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"Milkweed" redirects here. For other uses, see Milkweed (disambiguation).
Not to be confused with Asclepius.

Asclepias
Asclepias incarnata (swamp milkweed) flowers and a monarch butterfly
Scientific classification Edit this 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 its 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.

Etymology

[edit]

In the first century AD, Dioscorides described a plant called ἀσκληπιάς (asklepias) in Greek[8] and Pliny the Elder described a plant called asclepias in Latin.[9] Though usually explained as being derived from the name of the Greek medical god Asklepios, neither author mentioned the god in connection with this plant. Their plant has since been commonly identified as the plant now known as Vincetoxicum hirundinaria.[8] Some botanists have disagreed as, though the old descriptions are short and vague, Vincetoxicum hirundinaria does not have leaves like ivy.[10][11]

Linnaeus first described the genus Asclepias in his Genera Plantarum of 1737 without specifying any particular plants in the genus.[12] Linnaeus published his Flora Suecica in 1745[13] and Materia Medica in 1749.[14] The only Asclepias he mentioned in either book was the one he would later name as Asclepias vincetoxicum. Both books mentioned that in pharmacy this plant was called Hirundinariæ. In none of his works does Linnaeus give an origin for the name Asclepias, presumably because it was being used for these plants by some herbalists and botanists for centuries before.[10] These publications were before his 1753 innovation of names that were simply two words - genus and species - rather than a much longer description.[15]

When Linnaeus described the 18 species that he assigned to the genus Asclepias in his Species Plantarum of 1753, he included Asclepias vincetoxicum.[15] That plant is now called Vincetoxicum hirundinaria. Of those 18, 7 have been assigned to other genera in the years since: Xysmalobium undulatum, Calotropis gigantea, Gymnema lactiferum, Vincetoxicum hirundinaria, Vincetoxicum nigrum, Gomphocarpus fruticosus and Cynanchum thesioides. The species that remain in Asclepias from those Linnaeus described are: A. syriaca, A. amoena and A. purpurascens both now regarded as A. purpurascens, A. variegata, A. nivea, A. incarnata, A. curassavica, A. decumbens and A. tuberosa both now regarded as A. tuberosa, A. verticillata and A. rubra.[7]

Flowers

[edit]
A monarch butterfly on swamp milkweed
Asclepias syriaca seed pods, upper image from August and lower from December
Milkweed sprout, a few days after sowing
Chemical structure of oleandrin, one of the cardiac glycosides

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.[16]

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).[17] 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,[18] whereas monarch butterflies are poor pollinators of milkweed.[5]

Male Pepsis grossa, a typical milkweed-pollinating wasp
Honeybee on antelope horn (Asclepias asperula) showing pollinia attached to legs
Milkweed seeds dispersed by the wind.

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[19] (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.

Selected species

[edit]
For a complete list, see List of Asclepias species.
Image Scientific name Common name Native 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 from Quebec to Ontario and from Maine to Minnesota, south to Georgia and Mississippi[20]
Asclepias fascicularis Narrow-leaf milkweed Native from northeastern Washington and Idaho, south to Baja California and northern Arizona[21]
Asclepias hirtella Tall green milkweed Native to Canada and the Midwestern and Upper South United States
Asclepias humistrata Sandhill milkweed Native to southeastern United States
Asclepias incarnata Swamp milkweed Native from Manitoba to Quebec and Nova Scotia, from New England south to Georgia, west to Louisiana and Texas, and north to North Dakota.[22]
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 Native to New York, south to Florida, and west to Arizona and Texas[23]
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[24] Native to southwestern United States and Mexico
Asclepias sullivantii Sullivant's milkweed Native to the midwestern United States and Canada, ranging north to Minnesota, east to southern Ontario and Ohio, west to Nebraska, Kansas, and south to Oklahoma. It is considered rare in Minnesota, Wisconsin, and Ontario, and is known only from historical records in North Dakota.[25]
Asclepias syriaca Common milkweed Native to southern Canada and much of the United States east of the Rocky Mountains, excluding Florida and the drier parts of the prairies.[26]
Asclepias texana Texas milkweed Native from central Texas west to the Edwards Plateau and south into Coahuila and the Chihuahuan Desert as far south as Durango.[27]
Asclepias tuberosa Butterfly weed, pleurisy root Native from Ontario to Newfoundland and New England, south to Florida, west to Texas, and north through Colorado to Minnesota.[28]
Asclepias uncialis Wheel milkweed Native to Arizona, Colorado, New Mexico, Nevada, Oklahoma, Texas, Utah and Wyoming.[29]
Asclepias variegata White milkweed Native to New York and Connecticut, south to Florida, west to Texas and Oklahoma, and northeast to Illinois and Ohio.[30]
Asclepias verticillata Whorled milkweed Native from Massachusetts to Montana, south to Florida and eastern Texas; also southern Canada[31]
Asclepias viridiflora Green milkweed Native from Ontario to British Columbia and from Massachusetts and New York to Montana, south to Florida, Texas, and Arizona[32]
Asclepias viridis Green antelopehorn, spider milkweed Native from West Virginia to Nebraska, south to Florida and Texas[33]
Asclepias welshii Welsh's milkweed Native to southern Utah and northern Arizona[34]

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.

