monarch butterfly related articles

Monarch butterfly
Female
Male

Apparently Secure (NatureServe)[1]
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Nymphalidae
Genus: Danaus
Species:
D. plexippus
Binomial name
Danaus plexippus
Synonyms
  • Papilio plexippus Linnaeus, 1758
  • Danaus archippus (Fabricius, 1793)[2]
  • Danaus menippe (Hübner, 1816)[3]
  • Anosia plexippus Dyar, 1903

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 12–4 in)[7] A Müllerian mimic, 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]

Monarch butterfly Intro articles: 3

Taxonomy

White morph of the monarch in Hawaii called white monarch

The name "monarch" is believed to have been given in honor of King William III of England, as the butterfly's main color is that of the king's secondary title Prince of Orange.[11] The monarch was originally described by Carl Linnaeus in his Systema Naturae of 1758 and placed in the genus Papilio.[12] In 1780, 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".[13] In the 10th edition of Systema Naturae, at the bottom of page 467,[14] 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.[15]

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.[16]
  • D. cleophile, the Jamaican monarch, described by Jean-Baptiste Godart in 1819, ranges from Jamaica to Hispaniola.[17]

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.[18]
    • D. p. p. (as yet unnamed) – a color morph lacking some wing vein markings.[19]
  • 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:[20]
  • 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%. 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.[18]

Some taxonomists disagree on these classifications.[16][20]

Monarchs belong in the subfamily Danainae of the family Nymphalidae; Danainae was formerly considered a separately family Danaidae.[21]

A 2015 paper identified genes from wasp bracoviruses in the genome of the North American monarch[22] leading to articles about monarch butterflies being genetically modified organisms.[23][24]

Monarch butterfly Taxonomy articles: 43

Description

Monarch butterfly

The monarch's wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in).[7] The upper sides 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.[25] The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants.[26] 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] Monarchs are commonly and easily mistaken for the similar viceroy butterfly – the two species are Müllerian mimics.

Monarch flight has been described as "slow and sailing",[27] with a flight speed estimated at approximately 9 km/h or 5.5 mph.[28] For comparison, the average human jogs at a rate of 9.7–12.9 km/h (6–8 mph).

Adults are sexually dimorphic. Males are slightly larger than females[10][25] 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.[29]

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.[18]

The monarch has six legs like most adult insects, but uses only its middle legs and hindlegs in walking as its forelegs are small, as in all Nymphalidae, and held against its body.[30]

Monarch flying away from a Mexican sunflower

Detailed measurements

Male and female monarch measurements

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.[31] The study found significant differences in overall wing size and in the physical dimensions of wings. Males tended to have larger wings than females, and were heavier than females, on average. Both males and females had similar thorax dimensions (wing muscles are contained in the thorax). Female monarchs tended to have thicker wings, which is thought to convey greater tensile strength. This would make female wings less likely to be damaged during migration. Also, females had lower wing loading than males (wing loading is a value derived from the ratio of wing size to body mass), which would mean females require less energy to fly.[32]

Monarch butterfly Description articles: 6

Distribution and habitat

String of monarchs wintering at the Pismo State Beach Monarch Preserve, 2015

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]:(p18) However, no genetic differences between the western and eastern monarch populations exist;[33] reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species' range.[10]:(p19)

In the Americas, the monarch ranges from southern Canada through northern South America.[4] It has also been found in Bermuda, Cook Islands,[34] Hawaii,[35][36] Cuba,[37] and other Caribbean islands[10]:(p18) the Solomons, New Caledonia, New Zealand,[38] Papua New Guinea,[39] Australia, the Azores, the Canary Islands, Madeira, continental Portugal, Gibraltar,[40] the Philippines, and Morocco.[41] It appears in the UK in some years as an accidental migrant.[42]

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.[43][44] On the US East Coast, they have overwintered as far north as Lago Mar, Virginia Beach, Virginia.[45] 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.[46] 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.[47]

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.

