Each year, as fall’s cooler temperatures signal the coming of winter, monarch butterflies (Danaus plexippus, Nymphalidae) commence their journey south, flying 50 to 100 miles a day from breeding grounds in North America to their winter habitat in the oyamel fir forests of Central Mexico.¹ During their migration and summer breeding period, adult monarchs drink the nectar of a variety of plants and flowers, such as yarrow (Achillea millefolium, Asteraceae) and primrose (Primula vulgaris, Primulaceae),² but will depend most heavily on a single genus of plants — the milkweeds (Asclepias spp., Apocynaceae) — as the only food source for their offspring. Just as they have evolved to travel the same route every year using an internal compass, monarchs also have acquired the instinct to lay their eggs on no other plant.³

The milkweed genus Asclepias comprises about 130 species, although monarchs are thought to feed on just 27 milkweeds native to North America.^4-6 While this article focuses on species native to the United States, many species occur in tropical parts of the Americas and in Africa. The geographic distribution of US species varies, with some milkweeds flourishing in the Northeast and others being more abundant in West. They are hardy wildflowers that grow in diverse and oftentimes dry locales, such as prairies, pastures, and roadsides. As perennials, milkweeds stay dormant through the winter and grow back from root in the springtime. (Bloodflower [A. curassavica], also called tropical or scarlet milkweed, is the only annual milkweed species, although it can be grown as a perennial in locations with no frost year-round.)

Most milkweeds grow to at least one foot high, with some reaching more than six feet, and have tough, green stems full of strong fibers, two or three often long and narrow leaves at each node, and clusters of small flowers that vary in color depending on species, from vivid pinks to mellow oranges to soft whites.⁵ Milkweeds also produce pods full of seeds with fluffy white hairs that make them float in the wind for dispersal.

The name milkweed comes from the thick, white latex the plants secrete when punctured. Butterfly weed (A. tuberosa) — which is referred to as pleurisy root when just the root is being discussed³ — is the only milkweed species with clear sap. Milkweed species vary in toxicity, typically mild in humans. Milkweed has been known to harm some livestock, including cattle, goats, horses, fowl, and sheep.^4,6 While studies in which animals were intentionally fed concentrated milkweed preparations in order to test for toxicity found strong adverse effects,⁵ the real risk of such poisonings is rare.

“This generally only occurs under conditions of poor management,” said Carl Stenoien, a PhD student at the University of Minnesota’s Monarch Lab in St. Paul (email, September 9, 2013). “These animals have a natural aversion to the bitter taste and toxic effects of milkweeds. Ingestion may occur if the plants are incidentally dried and baled or in overgrazed pastures where preferred forbs or grasses have been consumed and milkweeds are the only remaining plants.”

Small flowers on the milkweed plant contain ample nectar, attracting butterflies, bees, hummingbirds, and other pollinators that collect large amounts of pollen (as opposed to tiny individual grains) stored in small sacs on the flowers.³ Thus, milkweed serves significant roles in supporting pollinators — especially bumblebees and honeybees⁵ — throughout the warmer months when other pollinator plants are not in bloom. It also supports non-harmful and beneficial insects such as mite-eating ladybeetles.³

Traditional Uses by Humans

It is not just the wondrous monarchs and other insects that derive benefits from the milkweed, as humans have used various Asclepias species for centuries to treat myriad health conditions. According to Volume II of The Useful Wild Plants of Texas, the Southeastern and Southwestern United States, the Southern Plains, and Northern Mexico — a text that dedicates 75 pages to the milkweeds — “The genus Asclepias, many species of which occur in Texas, have so many, such varied, and such significant uses that the genus is one of the most highly valued in this work…. The number of medicinal uses of Asclepias, including cures and treatments for almost every system of the body, is nothing short of phenomenal.”⁵

As is common with many medicinal plants, the active chemicals in milkweed are responsible for the plant’s toxic properties as well as its therapeutic benefits. Within the milkweed latex are steroids called cardenolides, which exhibit cardiotonic — and sometimes cardiotoxic — properties. Some milkweed species, such as inmortal (A. asperula), have high levels of cardenolides, while others, such as showy milkweed (A. speciosa) and swamp milkweed (A. incarnata), have lower levels.^5,6 Different populations within the same species can produce varying levels of cardenolides due to ecological factors. Additionally, the plant part containing the strongest cardenolide concentration often varies from species to species.

