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Botany

The genus Asclepias contains over 200 species distributed broadly across Africa, North America, and South America. The genus previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae. Asclepias syriaca, common milkweed, is native to southern Canada and much of the United States east of the Rocky Mountains, excluding the drier parts of the prairies. The flowers of common milkweed vary from white (rarely) through pinkish and purplish. Asclepias tuberosa, butterfly weed, has hairy stems that are not milky when broken.

Chemistry

Milkweed latex is a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Most species are toxic to humans and many other species, primarily due to the presence of cardenolides. Cardenolides are C(23)-steroids with methyl groups at C-10 and C-13 and a five-membered lactone at C-17. Cardenolides are toxic to animals through inhibition of the enzyme Na+/K+-ATPase, which maintains the sodium and potassium ion gradients across cell membranes. Cardenolides have not been found in Asclepias tuberosa, which contains some unique pregnane glycosides instead.

A pentane extract of the flowers of common milkweed has six major constituents, in order of relative abundance: benzaldehyde, (E)-β-ocimene, phenylacetaldehyde, benzyl alcohol, nonanal and (E)-2-nonenal. The volatile profile collected directly from fresh florets was dominated by (E)-β-ocimene, benzaldehyde and phenylacetaldehyde.

Ecology and uses

The monarch butterfly uses and requires certain milkweeds as host plants for its larvae. Monarch, queen and plain tiger butterflies ingest the cardenolides of the milkweeds they mostly feed on and sequester them as larvae for defense as adults. The cardenolide content in butterflies deters most vertebrate predators.

Common milkweed has been studied as a source of rubber from its latex and as a fiber source from its seed fluff. The compressed floss of common milkweed has a silk-like sheen. Milkweed has also been cultivated commercially to be used as insulation in winter coats. Some species can be made edible if properly processed.

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