The monarch is one of the best known butterfly species. Although native to North and South America, it has expanded its distribution to Australia, many Pacific islands, and a few places in Europe. Monarch larvae, like those of many other Danaini, feed on milkweeds of the family Asclepiadaceae, from which they sequester cardiac glycosides that often render them unpalatable to potential predators. The species is perhaps best known for its annual migrations, from eastern North America to the mountains of central Mexico, and from western North America to the coast of California. The butterflies rest for the cool winter months in large aggregations which may contain tens of millions of individuals.
Left: Monarch larva feeding on milkweed host plant at Poco Sol (Caribbean slope below Monteverde), Costa Rica. Right: Overwintering monarchs covering a tree branch at Alpha Monarch research site, Mexico. Images © Greg and Marybeth Dimijian
North American Ecology (US and Canada)
- Scott, J. A. 1986. The butterflies of North America. Stanford University Press.
Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) The Monarch butterfly is attaining near worldwide distribution in suitable places, but is primarily a species of the Americas. Autumnal migrants occasionally stray to Europe including England; the species has been spread widely in the Pacific Ocean area and is well-established in Australia. However, the seasonal ranges in North America are vastly different, by millions of square kilometers. Essential overwintering areas for North American populations are limited to a few dozen places in coastal California and the mountains of Mexico. Specifically that means that the summer range includes portions of up to 49 of the 50 US states and southern portions of all Canadian Provinces bordering the US where milkweeds occur. However the winter range includes none of Canada and in the US only a few dozen square kilometers in California, and at least some years a bit of south Florida. Florida winter populations may be non-migratory and if so contribut little to summer populations. Some winters a few may persist in sothern Arizona.
Monarch butterflies are found in North America, South America, the Caribbean, Australia, New Zealand, islands in the Pacific Ocean, Mauritius, the Canary Islands of the Atlantic, and Western Europe.
Both male and female monarchs are bright orange with black borders and black veins. The veins on the female are thicker than those of the male. Male monarchs also have a swollen pouch on both of their hind wings.
Monarchs are poisonous to vertebrates. Their poison comes from the milkweed they feed on.
Monarchs also use their appearance to ward off predators. Orange is considered a warning color, which will warn predators that monarchs are poisonous, and not to attack them. From a distance, monarchs can blend into their surroundings. Sometimes, their spots will appear to be the eyes of a larger animal, and will ward off predators.
Range wingspan: 8.6 to 12.4 cm.
Other Physical Features: ectothermic ; bilateral symmetry ; poisonous
Monarch butterflies have a wingspan of 3-4 1/2 inches (75-114 mm). The male is bright orange and has a black scent patch in the middle of the upper hind wing. The female is dull orange or brown with more thickly scaled black veins. The underside of the hind wing is light orange with black veins and a marginal black border with two rows of white spots. The upper front wing has a black apex (tip) with white spots. The sub-apex has light orange spots on a dark background. The underside of the front wing has white spots and yellow ovals in the outer portion.
- Opler, P. A. and A. Bartlett Wright. 1999. Western Butterflies. Peterson Field Guides. Houghton Mifflin Co., Boston, New York.
- Stewart, B., P. Brodkin, and H. Brodkin. 2001. Butterflies of Arizona. A Photographic Guide. West Coast Lady Press, Arcata, CA.
Comments: Habitat is a complex issue for this species. In general breeding areas are virtually all patches of milkweed in North America and some other regions. The critical conservation feature for North American populations is the overwintering habitats, which are certain high altitude Mexican conifer forests or coastal California conifer or Eucalyptus groves as identified in literature. It appears virtually all North American monarchs overwinter in one of these two areas. Lethal cold would preclude successful overwintering in places like the Gulf Coast and much of Florida some years and it appears thse are not major wintering regions as used to be assumed. In addition certain major coastal migratory stopovers may be important conservation sites especially those in along Delaware Bay in New Jersey including Cape May where adults may holdover for several days awaiting suitable conditions for crossing the Bay. There are major, but probably less important, roosting sites farther north such as east of New Haven and probably others farther south perhaps even as far as Cuba. Coastal regions are important flyways and so nectar (wild or in gardens) is an important reseource in such places. In places like Hawaii and some others the species simply breeds year round and is not really migratory.