Ecology

[edit]

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.[17][35] 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.[36] 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.[37][38][39]

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.[40] By contrast, some species of Asclepias are extremely poor sources of cardenolides, such as Asclepias fascicularis, Asclepias tuberosa, and Asclepias angustifolia.[citation needed]

Monarch butterfly conservation and milkweeds

[edit]

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.[41]

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.[42] This is thought to adversely affect migration patterns, and to cause a dramatic build-up of the dangerous parasite, Ophryocystis elektroscirrha.[43] 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.[44]

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.[45][46]

Monarch caterpillars do not favor butterfly weed (A. tuberosa), perhaps because the leaves of that milkweed species contain very little cardenolide.[47]

Uses

[edit]

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 Menominee 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.[48]

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.[49]

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.[50] 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.[51][52] Milkweed is grown commercially as a hypoallergenic filling for pillows[53] and as insulation for winter coats.[54] 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.[55]

A. syriaca grown in the St Lawrence Valley of Quebec is known as soie d'Amérique "Silk of America" using a term "silk"[56] applied by the naturalist Charles Sigisbert Sonnini in 1810, when the plant was used in France to produce a fiber for fabric. Milkweed floss can be used in thermal insulation and acoustic insulation. It is highly buoyant and water-repellent, but absorbs oil readily,[57] so it can be used for oil spill cleanup.[58][59][60]

Seeds of Asclepias syriaca (Common Milkweed)

Milkweed latex contains about two percent latex,[clarification needed] 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.[61]

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]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Asclepias

View on Grokipedia
from Grokipedia
is a genus of approximately 70 species of herbaceous perennial plants in the family Apocynaceae, native to North America and collectively known as milkweeds due to their characteristic milky latex sap exuded from injured tissues. These plants feature complex umbellate flower clusters with unique pollinia structures that facilitate pollination primarily by insects such as bees and butterflies, and they inhabit diverse environments ranging from prairies and wetlands to deserts and forest edges. Ecologically, Asclepias species serve as the sole host plants for the larvae of the monarch butterfly (Danaus plexippus), where the caterpillars feed exclusively on the foliage, sequestering cardiac glycosides (cardenolides) from the plants that render both larvae and adults toxic to most predators. This mutualistic relationship underscores the genus's pivotal role in supporting monarch migration and population dynamics, with declines in milkweed availability linked to observed reductions in butterfly numbers. While some species like Asclepias syriaca have been utilized historically for fiber and in traditional remedies, the plants' toxicity has also led to their classification as weeds in agricultural contexts.

Taxonomy and Classification

Etymology and History

The genus Asclepias derives its name from , the god of healing and medicine, a connection made by upon formalizing the genus in his on May 1, 1753. Linnaeus selected this epithet to reflect the plants' milky latex and documented uses in indigenous remedies, though many species contain toxic cardiac glycosides that limit practical medicinal application. He designated as the , based on North American specimens erroneously linked to Syrian origins by earlier observers. European botanists first encountered Asclepias species through explorations, with French and English collectors dispatching specimens to as early as the 1500s and 1600s, often under names like "milkweed" for the characteristic latex. Linnaeus's initial treatment encompassed a limited number of species, primarily from eastern , but subsequent works expanded the known diversity; for instance, Michaux described Asclepias longifolia in his Boreali-Americana published in 1803, drawing from field collections across the continent. These efforts highlighted the genus's predominance in the , prompting revisions to accommodate morphological variation observed in materials. Taxonomic history includes early 20th-century disputes over delimitation, notably surrounding Asclepias uncialis (wheel milkweed), first described by Edward Lee Greene in 1899 from specimens. Botanists debated its status as a distinct dwarf species versus variation within broader taxa like Asclepias cryptoceras, with fluctuations between recognizing one to four entities based on limited morphological and distributional data from 19th- and early 20th-century collections. Such controversies underscored challenges in classifying compact, low-growing milkweeds amid incomplete type specimens and regional .

Phylogenetic Position

Asclepias is classified within the family , subfamily , and tribe , a placement corroborated by molecular phylogenetic analyses employing non-coding chloroplast DNA sequences that demonstrate of the relative to Old World counterparts in the tribe. Key synapomorphies uniting Asclepias with other Asclepiadeae include the formation of pollinia—compact masses—and associated translator structures derived from the style head, which facilitate precise transfer via pollinators, distinguishing the tribe from basal lacking such specialized apparatuses. Molecular dating calibrated with fossils indicates that diversification within Asclepiadoideae, including the radiation of Asclepias, predominantly occurred during the epoch, approximately 23 to 5.3 million years ago, with the genus's stem divergence from relatives estimated around 10-15 million years ago based on and nuclear phylogenomic data. This temporal framework aligns with paleoclimatic shifts promoting arid adaptations in North American lineages, fostering endemism primarily in the continent. Phylogenetic reconstructions from the onward, incorporating high-throughput sequencing of genomes and transcriptomes, have delineated subgeneric within Asclepias, revealing polytomies in earlier studies resolved into supported branches corresponding to morphological groups such as Incarnatae and Syriacae, with implications for North American biogeographic patterns tied to vicariance and dispersal. These analyses underscore the genus's and highlight cryptic divergences in rare species, reinforcing the as a pivotal period for formation without reliance on pre-molecular classifications.