“In addition to the variation in cardenolides between species, there can be great variation within a species as well,” said Stenoien of the MonarchLab. “Whether plants are water-stressed, grown in sun or shade, fertilized, or have recently been grazed by herbivores all affect the concentration and localization of the cardenolides. Depending on these local factors, the toxicity of a single milkweed plant may fluctuate greatly on time scales ranging from minutes to months.”

Cardenolides, a type of cardiac glycosides, are found in many plants in addition to milkweeds, such as the highly toxic common oleander (Nerium oleander, Apocynaceae) and foxglove (Digitalis spp., Scrophulariaceae), which contains the important cardenolides digoxin, digitoxin, and lanoxin. Cardiac glycosides inhibit the sodium-potassium pump in the cell membranes, resulting in increased concentrations of intracellular sodium — which in turn causes increased calcium levels — and decreased potassium in the cells of the heart. According to Useful Wild Plants, “Combined with indirect vagal stimulation, this slows the heart and produces a more forceful beat.”⁵ These plant chemicals, which reportedly have been used in traditional medicine for about 1,500 years, are employed as pharmaceutical drugs to treat congestive heart failure and present potential as novel cancer therapeutic agents.⁷

Although a search of the PubMed database for “milkweed,” “Asclepias,” and “pleurisy root” suggests that no clinical research has been conducted on any of the milkweeds mentioned in this article,⁸ a few human trials in the 1960s and ’70s studied the potential asthma-reducing benefits of a species found in India known commonly as Indian ipecac (A. asthmatica), which was studied under its synonym, Tylophora indica.^9,10

Uses by Native Americans. Milkweeds have been used traditionally to treat a multitude of conditions, and several in vitro studies have supported the plant’s antimicrobial and antiseptic activity.⁵ Numerous native tribes throughout the Americas — including the Navajo, Iroquois, Cherokee, Blackfoot, Meskwaki, and others — extensively employed various parts of milkweed species. Anthropologist Daniel Moerman, PhD, who has spent decades studying the usage of plants by Native Americans, told HerbalGram that there are over 300 milkweed uses documented in the University of Michigan’s Native American Ethnobotany database (email, September 12, 2013). Crushed milkweed leaves were used externally to treat skin ulcers, skin cancers, wounds, ringworm, and headaches, while the root was made into a powder or juice and applied topically to cure tumors and to treat wounds, boils, and rashes. The sap was used externally for leprosy, to make warts and freckles disappear (due to its caustic properties), to lighten skin, and to treat ear infections (by the Maya). The seeds were sometimes used on sores.

Internally, milkweed was often relied upon by Native Americans and European settlers to treat snakebites.⁵ Some tribes are reported to have made various milkweed concoctions, while others such as the Choctaw simply chewed on the root of whorled milkweed (A. verticillata).⁵ Due to the plants’ reported ability to draw out snake venom, some also used milkweed root, latex, leaves, or boiled seeds externally for snakebites.

Many tribes chewed milkweed root to treat sore throats and rashes, while native peoples in Mexico drank the latex or ate the flowers, leaves, or stems to treat rabies in humans and animals.⁵ Some ingested root preparations for hemorrhaging, and many native and settler groups throughout the Americas took the root — especially that of A. tuberosa — to treat respiratory conditions, including pleurisy, lung inflammation, colds, etc.^5,11 Additional internal traditional uses included toothache; heart conditions; fever; headache; digestive conditions including gas, indigestion, diarrhea, and vomiting; and as a contraceptive and abortifacient (although the late, renowned Southwest herbalist Michael Moore contested the latter usage, claiming milkweed is strong enough to cause intense nausea and nothing more⁵).

