Some Uncommon Things About Common Milkweed

Common Milkweed, Asclepias syriaca, blooming on June 30, 2021, at Crow Hassan Park Reserve in Minnesota.

 

At times reviled as a nuisance of farm fields and pastures, Common Milkweed (Asclepias syriaca) has gained new respect as a plant that supports Monarch Butterfly larvae. That’s just part of the story, though. Here are a few things about milkweed that get less attention.

• The plant’s scientific name, Asclepias syriaca, is centuries old. It was given by Carl Linneaus, the Swedish botanist who in the 1700s developed the binomial system of nomenclature. That's the system that gives plants and other living things two names: a generic name – Asclepias, in this case – and a specific name, also called a specific epithet –syriaca for Common Milkweed.

• Linneaus is said to have been so impressed by the many medicinal uses of common milkweed that he named the plant after Asklepios, the Greek god of medicine. The specific epithet, syriaca, is from Linneaus’ mistaken belief that the plant came from Syria.  

• Syria is a long way from where common milkweed is naturally found. The plant is native to the Eastern and Great Plains regions of the U.S. and adjacent provinces of Canada. Like many plants, however, common milkweed has found its way overseas. It is now also found in southern and central Europe, where it invades grasslands and farm fields (1).

• One reason the plant isn’t always welcome is because it’s toxic to many animals, including humans. Like other milkweeds, the plant’s white latex contains cardiac glycosides, compounds that affect the function of the heart. Depending on the amount consumed, milkweed latex can cause symptoms ranging from nausea and vomiting to slowed heart rate, coma and even death (2).

• Some animals can eat milkweed safely. Milkweed bugs, for example, can isolate the cardiac glycosides they consume while they munch on leaves and other plant parts. The insects themselves then become toxic, which makes them unpalatable to predators. Their bright colors warn potential diners that eating them would be a mistake (3).

• Evidently, milkweed bugs have a lot of company. According to the U.S. Forest Service, common milkweed is a “mega food market” that feeds more than 450 kinds of insects. Some, like milkweed bugs, are destructive, but others merely sip the plant’s nectar or suck out its sap (4).

• Medicinal uses of common milkweed have waned, but not long ago, the plant saved lives in a different way. During World War II, milkweed pods were collected for the silks attached to their seeds. The buoyant, waterproof strands, called milkweed floss, were used to stuff life preservers when kapok, another plant fiber used for that purpose, could not be obtained from Indonesia.

• Milkweed floss was in such demand that school children were paid to gather the pods. The going rate was15 to 20 cents per onion bag or gunny sack filled with pods. Two bags provided enough floss to make one life preserver.

• Although it’s hard to imagine milkweed floss making much difference in the effort, the plant was abundant enough to have made an estimated 1.2 million life preservers. Milkweed was so valuable that the U.S. government considered it a “wartime strategic material” (5).

• After the war, common milkweed lost its status and was once again considered a weed. From reviled to revered and back again, shifting fortunes seem to define milkweed’s history.

References

(1) Asclepias syriaca (common milkweed). CABI Invasive Species Compendium. Viewed 6/30/21 at https://www.cabi.org/isc/datasheet/7249.

(2) Milkweed Plant Can Cause Serious Poisoning. Poison Control, National Capital Poison Center. Viewed on 6/30/2021 at https://www.poison.org/articles/milkweed-can-cause-serious-poisoning-204.

(3) Common Milkweed Insects. Susan Mahr, University of Wisconsin-Madison. Wisconsin Horticulture, Division of Extension. Viewed on 6/30/21 at https://hort.extension.wisc.edu/articles/common-milkweed-insects/.

(4) Plant of the week: Common milkweed (Asclepias syriaca).  David Taylor. U.S. Forest Service, USDA. Viewed on 6/30/2021 at Common Milkweed (fs.fed.us).

(5) A weed goes to war, and Michigan provides the ammunition. Gerald Wykes, from Michigan History magazine. Posted February 4, 2014, and updated January 20, 2019, on MLive. Viewed on 6/30/2021 at https://www.mlive.com/news/2014/02/a_weed_goes_to_war_and_michiga.html. 

Antsy Plants

From left: Wild Ginger, Nodding Trillium, and Bloodroot in early spring. 

Wild Ginger, Trillium and Bloodroot are done flowering, but that's not the end of their efforts. Now they must disseminate their seeds, and each has arrived at the same, six-legged solution to accomplish that task: Ants.

Myrmecochory (often pronounced MUR-mecco cor-ee), the dispersal of seeds by ants, is a convenient invention. Ants are found all over the world, from the tropics to the Arctic, so they are a ready resource. Similarly, plants that employ ants for seed dispersal live in diverse habitats, including the tropical rainforests of Latin America, the dry shrub communities of South Africa and Australia, and the eastern deciduous forests of Europe and the U.S. (1). Myrmecochory is thought to have developed independently more than 100 times, with more than 11,000 species of plants relying on these insects to spread their seeds (2).

That nature converged on the same solution in different, and distant, plants suggests that it works. Like any method of seed dispersal, though, myrmecochory has a cost. It demands adaptations, and in one group of ant-dispersed plants, that adaptation is in the form of a bribe.

To lure ants, myrmecochores attach small, fatty bodies called elaiosomes (e-lay-o-somes) to their seeds or fruits. Depending on the species, these mini nutritional packets are clear, white, brown, or other colors and shaped like worms, flags or amorphous dollops. Some may emit an odor like rotting insect carcasses, a trick to attract ants to take the seeds back to their nest, remove and feed the elaiosomes to the colony, and leave the seeds to germinate (1).

From left: Wild Ginger, Nodding Trillium, and Bloodroot seeds with their elaiosomes.

Both plants and ants are thought to benefit from this relationship. Plants benefit by reducing competition for light and nutrients between parent plants and their offspring. Moving seeds away from the parent plant also lessens the risk of local extinction: If one part of a population dies, another, more distant, part may survive. Another potential benefit is reduced seed predation. Perhaps better than any other animal, ants can disperse a cache of seeds meters away from the parent before mice, birds or other seed eaters find them.

One more potential benefit is improved seed germination. Seeds discarded in or near ant nests may end up in refuse piles, nutrient-rich microenvironments that can aid germination and seedling growth. The medium in ant nests may also retain more water or be better aerated, another potential aid to germination and growth. 

Ants benefit from myrmecochory, too, and it's likely they're adapted to the interaction. Not all species of ants forage for seeds that have elaiosomes, but those that do may have some yet-unknown characteristics that lead them to that behavior. Widespread though it is, this mutually beneficial relationship still has some secrets to share.

Myrmecochores of Minnesota

The ant-dispersed plants in this region tend to be early-blooming herbs of deciduous forests. Here are a few, based on personal observation or mention in references. 

Bloodroot (Sanguinaria canadensis)

Nodding Trillium (Trillium cernuum)

Wild Ginger (Asarum canadense)

Yellow Violet (Viola pubescens)

Spring Beauty (Claytonia virginica)

White Trout Lily (Erythronium albidum)

References

(1)    Handel, S.N., and Beattie, A.J. (1990). Seed dispersal by ants. Scientific American 263 (2): 76-83B.

(2)    Lengyel, S., Gove, A.D., Latimer, A. M., et al. (2010). Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: A global survey. Perspectives in Plant Ecology, Evolution and Systematics 12: 43-55.