Species Diversity and Distribution

The genus Asclepias encompasses approximately 140 of primarily herbaceous , though some tropical taxa exhibit shrubby growth. Over 90% of these originate from temperate , with roughly 110 documented across the continent, reflecting a core center of origin and radiation in this region. Diversity peaks in the and , where arid-adapted endemics contribute to hotspots of , often tied to localized edaphic and climatic niches. A smaller subset extends into Central and , with rare naturalized introductions in the , such as escaped ornamental plantings in , but no established wild populations there. Species exhibit varied growth forms, broadly categorized as upright perennials (e.g., A. syriaca, forming tall, rhizomatous clones), tuberous perennials (e.g., A. tuberosa, with deep storage roots enabling ), and low-decumbent or geophytic forms adapted to seasonal habitats. Fewer tropical species develop into shrubs or subshrubs, diverging from the herbaceous norm of northern taxa. Patterns of are pronounced, with many species restricted to narrow geographic ranges, such as wheel milkweed (A. uncialis) in the or cryptic lineages within A. tomentosa across southern U.S. states, underscoring microevolutionary divergence driven by isolation. Recent phylogenomic analyses have clarified taxonomic uncertainties, revealing deep genetic divergences and potential cryptic species without prompting widespread lumping or splitting of established names post-2020. Ongoing botanical surveys indicate stable overall diversity, with no evidence of genus-wide declines, though individual endemics face localized threats from unrelated to broad taxonomic shifts.

Morphology and Physiology

Vegetative Structure

Asclepias species are herbaceous perennials with root systems that vary between deep taproots and rhizomes, facilitating both persistence through seasonal dieback and clonal propagation in suitable soils. Many, such as showy milkweed (Asclepias speciosa), produce stems from widespread rhizomes, allowing colony formation, while others rely on taproots for anchorage in drier environments. Vegetative sprouts often emerge from underground roots, exhibiting robust growth compared to seedlings. Leaves are simple, typically arranged in opposite pairs or whorls along the stems, with entire margins and prominent venation featuring a central midvein from which secondary veins arch toward the tip. Leaf shape ranges from lanceolate to ovate, with sizes varying by species; for example, in Asclepias speciosa, blades measure 6–20 cm long. Surfaces may be glabrous or pubescent, contributing to water retention in arid-adapted taxa. Stems arise singly or in clusters, generally erect and unbranched to sparingly branched, attaining heights of 0.5–2 m across the genus. Pubescence on stems varies from glabrous to densely hairy, as seen in species like with minutely downy textures. In xeric species, such as Welsh's milkweed (Asclepias welshii), stems emerge from deeply buried rootstocks, supporting through access to subsurface moisture, with field observations confirming root depths exceeding surface layers.

Flowers and Inflorescences

The inflorescences of Asclepias species consist of umbel-like cymes, which are typically terminal or axillary clusters borne on peduncles that may be erect or drooping depending on the . Each comprises numerous small flowers, often numbering 20 to 130 per cluster in species like A. syriaca, arranged in a spherical or hemispherical form that facilitates exposure to pollinators. Individual flowers are 5-merous and radially symmetric, featuring five reflexed corolla lobes that are usually green to purple and conceal the five-parted calyx beneath. Above the corolla lies the gynostegium, a fused structure of stamens, styles, and stigmas forming a central column; this includes five hood-like appendages (corona) arising from the filament bases, each often bearing an internal horn that projects toward the center. The anthers produce waxy pollinia—paired sacs containing pollen masses—attached via translator arms to the corpusculum, a clip-like structure within the gynostegium slits, enabling removal and transfer by legs or mouthparts. Floral nectaries, located within the hoods, secrete sugary rewards, though access is restricted by the intricate morphology. Corolla and hood coloration varies widely across the genus, ranging from white and pale pink in A. incarnata to bright orange or yellow in A. tuberosa, with deeper reds or lavenders in species like A. curassavica. In A. tuberosa, yellow-flowered variants predominate west of the 100th meridian, while orange forms are more common eastward, reflecting regional polymorphism within populations. Asclepias species exhibit no , with hermaphroditic flowers producing both and ovules in each unit. Many taxa demonstrate , a genetic mechanism that blocks self-fertilization and promotes , as evidenced by zero fruit set from hand self-pollinations in species such as A. exaltata and A. viridiflora.