Uses by 18th and 19th Century Physicians. Meanwhile, medicinal milkweed preparations had somewhat widespread usage, documentation, and recommendation in the 1800s and early 1900s by Eclectic physicians, the Canadian pharmaceutical industry, and American botanists and medical physicians, such as the 1880 vice-president of the American Medical Association Francis Peyre Porcher, MD. According to the milkweed chapter of Dr. Moerman’s 1981 book Geraniums for the Iroquois: A Field Guide to American Indian Medicinal Plants, “Native American enthusiasm for the milkweeds was widely shared by Euro-Americans from the earliest times until the turn of the twentieth century. One species or another was listed in the USP [United States Pharmacopeia] from the first edition in 1820 until at least 1905.”¹¹

The 16th edition of The Dispensatory of the United States of America, published in 1888, documented numerous uses of pleurisy root, including as a diaphoretic, expectorant, and astringent; and to treat catarrh, pneumonia, pleurisy, consumption, diarrhea, dysentery, rheumatism, etc.¹² In 1863, Dr. Porcher reported that some doctors in South Carolina used “A. asperula and A. tuberosa roots in tea or powder preparations as a salivary stimulant. Apparently in the middle of the nineteenth century, the soreness of the mouth caused by using Asclepias roots was much less severe than the side effects of using the then popular mercurial preparations.”⁵ In 1828, The New-England Farrier and Family Physician suggested milkweed for dropsy (edema), dysentery, and piles (hemorrhoids), while Massachusetts physician and Revolutionary War surgeon James Thacher, MD, recommended the root powder for gas and indigestion in 1813.

The Eclectic physicians of the late 19th and early 20th centuries used pleurisy root to treat respiratory conditions, particularly pneumonia, as well as pleurisy root tinctures or teas to treat fever in children.¹³ In the pandemic influenzas of the early 1900s, according to a 2006 article in the journal Alternative Complementary Therapy, A. tuberosa preparations were used for “chest pain, lung inflammation, coughing, and to reduce bronchial symptoms.”⁸ The Eclectics also used swamp milkweed root primarily as a diuretic, especially for cardiac edema, and for intestinal parasites in the formula King’s Entozoic.¹⁴

Uses by Contemporary Herbalists. More recent herbalists have continued to use milkweed for respiratory problems. Daniel Gagnon, RH (AHG) — a medical herbalist and owner of the herbal medicine company Herbs, Etc. in Santa Fe, NM — said that he “truly, deeply” knows and uses just two milkweed species: inmortal and pleurisy root. He noted that many American herbalists are familiar with pleurisy root from studying the Eclectic texts, including the 1854 King’s American Dispensatory (King), the 1898 King’s American Dispensatory (Felter and Lloyd), the 1922 The Eclectic Materia Medica, Pharmacology and Therapeutics (Felter), and the 1915 American Materia Medica, Therapeutics and Pharmacognosy (Ellingwood). Additionally, herbalists who have studied with Michael Moore or other teachers who were taught by Moore know and use inmortal, which is native to the Southwest United States.

Gagnon sees these two species as medicines for treatment of inflammation of the pleural membranes surrounding the lungs, removal of accumulating fluids at the bottom of the lungs, activation of phlegm excretion from the lungs, and restoration of inflamed membranes surrounding the heart (pericardium) and stomach (peritoneum).

“I know there are many other places where these two roots have been used, but lungs clearly come to mind when I think of inmortal and pleurisy root,” said Gagnon (email, July 28, 2013). “I personally have been using inmortal and pleurisy root in my clinical practice for over 30 years and have never had a client experience negative effects from these two herbal medicines.”

Gagnon noted that pleurisy root is used in the Herbs, Etc. formulation called Respiratonic^®, which has been on the market for more than 40 years and consistently ranks in the company’s top-10 bestselling products.

“We have old-time wildcrafters that sell us inmortal root every fall,” he said. “One of my wildcrafters has been picking inmortal for over 60 years. At this time I don’t see that the adoption of these two herbs by the [conventional] medical profession will occur because of [the] way they use medicines. This plant/drug/therapy is good for this problem/disease/condition. One pill/one condition medical doctors are not trained to think of tissue conditions and working from an energetics point of view. I believe that herbalists are the ones that are going to keep using this medicine. So, in effect, I am saying that the use of this plant will remain relatively insignificant.”