Monarch butterflies prefer warmer climates; they cannot tolerate frost. They like open country. Females spend much of their time around searching for or staying near the main food plant for the caterpillars (see food habits section). Monarch butterflies require thick tree covering during the winter. In California, they live in eucalyptus trees. These eucalyptus trees are not normally found in California, but were put there to replace trees that had been cut down. Biomes: temperate forest and rainforest, temperate grassland, chaparral, tropical rainforest, tropical deciduous forest, tropical scrub forest, tropical savanna and grasslands, mountains
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Comments: Larval foodplants are milkweeds.
Monarch females lay their eggs on milkweed plants (Asclepias), these plants are then the main food for the caterpillars. Adult monarchs feed on nectar from many kinds of flowers.
Flowering Plants Visited by Danaus plexippus in Illinois
(observations are from Robertson, Graenicher, Betz et al., Reed, Hilty, Clinebell, Smith & Snow, Hapeman, Macior, Broyles & Wyatt, Willson & Bertin, Fothergill & Vaughn; this is the Monarch butterfly)
Apiaceae: Eryngium yuccifolium sn (Rb), Pastinaca sativa sn (Rb), Sium suave sn (Rb), Torilis arvensis sn (FV); Asclepiadaceae: Ampelamus albidus [plup sn] (H), Asclepias exaltata [plup sn] (BW), Asclepias incarnata [plpr sn] [plup sn] (Rb, Btz), Asclepias syriaca [plpr sn] (Rb), Asclepias tuberosa [plab] [plup] (Rb, Btz, H), Asclepias verticillata [plpr sn] [plup sn] (Rb, WB); Asteraceae: Aster drummondii sn (Gr), Aster furcatus sn (Gr), Aster laevis sn (Gr), Aster lanceolatus sn (Gr), Aster macrophyllus sn (Gr), Aster novae-angliae sn (Rb, Gr, H), Aster pilosus sn (Rb), Aster puniceus sn (Gr), Aster salicifolius sn (Rb), Bidens aristosa sn (Rb), Bidens cernua sn fq (Rb), Boltonia asterioides sn (Rb), Cirsium altissimum sn (Gr), Cirsium discolor sn (Rb, H), Cirsium hillii sn (Rb), Cirsium vulgare sn (Rb, Gr), Conoclinium coelestinum sn (Rb), Coreopsis tripteris sn (Rb), Echinacea pallida sn (Rb), Echinacea purpurea sn (Rb, H), Eupatoriadelphus purpureus sn (Rb, Gr, H), Eupatorium altissimum sn (H), Eupatorium perfoliatum sn (Rb, Gr), Euthamia graminifolia sn (Gr), Helianthus annuus sn (Rb, H), Helianthus grosseserratus sn fq (Rb), Helianthus mollis sn (H), Helianthus strumosus sn (Gr), Heliopsis helianthoides sn (Gr), Liatris aspera sn (Rb, H), Liatris pycnostachya sn (Rb), Liatris spicata sn (Gr), Oligoneuron rigidum sn (H, Re), Rudbeckia laciniata sn (Gr), Silphium integrifolium sn (H), Silphium laciniatum sn (Rb), Silphium perfoliatum sn (Rb), Silphium terebinthinaceum sn (H), Solidago canadensis sn (Rb, Gr), Solidago nemoralis sn (Rb), Taraxacum officinale sn (FV), Vernonia fasciculata sn (Rb); Boraginaceae: Mertensia virginica sn (Rb); Campanulaceae: Lobelia siphilitica sn (Rb); Caprifoliaceae: Symphoricarpos albus sn (Gr), Symphoricarpos occidentalis sn (Gr); Dipsacaceae: Dipsacus fullonum sn (Rb); Fabaceae: Melilotus alba sn (Rb), Robinia pseudoacacia sn np (Rb), Trifolium pratense sn (Rb), Trfiolium repens sn (FV); Lamiaceae: Agastache nepetoides sn (Rb), Blephilia hirsuta sn (Rb), Monarda fistulosa sn (Rb, Cl), Physostegia virginiana sn np (Rb), Salvia azurea sn np (H); Liliaceae: Lilium michiganense sn (Rb); Orchidaceae: Platanthera blephariglottis sn (SS), Platanthera ciliaris sn (SS), Platanthera peramoena sn (Hpm); Polemoniaceae: Phlox divaricata laphamii sn (Rb), Phlox glaberrima sn (Rb); Polygonaceae: Persicaria pensylvanica sn (H); Ranunculaceae: Delphinium tricorne sn np (Rb, Mc); Rosaceae: Crataegus crus-galli sn (Rb), Crataegus intricata sn (Rb), Malus coronaria sn (Rb), Prunus americana sn (Rb), Prunus serotina [flwr sn] (Rb); Rubiaceae: Cephalanthus occidentalis sn fq (Rb); Verbenaceae: Verbena stricta sn (Rb); Violaceae: Viola sororia sn (FV)
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: > 300
Comments: This refers to global occurrences for the species as a whole and includes introduced populations in Hawaii, Australia etc., as well as central and South America. In these places and where non-migratory native populations occur, habitat may be more or less used all year. North American populations also have a lot of breeding habitat, most of it patchy and suboptimally managed for Monarch survival, but still producing millions of adults each summer. These face an annual bottleneck in terms of dependence on a few dozen overwintering areas. Western North American populations may also be under pressure from scarcity of good breeding sites, especially in dry years, and incompatible management.