Latex and Secondary Metabolites

Asclepias species produce a milky exuded from anastomosing laticifer networks that traverse vascular tissues, enabling rapid release upon mechanical damage. This functions in sealing through quick , forming a physical barrier that prevents ingress and excessive fluid loss, while also delivering chemical defenses against herbivores. The primary constituents include cardenolides—steroidal glycosides such as voruscharin—along with proteins and resins, as identified through chromatographic separation. Cardenolide levels in often exceed those in leaves, with species-specific profiles; for example, A. curassavica contains high concentrations, whereas A. speciosa shows negligible amounts. High-performance liquid chromatography (HPLC) assays reveal intraplant and temporal variability in concentrations, with cardenolides in A. eriocarpa fluctuating across monthly samples from roots, stems, leaves, and , peaking in certain seasons due to physiological demands. Elevated levels occur under , such as , correlating with upregulated production in laticifers. The latex system traces evolutionary conservation to the family, where laticifers provide a shared defense architecture, but Asclepias displays genus-specific enhancements in cardenolide diversity and activity, reflecting adaptive escalation in chemical profiles across 53 examined . Cardenolides, exemplified by structures like (a related ), bind extracellularly in , contributing to its defensive potency prior to enzymatic degradation.

Reproduction

Pollination Mechanisms

Asclepias species exhibit a specialized characterized by pollinia, waxy pollen sacs connected to a clip-like translator apparatus consisting of a corpusculum and translator arms. When probe the floral hoods for , their appendages—typically legs in and wasps or proboscis in —become entangled in the slits adjacent to the anthers, attaching the entire pollinarium ( pair plus translator). Successful requires the insect to subsequently insert the pollinarium precisely into one of the five stigmatic slits on another flower, where the pollinia contact the receptive surface; misalignment prevents fertilization due to the plant's and precise morphology. Primary pollinators include hymenopterans such as bumble bees (Bombus spp.), (Xylocopa spp.), honey bees (Apis mellifera), and various wasps, alongside lepidopterans like , though effectiveness varies by species and behavior. Larger bees and wasps achieve higher pollinarium removal rates due to their robust appendages suiting the translator clips, while butterflies often remove fewer pollinia but may carry them longer. Transfer efficiency differs markedly: for instance, honey bees insert over 18% of removed pollinia in Asclepias incarnata, whereas other visitors achieve lower rates, with overall insertion success per visit averaging around 5% or less owing to the mechanical precision required. The architecture and pollinarium design promote despite opportunities for ( between flowers on the same plant), which occurs naturally but yields lower fruit set—approximately one-third that of pure outcross hand- in —due to partial and resource costs. Pollinaria dry and rotate post-removal, facilitating insertion only after inter-plant movement, and the low per-flower success (1-5% in removal experiments) selects against inefficient geitonogamous transfers, favoring fidelity to diverse plants.

Seed Dispersal and Germination

Asclepias species produce follicles that dehisce longitudinally upon maturity, releasing numerous flattened seeds each attached to a coma—a tuft of silky white hairs that functions as a plume for wind dispersal. This anemochorous mechanism enables seeds to travel considerable distances, with dispersal ability varying by coma length and seed mass ratio; studies on A. syriaca demonstrate that higher coma-to-seed mass ratios correlate with greater fall distances in still air and field observations of up to several kilometers under favorable wind conditions. Wind tunnel experiments confirm that plume morphology reduces terminal velocity, facilitating long-range transport while minimizing deposition in dense vegetation. Seed viability post-dispersal is influenced by physical dormancy imposed by the impermeable seed coat, necessitating scarification or cold stratification to achieve germination. Mechanical scarification, such as nicking the seed coat, or moist cold treatment at 4°C for 30–56 days breaks dormancy by simulating winter conditions and permitting water imbibition; without such treatments, germination rates remain below 20% for many species. Success rates vary by taxon, reaching 85–90% in stratified A. viridis and A. tuberosa seeds under controlled conditions, though field emergence from depths of 0.2–1.6 inches depends on soil moisture and temperature cues exceeding 10°C. Post-pollination limits hybridization potential, as the precise pollinia attachment mechanism ensures -specific pollen transfer, resulting in low viable hybrid seed set even in . Hybrid pollinia often exhibit reduced fertility, further constraining despite occasional natural crosses documented in genera like A. purpurascens and A. tuberosa. This specificity maintains integrity during seed production phases.

Distribution and Habitat

Native Range

The genus Asclepias is native to the and , with the vast majority of its approximately 140–200 occurring in the . Over 70 are endemic to the and , representing the core of the genus's diversity in temperate , while hosts around 75 , underscoring its role as a secondary center of . Additional extend into Central and , including tropical forms like A. curassavica, though numbers diminish southward. In , a smaller subset—estimated at fewer than 20 —is confined to sub-Saharan regions, such as A. fruticosa and A. woodii in . No Asclepias species are native to , temperate , or northern ; Old World occurrences are limited to rare naturalized populations of American species, such as escapes of A. curassavica in tropical regions outside its native range. records and GIS-based distribution mapping from sources like the database confirm this predominantly American focus, with roughly 90% of species endemics to north of . Phylogenetic analyses and fossil evidence indicate Paleogene origins for the broader Asclepiadoideae subfamily around 55 million years ago in the early Eocene, potentially in or , with subsequent dispersal to the driving the genus's modern diversification. Contemporary distribution models, informed by occurrence data, suggest relative stability in the genus's range over the , without major post-glacial expansions beyond its established American and African extents.