Fellow herbalist David Winston, RH (AHG) — director of the Herbal Therapeutics Research Library and president of Herbalist and Alchemist, Inc. in Washington, NJ— reiterated American herbalists’ usage of pleurisy root as a tea or tincture and explained that it is often used according to the Eclectic indication “that it hurts to breathe,” as well as for lung conditions accompanied by fever, congestion, or sharp pain when coughing; as a diaphoretic especially for fevers with bronchial involvement; and to help reduce emotional agitation or irritability in people with high fevers.

“It can also be used for intercostal neuralgia (when the chest muscles hurt), and it is actually quite effective for that,” he said.

Additionally, Winston noted that the sap of common milkweed (A. syriaca) is occasionally used as a topical treatment for warts. If overdose of internally ingested pleurisy root does occur, Winston noted that it can cause nausea, vomiting, and diarrhea. He also cautioned that overdose of inmortal — which he said should not be used with conventional cardiac medications or by people with overt heart disease — may cause nausea and vomiting as well.

“These are strong medicines, not tonics, and are, in my opinion, best left to people who are knowledgeable about their use,” said Winston. “Small amounts of pleurisy root are relatively safe, especially when used in a larger formula. None of these should be used internally during pregnancy.”

Food and Fiber Uses. Milkweed also has been used extensively for non-medicinal applications. As Useful Wild Plants documents, Native American and Canadian First Nation tribes — including the Hopi, Kiowa, Zuni, Cheyenne, Menominee, and others — used many parts of the milkweed plant for food, cooking the stems like one would asparagus and to season soups, cooking the leaves with meat or eating them raw, making the dried latex into chewing gum, and boiling the seed pods alone or with meat.⁵ Flowers also were used to make preserves, added to venison broth soup and cornmeal, or eaten raw. Modern foragers in the United States sometimes use the flowers, seed pods, and detoxified greens in various dishes, such as stir-fries, soups, or casseroles, although others avoid milkweed due to perceptions of potential toxicity.⁵

Native tribes also used milkweed as a dye for textiles and ceramics.⁵ Native Americans used the stem fiber to construct bowstrings, fish nets, baskets, twine, and other products. More recently, the fine-yet-durable seed pod fiber has been used for insulation and soundproofing and to absorb spilled oil, and a few select US companies use common milkweed seed floss to stuff hypoallergenic pillows and comforters.⁵ During World War II, the US government began subsidizing milkweed crops for the seed floss’s use as insulation in sleeping bags, helmets, pillows, and more. Hummingbirds use the seed fiber to build nests, and some have proposed that these tiny birds were the first to discover the insulation properties of the seeds.⁵ Unfortunately, despite its rich history as a traditional medicine and fiber product with bright commodity potential, milkweed is currently viewed by most farmers and ranchers as an obnoxious weed.

Importance of Milkweed for Monarchs, Impact of Decline, and Possibility of Restoration

Importance. Female monarchs lay their eggs on no other plants but milkweeds.¹⁵ The eggs hatch in just three to four days and monarch larvae — those pudgy caterpillars with thin stripes of yellow, black, and white — emerge to repeatedly shed their skin and grow larger, all while constantly feeding on the milkweed, particularly the leaves and flowers. Depending on the ambient temperature, the larvae will feed for up to two weeks and then enter the pupal stage during which they encase themselves in silk chrysalises. Two weeks later, adult monarch butterflies emerge. Three generations of monarchs will go through this process in North American breeding grounds until the fourth generation migrates south to Mexico. (Monarch populations west of the Rocky Mountains spend the winter in California, while populations on other continents and islands may or may not migrate.)

While feeding on milkweed, the larvae ingest cardenolides and store these chemicals in their tissues through the remaining stages of metamorphosis and into their adult life, thus exhibiting toxicity to potential vertebrate predators. If a bird consumes a monarch that has fed on milkweed with enough cardenolides, the butterfly will taste bitter and may even cause vomiting.^5,6 Thereafter, that bird will associate the monarch’s unique colors and patterns as a warning sign. Most of these toxins are concentrated in the wings, and studies have shown that a higher proportion of brightly colored, bitter monarchs survive bird attacks than do more palatable monarchs, potentially allowing these individual monarchs another chance to mate and pass on genes that confer toxicity and bright coloration.^6,11 Because milkweed species contain varying levels of cardenolides, monarchs can present different levels of toxicity to predators depending on their specific larval host plant.

