| Asclepias | |
|---|---|
 | |
| Asclepias incarnata (swamp milkweed) flowers and amonarch 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] | |
| Synonyms[3] | |
List
| |
Asclepias is agenus ofherbaceous,perennial,flowering plants known asmilkweeds, named for theirlatex, a milky substance containingcardiac glycosides termedcardenolides,exuded where cells are damaged.[4][5][6] Most species aretoxic 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 themonarch butterfly, which uses and requires certain milkweeds as host plants for theirlarvae.
TheAsclepias genus contains over 200 species distributed broadly across Africa, North America, and South America.[7] It previously belonged to thefamily Asclepiadaceae, which is now classified as thesubfamilyAsclepiadoideae of the dogbane family,Apocynaceae.
The genus was formally described byCarl Linnaeus in 1753,[8] who named it afterAsclepius, the Greek god of healing.[9]
Members of the genus produce some of the most complex flowers in the plant kingdom, comparable toorchids in complexity. Fivepetals reflex backwards revealing agynostegium surrounded by a five-membranecorona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holdingpollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genusAsclepias.[10]
Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures calledpollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visitinginsects, such asbees,wasps, andbutterflies, slip into one of the five slits in each flower formed by adjacentanthers. 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-bodiedhymenopterans (bees, wasps) are the most common and best pollinators, accounting for over 50% of allAsclepias pollination,[12] whereasmonarch butterflies are poor pollinators of milkweed.[5]
Asclepias species produce their seeds in pods termedfollicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as thecoma[13] (often referred to by other names such aspappus, "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by thewind. Some, but not all, milkweeds also reproduce by clonal (or vegetative) reproduction.
| Image | Scientific name | Common name | Distribution |
|---|---|---|---|
 | Asclepias albicans | Whitestem milkweed | Native to theMojave andSonoran deserts |
 | Asclepias amplexicaulis | Blunt-leaved milkweed | Native to central and easternUnited States |
 | Asclepias asperula | Antelope horns | Native toAmerican southwest and northernMexico |
.jpg) | Asclepias californica | California milkweed | Native to central and southernCalifornia |
 | Asclepias cordifolia | Heart-leaf milkweed | Native to theSierra Nevada andCascade Range up to 2,000 m (6,600 ft). |
 | Asclepias cryptoceras | Pallid milkweed | Native to the western United States. |
_W_IMG_1570.jpg) | Asclepias curassavica | Scarlet milkweed, tropical milkweed, bloodflower, bastard ipecacuanha | Native to the American tropics, introduced to other continents |
_(5771618293).jpg) | Asclepias curtissii | Curtiss's milkweed | Endemic to sandy areas ofFlorida |
 | Asclepias eriocarpa | Woollypod milkweed | Native to California,Baja California, andNevada |
 | Asclepias erosa | Desert milkweed | Native to California,Arizona, and Baja California |
.jpg) | Asclepias exaltata | Poke milkweed | Native to easternNorth America |
 | Asclepias fascicularis | Narrow-leaf milkweed | Native toWestern 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 fromTexas toNew 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 andCanada |
 | Asclepias rubra | Red milkweed | |
 | Asclepias solanoana | Serpentine milkweed | Native tonorthern 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 ofAsclepias in South America, among them:A. barjoniifolia,A. boliviensis,A. curassavica,A. mellodora,A. candida,A. flava, andA. pilgeriana.
Milkweeds are an importantnectar source for nativebees,wasps, and other nectar-seeking insects, though non-nativehoney bees commonly get trapped in the stigmatic slits and die.[11][15] Milkweeds are also the larval food source formonarch butterflies and their relatives, as well as a variety of other herbivorous insects (including numerousbeetles,moths, andtrue bugs) specialized to feed on the plants despite theirchemical defenses.[5]
Milkweeds use three primarydefenses to limit damage caused by caterpillars: hairs on the leaves (trichomes),cardenolide toxins, andlatex fluids.[16] Data from aDNA 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 highcardenolide content ofAsclepias linaria reduces the impact of theOphryocystis 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 ofAsclepias are extremely poor sources of cardenolides, such asAsclepias fascicularis,Asclepias tuberosa, andAsclepias angustifolia.[citation needed]
The leaves ofAsclepias species are a food source formonarch butterfly larvae and some othermilkweed butterflies.[5] These plants are often used inbutterfly 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 ofMexico andCentral America. Year-round plantings of this species in theUnited 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 littlecardenolide.[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. TheOmaha people fromNebraska, theMenomin fromWisconsin and upperMichigan, theDakota fromMinnesota, and thePonca people fromNebraska, traditionally used common milkweed (A. syriaca) for medicinal purposes.[citation needed]Thebast fibers of some species can be used for rope. TheMiwok people of northernCalifornia 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. DuringWorld War II, more than 5,000 t (5,500 short tons) of milkweed floss was collected in the US as a substitute forkapok in life jackets.[31][32] Milkweed is grown commercially as ahypoallergenic 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 ifmonoculture 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 theSaint Lawrence River inCanada. Milkweed floss can be used inthermal 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]
Milkweedlatex contains about two percent latex,[clarification needed] and during World War II bothNazi Germany and the US attempted to use it as a source ofnatural rubber, although no record of large-scale success has been found.[41]
Many milkweed species also containcardiac glycoside poisons that inhibit animal cells from maintaining a properK+, 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 milddermatitis in some who come in contact with them. Nonetheless, some species can be made edible if properly processed.[5]
Latinasclepias and Greekasklepias for the common swallowwort; Asclepius, Greek god of medicine, the worship of Asclepius was centered in Epidaurus. See W.K.C. Guthrie,The Greeks and Their Gods, 1950; Carl Linnaeus,Species Plantarum. 214. 1753 andGenera Plantarum. Ed. 5. 102. 1754.
In our study, the least preferred milkweed speciesA. tuberosa (no choice; Fig. 2) andA. verticillata (choice; Fig. 3A) both have low cardenolide levels recorded in the literature (Roeske et al. 1976, Agrawal et al. 2009, 2015, Rasmann and Agrawal 2011)
It is the least favored by monarch caterpillars though because it has very little toxin (cardiac glycosides) in its leaves, but other butterflies and adult monarchs love it as a nectar source..
Unlike other milkweeds, this plant has a clear sap, and the level of toxic cardiac glycosides is consistently low (although other toxic compounds may be present)..