Ecological Preferences

Asclepias generally favor open, sunny such as prairies, meadows, open woodlands, and disturbed areas including roadsides and forest clearings, where competition for light is minimal. These preferences correlate with their role in early successional stages, where they establish readily via wind-dispersed seeds and rhizomes but decline in later seral phases under developing closed canopies, as evidenced by persistence in disturbed plots over time. Soil tolerances span a wide edaphic range, from dry, sandy, or rocky substrates to moist loams and clays, with many species exhibiting adaptability to neutral to alkaline conditions often exceeding 7.0 and up to 8.0 in some cases. Abundance surveys link higher milkweed densities to elevated and lower in open sites. Climatically, the thrives across temperate to subtropical zones, corresponding to USDA hardiness zones 3–9 for many North American species, with specialized drought-resistant taxa like Asclepias erosa and A. subulata persisting in arid environments through morphological adaptations including deep taproots and foliar trichomes that reduce . These maintain viability in low-precipitation regimes without evident shifts to alternative photosynthetic pathways, relying instead on structural .

Ecology

Herbivory and Defenses

Asclepias species endure intense herbivory from specialist insects, including the red milkweed beetle (Tetraopes tetraophthalmus) and larvae (Danaus plexippus), which can cause substantial defoliation in field populations. These herbivores, adapted to tolerate the ' toxins, nonetheless exert selective pressure, with community-level herbivory documented to reduce plant fitness in Asclepias syriaca. A primary physical defense is the rapid exudation of pressurized milky latex from vascular tissues upon wounding, which coagulates in air to entrap feeding ' mouthparts and legs, immobilizing small herbivores. assays confirm this mechanism deters generalist caterpillars, with latex application causing entanglement and reduced feeding efficiency, though specialists like larvae often circumvent it via vein-cutting behaviors to stem latex flow. Herbivore damage triggers induced defenses, including elevated cardenolide concentrations in distal undamaged leaves across multiple Asclepias species, alongside increased latex production in response to jasmonic acid signaling. These responses, varying by species and herbivore type, enhance resistance to subsequent attacks, as quantified through chemical profiling of plant tissues post-damage. Investment in latex yield and cardenolide synthesis incurs growth costs, with studies on A. syriaca revealing negative correlations between defense traits and biomass accumulation via non-destructive growth measurements, aligning with resource allocation models predicting trade-offs under herbivory pressure. Such costs underscore the evolutionary balance between antagonism and tolerance in milkweed-herbivore interactions.

Interactions with Pollinators and Specialists

Asclepias species host specialist herbivores that exhibit high host-specificity and physiological adaptations to the plants' toxic defenses. The monarch butterfly (Danaus plexippus) relies almost exclusively on approximately 110 North American Asclepias species as larval host plants, with caterpillars sequestering cardenolides—cardiac glycosides produced by the plants—to render themselves unpalatable to predators. Similarly, longhorn beetles in the genus Tetraopes, such as T. tetrophthalmus on A. syriaca, are monophagous or oligophagous, feeding on roots, stems, and leaves while sequestering the same toxins for defense; these beetles demonstrate tissue-specific tolerance, being better adapted to root toxins than foliar ones. Co-evolutionary dynamics are evident in genetic adaptations enabling tolerance among these specialists. Monarchs have evolved stepwise reductions in cardenolide sensitivity through substitutions in the alpha-subunit of sodium-potassium (ATPα), allowing sequestration without self-toxicity; these changes occurred incrementally across milkweed , linking resistance to host specialization. Tetraopes similarly possess ATPα modifications conferring resistance, supporting reciprocal adaptations in the milkweed-herbivore interaction. In contrast, pollinators of Asclepias exhibit low fidelity to the genus, with diverse generalist insects—including bees (Bombus spp., Apis mellifera), wasps, and butterflies—visiting flowers primarily for variable nectar rewards rather than host specificity. Nectar production is often modest and temporally restricted; for example, A. syriaca secretes nectar correlating with inflorescence size, influencing visitation, while A. verticillata produces it mainly from 1800 to 2200 hours over 4-5 days. Transect-based observations reveal fluctuating visitation rates, as in A. incarnata where pollinator visits varied significantly between years and seasonally, underscoring opportunistic rather than specialized pollination interactions.

Role in Ecosystems

Asclepias species function as host plants for numerous specialist insects, including herbivores such as aphids, beetles, and moths that feed exclusively or primarily on milkweeds, thereby anchoring specific trophic links in native food webs. Empirical analyses of arthropod communities on Asclepias reveal that these plants support oligotrophic and monophagous species, which in turn attract predators and parasitoids, enabling trophic cascades that influence community structure. Despite this, Asclepias typically constitutes a minor component of vegetation biomass in most habitats, limiting its role to niche rather than dominant ecosystem engineering. These plants associate symbiotically with arbuscular mycorrhizal fungi (AMF), which facilitate and uptake, enhancing morphology, accumulation, and production—defensive traits that indirectly modulate pressure and plant fitness in competitive communities. Field and experiments demonstrate that native AMF consortia improve Asclepias establishment and growth by up to several-fold compared to sterile conditions, underscoring their contribution to belowground trophic interactions without direct capabilities. In native ranges, Asclepias presence correlates with elevated diversity of specialist arthropods, as quantified in studies, though null models indicate that host specificity drives these patterns more than broader community facilitation effects. Native species exhibit negligible invasive potential, integrating into grasslands and prairies without displacing dominants, whereas non-native introductions like demonstrate moderate to high invasiveness in subtropical regions such as , where they persist year-round and simplify visitor networks by altering resource seasonality.