In 2010, scientists writing in the journal Ecology Letters proposed that monarchs additionally choose to lay their eggs on milkweed due to a possible medicinal benefit for their offspring — behavior they refer to as “trans-generational medication.”¹⁶ A protozoan parasite is known to infect monarch larvae and develop into harmful parasitic spores that cover the bodies of adult butterflies. Previous studies have found that when monarchs lay their eggs on certain milkweed species with more cardenolides, such as A. curassavica (bloodflower), parasite infection in their young is greatly reduced. The Ecology Letters study echoed these previous results: “All tested parasite genotypes produced lower spore loads on monarchs reared on A. curassavica than those reared on A. incarnata. Consequently, infected monarchs reared on A. curassavica had longer life spans than those reared on A. incarnata.” A later study conducted by the scientists in 2012 had similar conclusions. The researchers wrote, “as our results indicate, monarchs can obtain tolerance to infection by using particular species of milkweed.”¹⁷

Decline. Monarch populations are measured according to the amount of land they occupy during their overwintering season in central Mexico.³ A recent survey conducted by World Wildlife Fund found a 59% decrease in colony size from 2012.¹⁸ While it is generally accepted that there are many contributing factors to monarchs’ decline, including the degradation of overwintering habitat, most experts cite dwindling milkweed populations and the rapidly changing global climate as the predominant threats.^3,18,19

Once widespread, milkweed populations are decreasing largely due to increased usage of herbicides/pesticides across American farmland alongside increasingly frequent planting of genetically modified (GM) crops.^3,20 As the farming of herbicide-resistant row crops has increased, so has the decline of milkweed and monarch populations. Twenty-one milkweed species are currently classified by some US states as endangered, threatened, or of special concern.

A loss of undeveloped countryside also has meant less habitat range for milkweeds. According to a presentation at the 2012 Monarch Biology and Conservation Meeting given by Orley Taylor, PhD, of the University of Kansas’s Monarch Watch, milkweed habitat has decreased by about 160 million acres in the past 17 years, which represents 20% of the monarch’s eastern US breeding area.²⁰ This land is now being used for corn and soybean farming, as well as expanding urban development. Planned and controlled removal of milkweed is also an issue. As MonarchLab explains the situation, “It’s unfortunate that the monarch host plant has the word ‘weed’ in its name, as there are some cities and counties that consider it a noxious weed, and actively remove the plant.”¹⁹

“I used to see large amounts of pleurisy root on Route 81 through Virginia,” said herbalist Winston, “and over the years the amount of pleurisy root declined to point where you hardly ever see it.”

Several herb companies and bulk herb distributors sell milkweed, most often pleurisy root, including Winston’s Herbalist and Alchemist, as well as Herb Pharm, Nature’s Answer, Herbs Etc., Blessed Herbs, and others. A few of these businesses, such as Strategic Sourcing, Inc., cultivate milkweed while others wild harvest it. Still, Winston noted that it is unclear what impact this has had on milkweed populations. The nonprofit United Plant Savers categorized A. tuberosa as a “to-watch” species on its May 2012 Species At-Risk list, although it does not indicate if the primary threats are harvesting, habitat destruction, eradication, etc.²¹

“While it is not one of the top 50 herbs in commerce, there is definitely a consistent market for it,” said Winston.

Additionally, because bees are the major pollinator of milkweed,⁵ the current decrease in bee colonies may also have a negative effect on milkweed.

Whatever the cause for milkweed decline, it is compromising monarch numbers. “Fewer plants may result in higher per plant monarch density, which can increase the risk of spreading disease and can result in competition for food between larvae,” according to the Monarch Joint Venture.⁴ And, if monarch larvae ingest too little of the cardenolides in milkweed, they will be more susceptible to predators.⁵

Considering the remarkable life cycle and annual, multi-generational migration of the monarch, a continued decline of these butterflies would be a major loss to the many who cherish their existence, as would a furthered waning of milkweed populations.