Monarch butterfly Distribution and habitat articles: 18

Life cycle

The monarch butterfly undergoes four stages of complete metamorphosis:

Eggs

The eggs are derived from materials ingested as larvae and from the spermatophores received from males during mating.[48] Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months.[49] 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 ​11000 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.[48] The number of eggs laid by a female, who may mate several times, ranges from 290 to 1180.[50] 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.[51][52] Eggs take 3 to 8 days to develop and hatch into larva or caterpillars.[10]:(p21) Monarchs will lay eggs along the southern migration route.[53]

Larvae

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

Fifth instar with the white spots visible on the prolegs

The first instar caterpillar that emerges from 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 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 an enormous appetite, being able to consume a large milkweed leaf in a day. Its length ranges from 2.5 to 4.5 cm.[4]

Size comparison between an eastern black swallowtail caterpillar (top), a monarch caterpillar (middle), and a queen caterpillar (bottom).

As the caterpillar completes its growth, it is 4.5 cm long (large specimens can reach 5 cm) and 7 to 8 mm wide, and weighs about 1.5 grams, compared to the first instar, which was 2 to 6 mm long and 0.5 to 1.5 mm wide. Fifth-instar larvae increase in weight 2000 times from first instars. Fifth-stage instar larva can chew through the petiole or midrib of milkweed leaves and stop the flow of latex. After this, they eat more leaf tissue. Before pupation, larvae must consume milkweed to increase their mass, after which they stop feeding and search for a pupation site.

In a laboratory setting, the fourth and fifth instar stages of the caterpillar showed signs of aggressive behavior with lower food availability. Attacked caterpillars were found to be attacked when it was feeding on milkweed leaves, and the caterpillar attacked when it was foraging for milkweed.[54] This demonstrates the aggressive behavior of monarch caterpillars due to the availability of milkweed.

Pupa

Monarch larvae begin to pupate alongside other monarch chrysalises
Adult monarch emerges from its chrysalis shell

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 turns around and 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 will suddenly straighten out its body and go into peristalsis some seconds before its skin splits behind its head. It then sheds its skin over a period of a few minutes, revealing a green chrysalis. At first, the chrysalis is long, soft, and somewhat amorphous, but over a few hours it compacts into its distinct shape – an opaque, pale-green chrysalis with small golden dots near the bottom, and a gold-and-black rim around the dorsal side near the top.[55] At first, its exoskeleton is soft and fragile, but it hardens and becomes more durable within about a day. At this point, it is about 2.5 cm (1") long and 10–12 mm (3/8–7/16") wide, weighing about 1.2 grams. At normal summer temperatures, it matures in 8–15 days (usually 11–12 days). During this pupal stage, the adult butterfly forms inside. Within a day or so before emerging is due, the exoskeleton first becomes translucent and the chrysalis more bluish. Finally, within 12 hours or so, it becomes transparent, revealing the black and orange colors of the butterfly inside before it ecloses (emerges).[56]

Adult

An adult butterfly emerges after about two weeks as a chrysalis, and hangs upside down for a few hours until its wings are dry. Fluids are pumped into the wings, which expand, dry, and stiffen. The monarch extends and retracts its wings, and once conditions allow, flies and feeds on a variety of nectar plants. During the breeding season, adults reach sexual maturity in four or five days. However, the migrating generation does not reach maturity until overwintering is complete.[57] Monarchs typically live for two to five weeks during their breeding season.[10]:(pp22–23) Larvae growing in high densities are smaller, have lower survival, and weigh less as adults compared with those growing in lower densities.[58] Monarch metamorphosis from egg to adult occurs during the warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During the development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive.[10]:(pp21–22) However, this is a natural attrition rate for most butterflies, since they are low on the food chain.

Reproduction

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Monarch butterfly mating

Females and males typically mate more than once. Females that mate several times lay more eggs.[59] Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.[60] Male search and capture strategies may influence copulatory success, and human-induced changes to the habitat can influence monarch mating activity at overwintering sites.[61]

Courtship occurs in two phases. During the aerial phase, a male pursues and often forces a female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes.[62] Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others.[63][64] During copulation, a male transfers his spermatophore to a female. Along with sperm, the spermatophore provides a female with nutrition, which aids her in egg laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more females' eggs.[65]

Pictorial life cycle

Monarch butterfly Life cycle articles: 17

Larval host plants

The host plants used by the monarch caterpillar include:

Swamp milkweed, one of many species of Asclepias milkweeds used by the monarch

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.[72] This is thought to adversely affect migration patterns, and to cause a dramatic buildup of the dangerous parasite, Ophryocystis elektroscirrha.[73] New research also has shown that monarch larvae reared on tropical milkweed show reduced migratory development (reproductive diapause), and when migratory adults are exposed to tropical milkweed, it stimulates reproductive tissue growth.[74]

Monarch butterfly Larval host plants articles: 28

Adult food sources

Nectaring on purple coneflower (Echinacea purpurea)

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.[44]

Monarch butterfly Adult food sources articles: 17

Migration

In North America, monarchs migrate both north and south on an annual basis, in a long-distance journey that is fraught with risks.[4] The population east of the Rocky Mountains attempts to migrate to the sanctuaries of the Mariposa Monarca Biosphere Reserve in the Mexican state of Michoacán and parts of Florida. The western population tries to reach overwintering destinations 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.[75] Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico,[76] though they have a much lower migratory success rate than wild monarchs do.[77] See section on captive-rearing below. Monarch overwintering sites have been discovered recently in Arizona.[78] Monarchs from the eastern US generally migrate longer distances than monarchs from the western US.[79]

Monarch butterfly Migration articles: 6

Vision

Physiological experiments suggest that monarch butterflies view the world through a tetrachromatic system.[80] 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.[81] 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.[80] The combination of filtered and unfiltered green opsins permits the butterflies to distinguish yellow from orange colors.[80] 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.[82]

These butterflies are capable of distinguishing colors based on their wavelength only, and not based on intensity; this phenomenon is termed "true color vision". This is important for many butterfly behaviors, including seeking nectar for nourishment, choosing a mate, and finding milkweed to lay eggs on. One study found that floral color is more easily recognized at a distance by butterflies searching for nectar than floral shape. This is may be because flowers have highly contrasting colors to the green background of a vegetative landscape.[83] On the other hand, leaf shape is important for oviposition so that the butterflies can ensure their eggs are being laid on milkweed.

Beyond the perception of color, the ability to remember certain colors is essential in the life of monarch butterflies. Researchers have found that these insects can easily learn to associate color and, to a lesser extent shape, with sugary food rewards. When searching for nectar, color is the first cue that draws the insect's attention toward a potential food source, and shape is a secondary characteristic that promotes the process. When searching for a place to lay one's eggs, the roles of color and shape are switched. There may also be a difference between male and female butterflies from other species in terms of the ability to learn certain colors; however, there is no differences between the sexes for monarch butterflies.[83]

Monarch butterfly Vision articles: 6

Interactions with 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. 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.[84]

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.[85]

Butterfly weed (Asclepias tuberosa) lacks significant amounts of cardiac glycosides, but instead contains other types of toxic glycosides, including pregnanes.[86] This difference may reduce the toxicity of monarchs whose larvae feed on that milkweed species, as a naturalist has reported that monarch caterpillars do not favor the plant.[87] Some other milkweeds may have similar characteristics.

Types of predators

While monarchs have a wide range of natural predators, none of these are suspected of causing harm to the overall population, or are the cause of the long-term declines in winter colony sizes.

Several species of birds have acquired 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.[88] The black-headed grosbeak, on the other hand, has developed an insensitivity to secondary plant poisons that 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 effectively reduces potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.[88] Other bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, and pinyon jays.[85]

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.[89]

Some mice, particularly the black-eared mouse (Peromyscus melanotis) are able to tolerate large doses of cardenolides and are able to eat monarchs.[90] 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 and black-eared mice will eat up to 40 monarchs per night.[43][90]

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).[91] The Chinese mantis (Tenodera sinensis) will consume the larvae once the gut is removed thus avoiding cardenolides.[92] Predatory wasps commonly consume larvae,[93] though large larvae may avoid wasp predation by dropping from the plant or by jerking their bodies.[94]

Aposematism

Chemical structure of oleandrin, one of the cardiac glycosides

Monarchs are foul tasting and poisonous due to the presence of cardenolides in their bodies, which the caterpillars ingest as they feed on milkweed.[60] Monarchs and other cardenolide resistant insects rely on a resistant form of the Na+/ K+-ATPase enzyme to tolerate significantly higher concentrations of cardenolides than nonresistant species.[95] 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.[96] 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.[97]