Chemical Composition and Toxicity

Cardiac Glycosides and Other Compounds

Cardenolides, the principal cardiac glycosides in Asclepias , feature a cardenolide aglycone—a steroidal core with a five-membered ring at C-17—glycosidically linked to sugars, enabling tight binding to the extracellular face of Na⁺/K⁺- and inhibition of its ion-pumping activity. Prominent examples include calotropin, asclepin, and voruscharin, isolated from species such as A. curassavica and A. syriaca, with voruscharin demonstrating high potency against Na⁺/K⁺-. These compounds accumulate in , leaves, and seeds, with quantified levels in A. curassavica seeds reaching up to several milligrams per gram dry weight for individual cardenolides like 12β,19-anhydrodeacetyltanghigenin and calactin. Concentrations of total cardenolides vary ontogenetically, often peaking in mature tissues and —up to 1-2% dry weight in some assays—due to developmental of biosynthetic genes and allocation to defense. Biosynthesis proceeds via the , yielding as a precursor, followed by oxidative modifications; transcriptomic analyses in Asclepias have identified candidate genes for cardenolide-related enzymes, while enzymatic validation in related confirms key steps like cardenolide formation from intermediates. Interspecific variation is pronounced, with tropical Asclepias species producing higher cardenolide concentrations—often twofold or more—than temperate counterparts, reflecting phylogenetic escalation in defense investment correlated with herbivore pressure. Beyond cardenolides, Asclepias contains such as glycosides, which contribute to UV protection and oviposition cues, and triterpenoids like derivatives identified in A. syriaca. biosynthesis follows the shikimate and early phenylpropanoid pathways, while triterpenoids derive from the mevalonate route via oxidosqualene cyclases, with qualitative detection via GC-MS in leaf and stem extracts.

Effects on Animals and Humans

Milkweed species (Asclepias spp.) produce cardenolides, which exert potent cardiotoxic effects on non-adapted animals, primarily through inhibition of Na+/K+-ATPase pumps in , leading to arrhythmias, , and potentially fatal . Livestock such as sheep, , and horses are particularly susceptible; ingestion of contaminated hay or exceeding 1% milkweed content can cause , rapid , , spasms, , and death, with narrow-leafed species additionally inducing neurotoxic symptoms like tremors and . Documented cases include poisoned via hay containing Asclepias spp., exhibiting progressive dyspnea and cardiovascular collapse at doses equivalent to substantial biomass relative to body weight. Specialist herbivores, notably monarch butterflies (Danaus plexippus), have evolved targeted adaptations to milkweed cardenolides rather than generalized , including substitutions conferring Na+/K+-ATPase insensitivity and selective sequestration of polar cardenolides in exoskeletal tissues for defense against predators. This sequestration process incurs metabolic costs, such as reduced larval growth on high-cardenolide hosts like tropical milkweed (), where monarchs convert burdensome s into less harmful forms but still face oviposition penalties due to inefficient uptake. Such evolutionary specialization exemplifies an , where tolerance is mechanistically linked to toxin repurposing rather than broad enzymatic breakdown, enabling survival on otherwise lethal plants. Human exposure to Asclepias is uncommon and typically mild, manifesting as gastrointestinal distress (, , , ) from ingestion or severe and ocular from contact, with symptoms onset within hours. Rare severe cases mimic , with detectable serum levels of cardioactive steroids leading to weakness, confusion, and potential arrhythmias, as in a reported incident involving Asclepias syriaca. Despite structural similarity to therapeutic cardiac glycosides like , milkweed-derived cardenolides lack a viable in modern medicine due to inconsistent potency, challenges, and overdose risks, rendering them unsuitable for clinical cardiac applications beyond historical folk uses.