Comments: At least millions of individuals, probably more than a billion in many years, worldwide.
Can contain poison derived from their larval foodplant that helps avoid predation by birds (Scott, 1986).
Life History and Behavior
- Scott, J. A. 1986. The butterflies of North America. Stanford University Press.
Small caterpillars hatch from eggs laid by female Monarchs. They grow, shedding their skin to get bigger. Eventually each caterpillar stops growing and forms a case around itself called a chrysalis. Inside the chrysalis it changes its body its body in a process called metamorphosis. When it is done it emerges as an adult butterfly.
Development - Life Cycle: metamorphosis
In Baja California may be encountered throughout the year (November-April in coastal regions and June-October in the Mountains) (Brown et al., 1992)
The mating period occurs in the spring, just prior to migration from the overwintering sites. The courtship of Danaus plexippus 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 and involves the transfer of a spermatophore from the male to female. Along with sperm, the spermatophore is thought to provide the female with energy resources that aid her in carrying out reproduction and remigration.
Breeding interval: Monarch butterflies mate in the spring before they migrate.
Key Reproductive Features: semelparous ; sexual ; fertilization (Internal ); oviparous
Molecular Biology and Genetics
Barcode data: Danaus plexippus
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
-- end --
Download FASTA File
Statistics of barcoding coverage: Danaus plexippus
Public Records: 18
Specimens with Barcodes: 82
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: N5B - Secure
Rounded National Status Rank: N5B,N2N : N5B: Secure - Breeding, N2N: Imperiled - Nonbreeding
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: The Monarch is globally secure, because it is doing well in many places where populations are not native and/or not strongly migratory, which drives the G5 rank. However, this is misleading in North America where the Monarch is no longer secure because of serious threats to their obligate overwintering areas in Mexico (mostly) and and a recent order of magnitude decline in California based population, which apparently reflects threats in the western breeding range. Threats also exist in the eastern spring migration route (especially in Texas). The migratory North American populations have been declining in recent years with 2009 among the worst ever (2010 data not evaluated) for California winter population. Deteriorating spring climate conditions in Texas appear to be a major contributor to recently declining numbers in the eastern US in the 2000s. Western North American populations are probably more threatened than eastern ones because suitable milkweeds are less reliable in generally more arid western regions in dry periods. Rank Calculator v3.0 is also G5. Regardless of the fate of North american populations, the species as a whole is globally secure.
Environmental Specificity: Broad. Generalist or community with all key requirements common.
Comments: Very broad as a breeder everywhere and globally overall, but very narrow for overwintering populations in North America.
Other Considerations: Elimination of Mexican sites would mean virtual extinction of eastern North American populations. Nearly all western individuals had been thought to winter in California, but some of these also go to th mountains of Mexico. This species has become a significant ecotourism resource locally in Mexico.
The places where monarchs spend the winter are threatened. The United States and Mexico are trying to help the areas where monarch butterflies migrate.
IUCN Red List of Threatened Species: no special status
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
Global Short Term Trend: Increase of 10-25% to decline of 30%
Comments: Difficult to assess globally. Stable to increasing in many places where it is not native and in some Neotropical places where it is. North American populations, which are still a large portion of the global total, are well documented to be declining, but fluctuate from year to year. California based populations may be down more than 90% from 1997-2009.