Additional studies have shown that different species of milkweed have different effects on growth, virulence, and transmission of parasites.[98] One species, Asclepias curassavica, appears to reduce the symptoms of Ophryocystis elektroscirrha (OE) infection. There are two possible explanations for this: 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 OE parasites.[98] A. curassavica does not cure or prevent the infection with OE, it merely allows infected monarchs to live longer, and this would allow infected monarchs to spread the OE spores for longer periods. For the average home butterfly garden, this scenario will only add more OE to the local population.[99]

After the caterpillar becomes a butterfly, the toxins 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.[96] In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.[100]

Mimicry

Monarch (left) and viceroy (right) butterflies exhibiting Müllerian 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.[101]

Monarch butterfly Interactions with predators articles: 31

Human interaction

The monarch is the state insect of Alabama,[102] Idaho,[103] Illinois,[104] Minnesota,[105] Texas,[106] Vermont,[107] and West Virginia.[108] Legislation was introduced to make it the national insect of the United States,[109] but this failed in 1989[110] and again in 1991.[111]

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. [112]

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.[113]

Sanctuaries and reserves have been created at overwintering locations in Mexico and California to limit habitat destruction. These sites can generate significant tourism revenue.[114] However, with less tourism, monarch butterflies will have a higher survival rate because they show more protein content and a higher value of immune response and oxidative defense.[115]

Organizations and individuals participate in tagging programs. Tagging information is used to study migration patterns.[116]

The 2012 novel by Barbara Kingsolver, Flight Behavior, deals with the appearance of a large population in the Appalachians.[117]

Captive rearing

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 one hand there are many positive aspects of captive rearing. Monarchs are bred in schools and used for butterfly releases at hospices, memorial events and weddings.[118] Memorial services for the September 11 attacks include the release of captive-bred monarchs.[119][120][121] Monarchs are used in schools and nature centers for educational purposes.[122] Many homeowners raise monarchs in captivity as a hobby and for educational purposes.[123]

On the other hand this practice becomes problematic when monarchs are "mass-reared". Stories in the Huffington Post in 2015 and Discover magazine in 2016 have summarized the controversy around this issue.[124][125] The frequent media reports of monarch declines has encouraged many homeowners to attempt to rear as many monarchs as possible in their homes and then release them to the wild in an effort to "boost the monarch population". Some individuals have taken this practice to the extreme, with massive operations that rear thousands of monarchs at once, like one in Linn County, Iowa.[126] However, the practice of rearing "large" numbers of monarchs in captivity for release into the wild is not condoned by monarch scientists, because of the risks of genetic issues and disease spread.[127] One of the biggest concerns of mass-rearing is the potential for spreading the monarch parasite, Ophryocystis elektroscirrha, into the wild. This parasite can rapidly build up in captive monarchs, especially if they are housed together. The spores of the parasite also can quickly contaminate all housing equipment, so that all subsequent monarchs reared in the same containers then become infected. One researcher stated that rearing more than 100 monarchs constitutes "mass-rearing" and should not be done.[128]

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. Homeowners often raise monarchs in plastic or glass containers in their kitchens, basements, porches, etc., and under artificial lighting and controlled temperatures. Such conditions would not mimic what the monarchs are used to in the wild, and may result in adult monarchs that are unsuited for the realities of their wild existence. In support of this, a recent study by a citizen scientist found that captive-reared monarchs have a lower migration success rate than wild monarchs do.[77]

A study published in 2019 shed light on the fitness of captive-reared monarchs, by testing reared and wild monarchs on a tethered flight apparatus that assessed navigational ability.[129] In that study, monarchs that were reared to adulthood in artificial conditions showed a reduction in navigational ability. This happened even with monarchs that were brought into captivity from the wild for a few days. A few captive-reared monarchs did show proper navigation. This study revealed the fragility of monarch development: if the conditions are not suitable, their ability to properly migrate could be impaired. The same study also examined the genetics of a collection of reared monarchs purchased from a butterfly breeder, and found they were dramatically different from wild monarchs, so much so that the lead author described them as "franken-monarchs".[130]

An unpublished study in 2019 compared behavior of captive-reared versus wild monarch larvae.[131] The study showed that reared larvae exhibited more defensive behavior than wild larvae. The reason for this is unknown, but it could relate to the fact that reared larvae are frequently handled and/or disturbed.

Monarch butterfly Human interaction articles: 22