Human Uses and Cultivation

Historical and Traditional Applications

Indigenous North American tribes employed roots and latex of Asclepias , such as A. syriaca and A. tuberosa (known as pleurisy root), as emetics, laxatives, and remedies for pulmonary ailments including , coughs, and infections. Tribes like the , , and prepared infusions or decoctions from these roots to alleviate respiratory distress and promote expectoration, while the sap served topically for , ringworm, stings, and skin irritations. Stems provided strong fibers twisted into cordage for belts, fishing lines, nets, and bowstrings, valued for their durability among groups in the northeastern and midwestern regions. The floss from seed pods, a silky , found traditional use for stuffing textiles and mats, later scaled during as a buoyant substitute for imported kapok in life preservers, with U.S. citizens collecting over 11 million pounds between 1943 and 1945 to meet military demands disrupted by Pacific conflicts. Early 20th-century experiments explored extracting rubber from Asclepias stems as a domestic alternative amid global shortages, but yields proved insufficient for commercial viability, leading to abandonment by the 1940s. European settlers in adopted indigenous medicinal practices, brewing root teas for typhus, asthma, and diarrhea, with Asclepias entries appearing in the U.S. Pharmacopeia during the 1880s for their diaphoretic and expectorant properties. However, tinctures and internal remedies were largely discontinued by the early due to documented toxicity from cardiac glycosides, which caused vomiting, cardiac arrhythmias, and fatalities in overdoses. No randomized controlled trials validate the efficacy of these applications; historical accounts rely on anecdotal reports, with potential benefits attributable to emetic effects or rather than targeted pharmacological action, compounded by risks outweighing unproven gains in modern assessments.

Industrial and Modern Uses

During , milkweed floss from pods was harvested as a buoyant substitute for imported kapok in U.S. Navy life preservers, with approximately one pound sufficient to support a 150-pound person afloat due to its waterproof and low-density properties. Civilians, including schoolchildren, collected over 1,000 tons annually through government campaigns, processing facilities in states like extracting the floss for military use. Post-war, synthetic fibers supplanted floss in insulation and filling applications, rendering large-scale production uncompetitive as kapok imports resumed and cheaper alternatives emerged. Efforts to extract latex from Asclepias species for low-grade rubber peaked in the amid wartime shortages, with U.S. Department of experiments yielding polymers but at inefficient concentrations—typically under 5% latex content—prohibiting commercial viability. Pioneered by figures like , these initiatives were abandoned by the late as synthetic rubber from proved more scalable and cost-effective. In contemporary applications, milkweed floss finds niche use blended with down in products like jackets and pillows, comprising up to 20% of fillings for enhanced insulation without widespread adoption due to harvesting challenges and competition from synthetics. Proposals for production from Asclepias seed oils have been evaluated, yet yields remain low—averaging 100-200 kg/ha—falling short of dedicated crops like soybeans, limiting economic feasibility. Similarly, cardiac glycosides from the plants hold pharmaceutical potential akin to , but extraction inefficiencies and toxicity concerns have precluded industrial-scale development in favor of established sources. Experimental trials in the explore Asclepias for heavy metal uptake in contaminated soils, though field-scale efficacy remains unproven amid variable data.

Horticultural Cultivation

Asclepias are propagated primarily through seeds or vegetative divisions, with methods varying by to achieve reliable and establishment in garden settings. Seeds of most temperate , such as A. syriaca and A. tuberosa, require moist stratification for 30-60 days at 1-5°C to break and mimic winter conditions, often followed by via light sanding or acid treatment for hard-coated varieties to enhance permeability and rates exceeding 50%. Direct sowing in autumn allows natural stratification, while spring planting after treatment yields seedlings ready for transplant within 4-6 weeks under controlled conditions. Vegetative via root cuttings, typically 5-10 cm segments taken in fall or early spring, is effective for clonal reproduction in like A. tuberosa, promoting faster establishment than seeds but requiring well-drained media to prevent rot. Cultivated Asclepias thrives in full sun exposure of at least 6-8 hours daily, which promotes robust growth, flowering, and production essential for horticultural appeal. Well-drained soils are critical to avoid , with sandy or loamy substrates preferred over heavy clays, though tolerant species like A. incarnata adapt to moist, neutral to slightly acidic conditions (pH 6.0-7.0). Hardiness spans USDA zones 3-9 for many North American natives, such as A. syriaca (zones 3-9) and A. tuberosa (zones 3-9), with tropical A. curassavica limited to zones 8-11 and requiring winter protection northward. Water needs are moderate post-establishment, with developing after 1-2 years, but consistent moisture during the first season supports tillering and seed set. Pest management in horticultural settings emphasizes cultural and biological controls to minimize harm to beneficial insects, including monarch larvae that feed on foliage. Common pests include oleander aphids (Aphis nerii), which cluster on tender growth and can stunt plants, managed by high-pressure water blasts or introduction of predators like lady beetles and syrphid flies that naturally suppress populations without residues. Milkweed beetles (Tetraopes spp.) and bugs (Lygaeus kalmii) defoliate leaves and seeds; handpicking adults and eggs, combined with or , reduces infestations, as heavy use risks contaminating plants with systemic toxins lethal to caterpillars. Certain species, notably A. syriaca, pose challenges due to rhizomatous spread, potentially naturalizing aggressively in unmanaged gardens and outcompeting ornamentals via underground stolons extending 1-2 meters annually. Control involves preemptive division of clumps every 2-3 years, removal of seed pods before maturity to curb wind-dispersed dispersal, and regular mowing of shoots at level during vegetative growth to deplete root reserves without herbicides. These practices maintain tidy borders while preserving plant viability for successive seasons.