“I would venture to say that we need to protect this plant from herbicides and its eradication,” said herbalist Gagnon. “Losing this herb would mean that we would lose two of the best plants for inflammation of the lungs, heart, and intestines where the serous membranes are seriously compromised. I do not know of other herbal medicines that work as effectively for that purpose.”

Restoration. Current monarch conservation efforts in North America seek to address several causes of monarch decline, including the restoration of milkweed populations. Various monarch organizations in the United States are leading campaigns to increase the supply and demand of native plants and seed; produce more native seed in milkweed states; collect, process, and provide milkweed seed; raise awareness of the value and potential markets for milkweed; encourage landowners, gardeners, and other interested individuals to plant native milkweed; and map milkweed populations in the western United States.

More information on monarchs and milkweed, and tips on how to create a milkweed habitat to attract monarchs, is available at the Monarch Joint Venture website at www.monarchjointventure.org.

References

1. Monarch migration. The Monarch Joint Venture website. Available at: www.monarchjointventure.org/monarch-biology/monarch-migration/. Accessed August 2, 2013.
2. Good nectar sources. Monarch Watch: butterfly gardening website. Available at: www.monarchwatch.org/garden/nectar.htm. Accessed September 6, 2013.
3. Borders B, Casey A, Row J, et al. Pollinator plants of the Central United States: native milkweeds (Asclepias spp.). The Xerces Society for Invertebrate Conservation. June 2013. Available at: http://monarchjointventure.org/images/uploads/documents/Milkweeds-of-Central-US_plus-vendors_XercesSociety.pdf. Accessed September 6, 2013.
4. Habitat needs. Monarch Joint Venture website. Available at: http://monarchjointventure.org/monarch-biology/habitat-needs/. Accessed September 6, 2013.
5. Cheatham S, Johnston M, Marshall L. The Useful Wild Plants of Texas, the Southeastern and Southwestern United States, the Southern Plains, and Northern Mexico. Volume II. Austin, TX: Useful Wild Plants, Inc; 2000.
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9. PubMed search: Tylophora indica. Article type: “clinical trial”. Search performed on August 5, 2013.
10. Asclepias asthmatica. The Plant List website. Available at: www.theplantlist.org/tpl/record/kew-2654607. Accessed August 5, 2013.
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13. Oliff H. Re: Historical perspective of botanicals used by Eclectic physicians in pediatrics. Winston D. The use of botanicals in eclectic pediatrics. J Am Herbalists Guild. 2004;Spring/Summer:59-64. Available at: http://cms.herbalgram.org/herbclip/284/review44198.html.
14. Felter H, Lloyd J. King’s American Dispensatory, 19th ed, Volume II. Cincinnati, OH: The Ohio Valley Company; 1898.
15. Monarch life cycle. MonarchLab website. Available at: www.monarchlab.org/Lab/Biology/MonarchLifeCycle/Default.aspx. Accessed September 10, 2013.
16. Lefevre T, Oliver L, Hunter MD, de Roode JC. Evidence for trans-generational medication in nature. Ecology Letters. 2010. doi: 10.1111/j.1461-0248.2010.01537.
17. Sternberg ED, Lefevre T, Li J, et al. Food plant derived disease tolerance and resistance in a natural butterfly-plant-parasite interactions. Evolution. 2012;66(11):3367–3376.
18. Monarch butterfly survey indicates lowest numbers in 20 years [press release]. Mexico: World Wildlife Fund; March 14, 2013. Available at: http://worldwildlife.org/stories/monarch-butterfly-survey-indicates-lowest-numbers-in-20-years. Accessed August 2, 2013.
19. Monarch conservation. MonarchLab website. Available at: www.monarchlab.org/lab/research/topics/conservation/default.aspx. Accessed August 2, 2013.
20. Taylor O. Conservation and restoration of monarch butterfly habitats. Presentation at 2012 Monarch Biology and Conservation Meeting. Available at: www.monarchlab.org/mn2012/Admin/upload/147.pdf. Accessed September 10, 2013.
21. Species at-risk. United Plant Savers website. May 8, 2012. Available at: www.unitedplantsavers.org/content.php/121-species-at-risk. Accessed September 24, 2013.