Global Long Term Trend: Increase of >25% to decline of 50%
Comments: See short term.
Degree of Threat: Medium
Comments: The species as a whole is not seriously threatened, but the typical subspecies may be secure mostly where it is not native. The monarch appears to be doing well or even increasing in may places, but not in its core North American range. The native eastern North American populations are in trouble at their overwintering grounds. Overwintering habitats in Mexico are primarily in a few hectares each and have been under pressure from logging, agricultural and urban development. California based populations are declining more due to problems in the breeding areas. Climate change is emerging as a current, rather than an expected, threat in Mexico, California, and Texas at least. There was some improvement in the eastern, as well as western, USA in 2010 so at least populations can still respond quickly to temporarily good conditions. Regardless of what happens in North America this species is at virtually no risk of global extinction in the forseeable future.
Biological Research Needs: The basics of Monarch biology, migration, etc. are now rather well known, although some of the details regarding western US populations may still need resolution. Main needs now appear to be understanding impacts of and how to prevent or mitigate habitat changes and climate change primarily in overwintering areas and during spring migration particularly in Texas. Accurate information for Central and South America needs to be compiled, because this is globally significant to the species even if unrelated to its status in North America.
Global Protection: Unknown whether any occurrences are appropriately protected and managed
Comments: Too difficult to assess because needs vary so widely with local ecology.
Needs: Winter habitats in California and Mexico need protection from logging, development, etc. This need has been known for decades. In the near term mitigation of climate change impacts could become an urgent need.
Relevance to Humans and Ecosystems
Economic Importance for Humans: Positive
Some milkweed plants are poisonous to cattle. Monarch butterflies help reduce the amount of these plants.
Compared to bees, butterflies are often less efficient at transferring pollen between plants because pollen often does not stick to their bodies and they lack specialized structures for collecting pollen. However, on the occasions in which pollen does stick to the butterflies' bodies, it is often inadvertently transferred to another flower while the butterflies are nectaring, earning them the title of pollinators.
Research has shown that most butterfly species do indeed end up pollinating some of the flowers they visit, and because monarch butterflies have such a long migration route they are potential pollinators to plants across the country! Studies have found that monarch butterflies do carry pollen from swamp milkweed (Asclepias incarnata) and are, therefore, potential pollinators. Monarch butterflies are known pollinators of common milkweed (Asclepias syriaca), butterfly weed (Asclepias tuberosa), and rattlesnake master (Eryngium yuccifolium). Additionally, monarch butterflies feed on the nectar of many other plants, like rabbitbrush (Ericameria nauseosa), desert broom (Baccharis spp.), aster (Aster spp.), sunflower (Helianthus spp.), goldenrod (Solidago spp.), verbena (Verbena spp.), clover (Trifolium spp.), and other milkweed species (Asclepias spp.).
Although adult monarchs feed on the nectar of many flowers, they will breed only where milkweed plants are found. That is because monarch larvae feed exclusively on milkweed plants. Milkweed plants contain compounds called cardenolide alkaloids that are toxic to most vertebrate herbivores if ingested. However, while monarch larvae feed on milkweed plants they also ingest the toxins. They then sequester the compounds in their bodies, wings, and exoskeletons, making both the larvae and adult butterflies toxic to many predators.
- The Monarch Butterfly in North America, US Forest Service
- Diversity and temporal change in the effective pollinators of Asclepias tuberosa, D. L. Venable, Ecology, Vol. 77, No. 4, 1996, p. 1061-1073, Pollinator Conservation Digital Library
- Migratory Pollinators Program: Monarch Butterfly (Danaus plexippus), Arizona-Sonora Desert Museum
- Reproductive biology of Eryngium yuccifolium (Apiaceae), a prairie species, Brenda Molano-Flores, Journal of the Torrey Botanical Society, Vol. 128, No. 1, 2001, p. 1-6, Pollinator Conservation Digital Library
The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the most familiar North American butterfly. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in). (The viceroy butterfly is similar in color and pattern, but is markedly smaller, and has an extra black stripe across the hind wing.)
The eastern North American monarch population is notable for its multi-generational southward late summer/autumn migration from the United States and southern Canada to Mexico, covering thousands of miles. The western North American population of monarchs west of the Rocky Mountains most often migrate to sites in California but have been found in overwintering Mexico sites.