Conservation Implications and Debates

Monarch Butterfly Association

The monarch butterfly (Danaus plexippus) depends exclusively on plants in the genus Asclepias as host plants for its larval stage, with caterpillars feeding solely on milkweed foliage throughout development. This obligate relationship is central to the monarch's life cycle, as larvae cannot survive on alternative plants, leading to host-specific adaptations including tolerance to milkweed's toxic cardenolides. Adult monarchs retain these sequestered cardenolides, which deter predators by inducing emesis upon consumption, providing a chemical defense derived directly from the host plant. Female monarchs exhibit preferences for certain Asclepias species during oviposition, with A. syriaca (common milkweed) and A. incarnata (swamp milkweed) commonly selected in North American habitats. Oviposition decisions are influenced by plant volatiles, as demonstrated in choice tests where females discriminate among milkweed species based on chemical cues, favoring those emitting specific attractant compounds while avoiding high-toxicity variants. These preferences align with larval performance, though not perfectly, as some preferred species support higher survival rates in field and lab assays. Monarch populations experienced a peak in the late , with overwintering numbers exceeding 1 billion individuals, followed by declines to historic lows in the , including a around 2013-2014 with fewer than 35 million. Tagging and recapture data from long-term monitoring programs reveal that annual fluctuations are not attributable solely to milkweed availability in breeding grounds, as recovery rates and migration success vary significantly with conditions, predation, and en route mortality, independent of summer host plant density. This indicates multifaceted drivers in , with breeding habitat as one but not the exclusive factor.

Effectiveness of Milkweed Planting

Efforts to conserve butterflies (Danaus plexippus) through widespread planting of native Asclepias species aim to restore breeding amid perceived declines, yet genomic analyses indicate no significant reduction in effective sizes for either or milkweeds over the past 75 years, suggesting observed variations reflect natural fluctuations rather than a long-term collapse driven by loss.00996-X) A 2023 study sequencing DNA and Asclepias genomes found concurrent historical expansions in both species, with recent counts potentially reverting to pre-agricultural baselines influenced by factors like and predation rather than milkweed scarcity alone. This challenges the premise that U.S.-based planting alone can reverse trends, as bottlenecks during overwintering—exacerbated by and storms—exert greater control. Non-native tropical milkweed (A. curassavica) exacerbates risks by allowing year-round breeding, which accumulates the protozoan parasite Ophryocystis elektroscirrha (OE) and disrupts migratory cues, leading to higher infection rates and non-migratory phenotypes with reduced fitness. In contrast, planting shows mixed outcomes; while it can support local oviposition, studies indicate monarchs prefer certain natives like common (A. syriaca) and swamp (A. incarnata) milkweed, but overall remains low due to predation and defenses. Placement matters: milkweed at garden edges or in disturbed areas receives more eggs, as females favor accessible, low-vegetation sites, though quantitative boosts vary and do not scale to population-level recovery. Captive rearing of monarchs, often paired with planting initiatives, introduces harms including , weakened migratory instincts, and pathogen spread to wild populations upon release, with advising against it as counterproductive to genetic health. practices like strategic mowing of roadside or field milkweed promote regrowth of tender leaves attractive for oviposition, suppressing predators temporarily and increasing egg-laying by providing fresh foliage post-disturbance, though benefits are site-specific and diminish without addressing overwintering threats. Large-scale efficacy of planting remains unproven empirically, as U.S. enhancements yield marginal gains against dominant extrinsic pressures like variability and international degradation.

Broader Ecological and Management Considerations

Asclepias species present toxicity risks to livestock in pastoral systems, primarily through cardiac glycosides that induce symptoms such as weakness, irregular heartbeat, and digestive distress in cattle and sheep upon ingestion of substantial quantities. Management strategies include targeted herbicide applications, such as 2,4-D combined with picloram at rates of 0.5 kg ae/ha, or repeated mowing to prevent seed set and clonal spread, though efficacy diminishes in dense stands due to belowground rhizomes. These interventions, while safeguarding animal health, entail trade-offs for biodiversity, as broad-spectrum herbicides can suppress co-occurring native flora and non-target invertebrates, potentially reducing habitat heterogeneity in rangelands despite milkweed's role as a nectar source for generalist pollinators. Climate-induced stressors like and elevate cardenolide concentrations in Asclepias foliage, enhancing chemical defenses against herbivores but imposing selective pressures on toxin-adapted specialists through reduced palatability and nutritive value. Phenological responses, including advanced flowering by up to several days per decade in species like A. syriaca, reflect adaptive shifts without precipitating range contractions; distribution models forecast stasis or poleward expansions for many taxa by 2070, contingent on edaphic tolerances rather than limits alone. Management policies advocating widespread Asclepias propagation risk causal misattribution by prioritizing domestic augmentation over extraterritorial threats like at southern refugia, where precipitation deficits and land-use intensification dominate variance in viability. Designations such as milkweed's selection as Wisconsin's 2025 Rare Plant of the Year by the Department of Natural Resources exemplify promotional efforts amid observational indicating persistent or rebounding abundances post-herbicide eras, emphasizing integrated approaches that weigh empirical stressor hierarchies against supplementation.

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