Asclepias

Asclepias
Asclepias incarnata (swamp milkweed) flowers and a monarch butterfly
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Asterids
Order:	Gentianales
Family:	Apocynaceae
Subfamily:	Asclepiadoideae
Tribe:	Asclepiadeae
Subtribe:	Asclepiadinae
Genus:	Asclepias L.[1]
Type species
Asclepias syriaca L.
Species[2]
See List of Asclepias species
Synonyms[3]
List Acerates Elliott Acerotis Raf. Acerates Stopp Anantherix Nutt. Anthanotis Raf. Asclepiodella Small Asclepiodora A.Gray Biventraria Small Crassa Ruppius Dassovia Neck. Odontostelma Rendle Oligoron Raf. Onistis Raf. Otanema Raf. Otaria Kunth Oxypteryx Greene Podostemma Greene Podostigma Elliott Polyotus Nutt. Schizonotus A.Gray Solanoa Greene Solanoana Kuntze Stylandra Nutt. Trachycalymma (K.Schum.) Bullock

Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged.^[4][5][6] Most species are toxic to humans and many other species, primarily due to the presence of cardenolides. However, as with many such plants, some species feed upon milkweed leaves or the nectar from their flowers. A noteworthy feeder on milkweeds is the monarch butterfly, which uses and requires certain milkweeds as host plants for their larvae.

The Asclepias genus contains over 200 species distributed broadly across Africa, North America, and South America.^[7] It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae.

The genus was formally described by Carl Linnaeus in 1753,^[8] who named it after Asclepius, the Greek god of healing.^[9]

Members of the genus produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.^[10]

Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects, such as bees, wasps, and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die).^[11] Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans (bees, wasps) are the most common and best pollinators, accounting for over 50% of all Asclepias pollination,^[12] whereas monarch butterflies are poor pollinators of milkweed.^[5]

Asclepias species produce their seeds in pods termed follicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as the coma^[13] (often referred to by other names such as pappus, "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. Some, but not all, milkweeds also reproduce by clonal (or vegetative) reproduction.

Image	Scientific name	Common name	Distribution
	Asclepias albicans	Whitestem milkweed	Native to the Mojave and Sonoran deserts
	Asclepias amplexicaulis	Blunt-leaved milkweed	Native to central and eastern United States
	Asclepias asperula	Antelope horns	Native to American southwest and northern Mexico
	Asclepias californica	California milkweed	Native to central and southern California
	Asclepias cordifolia	Heart-leaf milkweed	Native to the Sierra Nevada and Cascade Range up to 2,000 m (6,600 ft).
	Asclepias cryptoceras	Pallid milkweed	Native to the western United States.
	Asclepias curassavica	Scarlet milkweed, tropical milkweed, bloodflower, bastard ipecacuanha	Native to the American tropics, introduced to other continents
	Asclepias curtissii	Curtiss's milkweed	Endemic to sandy areas of Florida
	Asclepias eriocarpa	Woollypod milkweed	Native to California, Baja California, and Nevada
	Asclepias erosa	Desert milkweed	Native to California, Arizona, and Baja California
	Asclepias exaltata	Poke milkweed	Native to eastern North America
	Asclepias fascicularis	Narrow-leaf milkweed	Native to Western United States
	Asclepias hirtella	Tall green milkweed
	Asclepias humistrata	Sandhill milkweed	Native to southeastern United States
	Asclepias incarnata	Swamp milkweed	Native to wetlands of North America
	Asclepias lanceolata	Lanceolate milkweed (Cedar Hill milkweed)	Native to coastal plain of eastern United States from Texas to New Jersey
	Asclepias linaria	Pine needle milkweed	Native to Mojave and Sonoran deserts
	Asclepias meadii	Mead's milkweed	Native to midwestern United States
	Asclepias nyctaginifolia	Mojave milkweed	native to the American southwest
	Asclepias purpurascens	Purple milkweed	Native to eastern, southern, and midwestern United States
	Asclepias prostrata	Prostrate milkweed	Native to Texas and northern Mexico
	Asclepias quadrifolia	Four-leaved milkweed	Native to eastern United States and Canada
	Asclepias rubra	Red milkweed
	Asclepias solanoana	Serpentine milkweed	Native to northern California
	Asclepias speciosa	Showy milkweed	Native to western United States and Canada
	Asclepias subulata	Rush milkweed	Native to southwestern North America
	Asclepias subverticillata	Horsetail milkweed[14]
	Asclepias sullivantii	Sullivant's milkweed
	Asclepias syriaca	Common milkweed
	Asclepias texana	Texas milkweed
	Asclepias tuberosa	Butterfly weed, pleurisy root
	Asclepias uncialis	Wheel milkweed
	Asclepias variegata	White milkweed
	Asclepias verticillata	Whorled milkweed
	Asclepias viridiflora	Green milkweed
	Asclepias viridis	Green antelopehorn, spider milkweed
	Asclepias welshii	Welsh's milkweed

There are also 12 species of Asclepias in South America, among them: A. barjoniifolia, A. boliviensis, A. curassavica, A. mellodora, A. candida, A. flava, and A. pilgeriana.