- 1 Range
- 2 Habitat
- 3 Status
- 4 Reproduction
- 5 Life cycle
- 6 Taxonomy
- 7 Sexual dimorphism
- 8 Food plants
- 9 Adult food sources
- 10 See also
- 11 Origin of name
- 12 Description
- 13 Migration
- 14 Defense against predators
- 15 Human Interactions
- 16 Threats
- 17 Genome
- 18 Conservation
- 19 See also
- 20 References
- 21 External links
In North America, the monarch ranges from southern Canada to northern South America. It rarely strays to western Europe (rarely as far as Greece) from being transported by US ships or by flying there if weather and wind conditions are right. It has also been found in Bermuda, Cook Islands, Hawaii, the Solomons, New Caledonia, New Zealand, Australia, New Guinea, Sri Lanka, India, Nepal, the Azores, the Canary Islands Phillippines, and Honolulu.
Overwintering populations of D. plexippus are found in Mexico, California, along the Gulf coast, year-round in Florida, and in Arizona where the habitat provides the specific conditions necessary for their survival. The overwintering habitat typically provides access to streams or damp soil, cool temperatures, necessary levels of humidity, periods of sunlight (for body temperatures that allow flight), appropriate vegetation on which to roost, relatively free of predators. Forest sanctuaries but can be found in agricultural fields and pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides. The eastern North American overwinters in Mexican conifer groves. Roosting butterflies have been seen on sumacs, locusts, basswood elm, oak, osage orange, mulberry, pecan, willow, cottonwood, and mesquite.
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.
Males who are fit are more likely to mate. Females and males typically mate more than once. Females that mate several times lay more eggs. Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.  Courtship occurs in two phases. During the aerial phase, the male pursues and often forces the female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes.  Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating though some have more success than others.   During copulation, the male transfers the spermatophore to the female. Along with sperm, the spermatophore provides the female with nutrition to aid her in egg-laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more female's eggs.
The monarch undergoes complete metamorphosis consisting of four stages:
- The eggs are laid during the spring and summer months onto the leaves of milkweed plants.
- The eggs hatch (after four days), into larvae, or caterpillars. The caterpillar consumes its egg case then begins to feed on milkweed and sequester cardenolides, a type of cardiac glycoside. During the larval stage, monarchs store energy in the form of fat and nutrients to carry them through the nonfeeding pupal stage. The larval stage lasts around two weeks during which it molts five times. Each molt is called an instar.
- In the pupa or chrysalis stage, the caterpillar spins a silk pad onto a horizontal substrate. It then hangs from the pad by the last pair of prolegs upside down resembling the letter 'J'. It sheds its skin leaving itself encased in an articulated green exoskeleton. During this pupal stage, the adult butterfly forms inside. The exoskeleton becomes transparent before it ecloses (emerges), and its adult colors can be seen.
- The adult butterfly emerges after about two weeks, and hangs until its wings are dry. Fluids are pumped into wings and they expand and stiffen. The monarch expands and retracts its wings, and when conditions allow it then flies to feed on a variety of nectar plants. In about three days the adult reaches reproductive maturity.
Adult monarchs live two to eight weeks during the breeding season.
The name 'monarch' may be in honor of King William III of England. The monarch was originally described by Linnaeus in his Systema Naturae of 1758 and it was placed in the genus Papilio. In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus; Danaus. There are three species of Monarch butterflies:
- D. plexippus, described by Linnaeus in 1758, is the species known most commonly as the monarch butterfly of North America. Its range actually extends worldwide and can be found in Hawaii, Australia, New Zealand, Spain and on Oceanic Islands.
- D. erippus, the southern monarch was described by Cramer in 1775, This species is found in tropical and subtropical latitudes of South America, mainly in Brazil, Uruguay, Paraguay, Argentina, Bolivia, Chile and southern Peru. The south American monarch and the North American monarch may have been one species at one time. Some researchers believe the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. Sea levels were higher then and the entire Amazonas lowland was a vast expanse of brackish swamp that offered limited butterfly habitat.
- D. cleophile Jamaican monarch described by Godart in 1819, the Lepidoptera Specialist Group in 1996, and the IUCN Red List of Threatened Species, Version 2014.1, in 2014. It ranges from Jamaica to Hispaniola.
Three subspecies and two color morphs of D. plexippus have been identified:
- D. p. plexippus, the nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America.
