Asclepias – Milkweed

by Adriana Lopez-Villalobos and Amelie Mahrt-Smith

If you’ve ever thought of planting flowers to attract pollinators to your garden, milkweed should be on top of your list – plants in the genus Asclepias are known for attracting all kinds of insects, most notably the monarch butterfly. The common milkweed, A. syriaca, is estimated to provide food to over 450 different species of insects! [3] Asclepias is a member of the Apocynaceae family, also known as the dogbane family. While they may provide adequate food for pollinators, some taxa are poisonous to animals – the family gets its colloquial name from those taxa that were historically used as dog poison [6]. The genus name, Asclepias, comes from the Greek god Asklepios the god of medicine. Milkweed has had a variety of uses in human culture over the years; medicine is just the beginning.

Common milkweed, Asclepias syriaca, growing by a riverbank in Kingston, ON.

The flowers of Asclepias are morphologically distinct. They are clustered in heads called umbels, with 30-50 individual flowers in each. The image below shows a close up of several A. syriaca flowers and their characteristics. The petals (a) are reflexed downward toward the stem, and a petal-like shape is made by the corona, which is made up of five nectar-secreting hoods (b) and incurving horns (c). The floral corona helps to attract pollinators. Pollen is produced in the anther (d) and received by the stigmatic disc (e). In Asclepias, these structures are fused into a single structure called the gynostegium. Between two adjacent anthers forms the anther wings (f), which enclose a stigmatic chamber. Above this chamber is the pollinium gland (g), where pollen can be retrieved[4]. All of the flower buds in an umbel will open within 2-3 days of each other, and fade in colour and begin to shrivel shortly after being pollinated [5]. Not every flower will produce seeds, even if they have been pollinated – one stem may have 50 flowers but still only produce one or two seed pods.

LEFT: Close up of A. syriaca inflorescence, showing the different parts of the individual flower. RIGHT: Seed pods ripening on an A. syriaca individual.

Pollinators searching for nectar on the unstable flowers can get pollinia, a mass of pollen grains, stuck to the adhesive pads on their feet that help them climb. These pollen grains may be deposited on the stigmatic disc of another flower as the insect continues to forage [4]. While they are visited by bumblebees, wasps, ants, and flies as well, milkweed is most commonly associated with the butterflies that are attracted to the sweet-smelling flowers. The iconic monarch butterfly relies on milkweed for its entire life cycle. The eggs are laid on the underside of milkweed leaves, which the caterpillars eat from when they hatch. The poisonous compounds in Asclepias, chemicals called cardiac glycosides, are actually used by the caterpillars as a defence mechanism against predators. Insects like the monarch caterpillar that are adapted to feeding on Asclepias plants store these compounds in their body instead of metabolizing them, which effectively makes them poisonous to those looking for a tasty grub to snack on [1]. After metamorphosis, the monarch butterfly may eat the nectar from milkweed flowers in addition to many other species, but it will always return to milkweed to lay its eggs.

Although toxic in large quantities, the compounds in milkweed have given them traditional medicinal uses in human culture. A. syriaca, the common milkweed, was used by colonial settlers as an expectorant, an emetic, and a remedy for asthma. A related plant, A. tuberosa, has been used to induce perspiration and in the treatment of lung ailments. The milky latex produced by the plant, which can be seen oozing from the stem if one breaks off a leaf or flower, was investigated as a rubber precursor but was never profitable. The seed hair fibres were used as a wartime substitute for kapok to make life jackets [2]. Although not profitable for humans, milkweed is still a very important plant for pollinators like the monarch butterfly.

Asclepias syriaca is native to North America and is commonly found invading fields and roadsides. This specimen was collected at QUBS in 1960 by the former curator, Roland Beschel. You can find A. syriaca in flower all around QUBS from July to early August!

A specimen of A. syriaca collected at QUBS in 1960.

References

  1. Lewis, D.R. Iowa State University. The Milkweed Insects. https://www.extension.iastate.edu/news/2005/jul/072201.htm#:~:text=The%20monarch%20butterfly%20is%20one,feed%20on%20the%20common%20milkweed.&text=Plants%20can%20be%20interesting%2C%20especially,seeing%20them%20eaten%20by%20insects. Downloaded on 03 August 2020.
  2. Simpson, M.G. 2010. Plant Systematics., 2nd. Ed. Academic Press, Elsevier. ISBN 9780123743800
  3. Moore, R.J. 1946. Investigations on rubber-bearing plants. V. Notes on the flower biology and pod yield of Asclepias syriaca L. Can. Field Natur. 61: 40 – 46.
  4. Macior, L.W. 1965. Insect adaptation and behavior in Asclepias pollination. Bull. Torrey Bot. Club, 92:114 – 126.
  5. Gaertner E.E. 1979. The history and use of milkweed (Asclepias syriaca L.). Economic Botany 33: 119-123.
  6. Erickson, J.M. 1973. The utilization of various Asclepias species by larvae of the monarch butterfly, Danaus plexippus. Psyche A Journal of Entomology 80(3) DOI: 10.1155/1973/28693

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