Milkweeds are an important nectar source for native bees, wasps, and other nectar-seeking insects, though non-native honey bees commonly get trapped in the stigmatic slits and die.^[11][15] Milkweeds are also the larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects (including numerous beetles, moths, and true bugs) specialized to feed on the plants despite their chemical defenses.^[5]

Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves (trichomes), cardenolide toxins, and latex fluids.^[16] Data from a DNA study indicate that, generally, more recently evolved milkweed species ("derived" in botany parlance) use these preventive strategies less but grow faster than older species, potentially regrowing faster than caterpillars can consume them.^[17][18][19]

Research indicates that the very high cardenolide content of Asclepias linaria reduces the impact of the Ophryocystis elektroscirrha (OE) parasite on the monarch butterfly, Danaus plexippus. The OE parasite causes holes to form in the wings of fully developed monarch butterflies. This causes weakened endurance and an inability to migrate. The parasite only infects monarchs when they are larvae and caterpillars, but the detriment is when they are in their butterfly form.^[20] By contrast, some species of Asclepias are extremely poor sources of cardenolides, such as Asclepias fascicularis, Asclepias tuberosa, and Asclepias angustifolia.^{[citation needed]}

The leaves of Asclepias species are a food source for monarch butterfly larvae and some other milkweed butterflies.^[5] These plants are often used in butterfly gardening and monarch waystations in an effort to help increase the dwindling monarch population.^[21]

However, some milkweed species are not suitable for butterfly gardens and monarch waystations. For example, A. curassavica, or tropical milkweed, is often planted as an ornamental in butterfly gardens outside of its native range of Mexico and Central America. Year-round plantings of this species in the United States are controversial and criticised, as they may lead to new overwintering sites along the U.S. Gulf Coast and the consequent year-round breeding of monarchs.^[22] This is thought to adversely affect migration patterns, and to cause a dramatic build-up of the dangerous parasite, Ophryocystis elektroscirrha.^[23] 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.^[24]

Because of this, it is most often suggested to grow milkweeds that are native to the geographical area they are planted in to prevent negative impacts on monarch butterflies.^[25][26]

Monarch caterpillars do not favor butterfly weed (A. tuberosa), perhaps because the leaves of that milkweed species contain very little cardenolide.^[27] Some other milkweeds may have similar characteristics.

Milkweeds are not grown commercially in large scale, but the plants have had many uses throughout human history.^[5] Milkweeds have a long history of medicinal, every day, and military use. The Omaha people from Nebraska, the Menomin from Wisconsin and upper Michigan, the Dakota from Minnesota, and the Ponca people from Nebraska, traditionally used common milkweed (A. syriaca) for medicinal purposes.^{[citation needed]} The bast fibers of some species can be used for rope. The Miwok people of northern California used heart-leaf milkweed (A. cordifolia) for its stems, which they dried and used for cords, strings and ropes.^[28]

The fine, silky fluff attached to milkweed seeds, which allows them to be distributed long distances on the wind, is known as floss. Milkweed floss is incredibly difficult to spin due to how short and smooth the filaments are, but blending it with as little as 25% wool or other fiber can produce workable yarn.^[29]

A study of the insulative properties of various materials found that milkweed floss was outperformed by other materials in terms of insulation, loft, and lumpiness, but it scored well when mixed with down feathers.^[30] The milkweed filaments from the coma (the "floss") are hollow and coated with wax, and have good insulation qualities. During World War II, more than 5,000 t (5,500 short tons) of milkweed floss was collected in the US as a substitute for kapok in life jackets.^[31][32] Milkweed is grown commercially as a hypoallergenic filling for pillows^[33] and as insulation for winter coats.^[34] Using milkweed floss for these purposes could provide a plant-based alternative to down and promote the growth of milkweed in areas where it has declined, though there is some concern that the environmental impacts could be negative if monoculture is used.^[35] Asclepias is also known as "Silk of America"^[36] which is a strand of common milkweed (A. syriaca) gathered mainly in the valley of the Saint Lawrence River in Canada. Milkweed floss can be used in thermal insulation and acoustic insulation. The floss is also highly buoyant and water-repellent, but absorbs oil readily.^[37] Due to its oil-absorbing properties, it can be used for oil spill cleanup.^[38][39][40]

Milkweed latex contains about two percent latex, and during World War II both Nazi Germany and the US attempted to use it as a source of natural rubber, although no record of large-scale success has been found.^[41]

Many milkweed species also contain cardiac glycoside poisons that inhibit animal cells from maintaining a proper K⁺, Ca²⁺ concentration gradient.^[6] As a result, many peoples of South America and Africa used arrows poisoned with these glycosides to fight and hunt more effectively. Some milkweeds are toxic enough to cause death when animals consume large quantities of the plant. Some milkweeds also cause mild dermatitis in some who come in contact with them. Nonetheless, some species can be made edible if properly processed.^[5]

• Everitt, J. H.; Lonard, R. L.; Little, C. R. (2007). Weeds in South Texas and Northern Mexico. Lubbock: Texas Tech University Press. ISBN 978-0-89672-614-7.

• Milkweed test-cultivated for the insulation value of floss
• Milkweed in Handbook of Energy Crops
• Common milkweed production research at Western Illinois University
• UVSC Herbarium — Asclepias