- D.p.m. (as yet unnamed), a color morph lacking wing vein markings.
- D. p. nigrippus named in 1909 by Richard Haensch as forma: Danais [sic] archippus f. nigrippus. Hay-Roe et al. in 2007 identified this taxon as a subspecies:
- D. p. megalippe, named in 1826 by Jacob Hübner, is a nonmigratory subspecies, and is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River. Three local forms were at first considered other subspecies, but are considered color varieties of D. p. megalippe These are:
- D. p. m. leucogyne, named by Arthur G. Butler in 1884
- D. p. m. portoricensis, named in 1941 by A.H. Clark
- D. p. m. tobagi, also named in 1941 by A.H. Clark
The levels of cardiac glycosides in white monarchs in Hawaii are low because their host plant Calotropis gigantea (crown flower) contains a low level of these toxins. The percentage the white morph in Oahu, is nearing 10%. On other Hawaiian islands, the white morph occurs at a relatively low frequency. White Monarchs (nivosus) have been found throughout the world, including Australia, New Zealand, Indonesia, and the United States.
Larvae host plants
The host plants used by the monarch caterpillar include:
- Asclepias nivea – Caribbean milkweed
- Asclepias syriaca – Common Milkweed
- Asclepias incarnata – Swamp Milkweed
- Asclepias verticillata – Whorled Milkweed
- Asclepias tuberosa – Butterfly Weed
- Asclepias exaltata – Poke Milkweed
- Asclepias viridis – Green Antelopehorn Milkweed
- Asclepias asperula - Antelopehorns Milkweed
- Asclepias oenotheroide – Zizotes Milkweed
- Asclepias variegata – White Milkweed
- Asclepias perennis – Aquatic Milkweed
- Asclepias humistrata – Sandhill/Pinewoods Milkweed
- Asclepias fascicularis – Mexican Whorled Milkweed
- Asclepias speciosa – Showy Milkweed
- Asclepias subulata – Rush Milkweed
- Asclepias erosa - Desert Milkweed
- Asclepias californica – California Milkweed
- Asclepias cordifolia – Heartleaf Milkweed
- Asclepias vestita – Woolly Milkweed
- Asclepias eriocarpa – Woolly Pod Milkweed
- Sarcostemma clausa – white vine
Asclepias curassavica has been planted as an ornamental and naturalized. Its distribution is probably wordwide. Year-round plantings may be the cause of new overwintering sites along the Gulf coast and in Spain.
Adult food sources
Although larvae eat only milkweed, adult monarchs feed on the following nectar plants:
- Apocynum cannabinum – Indian hemp
- Asclepias californica – California milkweed
- Asclepias incarnata – swamp milkweed
- Asclepias syriaca – common milkweed
- Asclepias tuberosa – butterfly weed
- Aster sp. – asters
- Cirsium sp. – thistles
- Daucus carota – wild carrot
- Dipsacus sylvestris – teasel
- Erigeron canadensis – horseweed
- Eupatorium maculatum – spotted joe-pye weed
- Eupatorium perfoliatum – common boneset
- Hesperis matronalis – dame's rocket
- Medicago sativa – alfalfa
- Solidago sp. – goldenrod
- Syringa vulgaris – lilac
- Trifolium pratense – red clover
- Vernonia altissima – tall ironweed
Origin of name
In Homeric Greek plexippos (πληξιππος) means "one who urges on horses", i.e. "rider or charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467, Linnaeus wrote that the names of the Danai festivi, the division of the genus to which Papilio plexippus belonged, were derived from the sons of Aegyptus.
The monarch’s wingspan ranges from 8.9–10.2 cm (3½–4 in). The upper side of the wings is tawny-orange, the veins and margins are black, and in the margins are two series of small white spots. The fore wings also have a few orange spots near the tip. The underside is similar, but the tip of the fore wing and hind wing are yellow-brown instead of tawny-orange and the white spots are larger.
The male has a black patch of androconial scales on either hind wing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs), and the black veins on its wing are narrower than the female’s. The male is also slightly larger. One variation has been observed in Australia, New Zealand, Indonesia and the United States termed nivosus by lepidopterists. It is grayish-white in all areas of the wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist on Oahu in Hawaii, possibly due to selective predation.
Like all insects, the monarch has six legs, but uses the four hindlegs as it carries its two front legs against its body.
The eggs are creamy white and later turn pale yellow. They are elongated and subconical, with about 23 longitudinal ridges and many fine traverse lines. A single egg weighs about 0.46 mg (0.0071 gr), and measures about 1.2 mm (47 mils) high and 0.9 mm (35 mils) wide.
The caterpillar is banded with yellow, black, and white stripes. The head is also striped with yellow and black. Two pairs of black filaments are seen, one pair on each end of the body. The caterpillar reaches a length of 5 cm (2 in).
The chrysalis is blue-green with a band of black and gold on the end of the abdomen. Other gold spots occur on the thorax, the wing bases, and the eyes.
The eastern population migrates both north and south on an annual basis. The population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve.in Mexico. The western population overwinters in various coastal sites in central and southern California.
The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring.
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.
Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed. By ingesting a large amount of plants in the genus Asclepias, primarily milkweed, monarch caterpillars are able to sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis. It has been found that monarchs are able to sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content.
Additional studies have shown that different species of milkweed have differing effects on growth, virulence, and transmission of parasites. One species, Asclepias curassavica, appears to reduce the proportion of monarchs infected by parasites. There are two possible explanations for the positive role of A. curassavica on the monarch caterpillar: that it promotes overall monarch health to boost the monarch's immune system; or that chemicals from the plant have a direct negative effect on the parasites.
After the caterpillar becomes a butterfly, the toxin shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste, and may prevent them from ever ingesting the body of the butterfly. In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.
Monarchs share the defense of noxious taste with the similar-appearing viceroy butterfly in what is perhaps one of the most well-known examples of mimicry. Though long purported to be an example of Batesian mimicry, the viceroy is actually reportedly more unpalatable than the monarch, making this a case of Müllerian mimicry.
The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, Vermont, and West Virginia. It was nominated in 1990 as the national insect of the United States of America. but the legislation did not pass.
Monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these Monarch Waystations. Monarchs are raised as a hobby and for educational purposes. Butterfly farmers raise Monarchs and ship them to individuals and organizations to be released at a wedding or funeral, for example. The release of captive bred Monarchs remains controversial.
Some organizations, such as the Cape May Bird Observatory, have monarch identification tagging programs. Plastic stickers are placed on the wing of the insect with identification information. Tracking information is used to study their migration patterns, including how far and where they fly.
Larva feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepias sp.vary. Not all monarchs are unpalatable, but exibit Batesian or automimics. Cardiac glycosides levels are higher in the abdomen and wings. Some predators can differentiate betwwen these parts and consume the most palatable ones. Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, pinyon jays,Black-headed Grosbeak, and orioles.
Some mice are able to withstand large doses of the toxin. Overwintering adults become less toxic over time making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice.
In North America, eggs and first instar larvae of the monarch are eaten by larvae and adults of the introduced Asian lady beetle (Harmonia axyridis). The Chinese mantid ("Tenodera sinensis") will consume the larvae once the gut is removed thus avoiding cardenolides. Wasps commonly consume larvae.
Several birds have also adapted various methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides. The oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The grosbeak, on the other hand, has adapted the ability an insensitivity to secondary plant poisons which allows it to ingest monarchs without vomiting. As a result, orioles and grosbeaks will periodically have high levels of cardenolides in their bodies, and they will be forced to go on periods of reduced monarch consumption. This cycle of predation effectively reduces the potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.
On Oahu, a white morph of the monarch has emerged. This is because of the introduction, in 1965 and 1966, of two bulbul species, Pycnonotus cafer and Pycnonotus jocosus. They are now the most common insectivore birds, and probably the only ones preying on insects as large as the monarch. Monarchs in Hawaii are known to have low cardiac glycoside levels, but the birds may also be tolerant of the chemical. The two species hunt the larvae and some pupae from the branches and undersides of leaves in milkweed bushes. The bulbuls also eat resting and ovipositing adults, but rarely flying ones. Because of its colour, the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e. the birds have learned the orange monarchs can be eaten), because of camouflage (the white morph matches the white pubescence of milkweed or the patches of light shining through foliage), or because the white morph does not fit the bird's search image of a typical monarch, so is thus avoided.
Parasites include the tachinid flies Sturmia convergens and Lespesia archippivora. Lesperia-parasitized butterfly larvae complete the formation of their crysalid but die before they emerge as an adult. Before pupation is complete, one white maggot comes out of the chrysalid. The maggot forms a brown pupa on the ground then emerges as an adult.
The bacterium Micrococcus flacidifex danai also infects larvae. Just before pupation, the larvae migrate to a horizontal surface and die a few hours later, attached only by one pair of prolegs, with the thorax and abdomen hanging limp. The body turns black shortly after. The bacterium Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory-reared insects.
The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the 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.
The yearly decrease in the monarch butterfly population has been linked to the decrease in the milkweed plant (Asclepias)—a primary food for monarchs—from herbicide use in the butterfly’s reproductive and feeding areas. The destruction of common milkweed has effectively eliminated the food source from most of the habitat monarchs used to use. Common milkweed is susceptible to the use of herbicides. Varietals do exist, however, (see Human Interactions) that can be successfully planted in gardens and other areas to help mitigate habitat loss in the wild.
Genetically modified crops
Conservationists attribute the disappearance of mikweed speices to agricultural practices in the Midwest, where genetically modified seeds are bred to resist herbicides that eliminate milkweed nearby. Growers eliminate milkweed that previously grew between the rows of food crops. Corn and soybeans are resistant to the effect of the herbicide glyphosate. The increased use of these crop strains is correlated with the decline in Monarch populations between 1999 and 2010. Chip Taylor, director of Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120–150 million acres lost. To help fight this problem, Monarch Watch encourages the planting of “Monarch Waystations.”  Garden-friendly milkweed not only helps fight the declining Monarch population, but also encourages other pollinators as well.
Loss of overwintering habitat
The area of forest occupied by overwintering monarch butterflies in Mexico reached its lowest level in two decades in 2013. According to a survey carried out during the 2012–2013 winter season by the WWF-Telcel Alliance, and Mexico’s National Commission of Protected Areas (CONAP), the nine hibernating colonies occupy a total area of 2.94 acres of forest—representing a 59% decrease from the 2011–2012 survey of 7.14 acres.
The same survey in 2012–2013 showed the decline is continuing. There were only seven colonies occupying 0.67 hectares (1.66 acres), the third consecutive record low since record-keeping began in 1995–1996. It represents a 44% decrease from the previous year, a 76% decrease from 2011–2012 and a 92% decrease compared to the 1996–1997 count.
Mexican environmental authorities continue to monitor illegal logging of the oyamel trees. The Oyamel is a major species of evergreen on which the overwintering butterflies spend a significant time during their winter diapause.
Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth and the survival of migrating adult butterflies.Omar Vidal, director general of WWF-Mexico, said "The monarch’s lifecycle depends on the climatic conditions in the places where they breed. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate." 
The 273-million base pair draft sequence includes a set of 16,866 protein-coding genes. The genome provides researchers insights into migratory behavior, the circadian clock, juvenile hormone pathways and microRNAs that are differentially expressed between summer and migratory monarchs.
The Center for Biological Diversity, The Center for Food Safety, The Xerces Society and Lincoln Brower have filed a petition to the Interior Department (USA) to protect the monarch by having it declared as an endangered species.
Conservationists are lobbying transportation departments and utilities to reduce| their use of herbicides and specifically encourage milkweed to grow along roadways and power lines. The goal is to reduce roadside mowing and application of herbicides during the butterfly breeding season. Environmental conservationists are lobbying large-scale agriculture companies to leave small areas of cropland unsprayed to allow the butterflies to breed.
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- Robert Michael Pyle suggested Danaus is a masculinised 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 (Pyle, Robert Michael (2001). Chasing Monarchs: Migrating with the Butterflies of Passage. Houghton Mifflin Books. pp. 148–149. ISBN 0-618-12743-7. Retrieved 2013-03-20.). He masculinized the genus name because it had to agree in gender with the species name. If the species-group name is not a noun in apposition, Pyle could have been right and the genus name and specific epithet have to agree in gender, but in that case it is the specific epithet and not the genus name, that is to be altered. In the case of Danaus plexippus, however, the specific epithet is a noun in apposition, formed from a personal name in the nominative case, which should not be altered (see ICZN art. 31.1 and art. 32.3). If, instead of Danaus, Danaë had been intended, the name would simply have been Danae plexippus. Moreover, in Systema Naturae, there is a very strong connection of the names with Danaus, and not a single one with Danaë.
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