Common Milkweed

Common milkweed, Asclepias syriaca, grows best in disturbed, open, sunny habitats including fields, roadsides, pastures, and gardens.  There are 150 milkweed species worldwide, mostly found in the Western hemisphere with 22 in the United States.  They grow 3′ to 5′ tall with soft, pillowy leaves grown in tiers at right angles to the stem.  Pink-lavender clusters containing dozens of flowers grow from the top of the stems.  Each single flower has five bent-back petals that act as caps to individual nectar cups.  Nectar is secreted mainly in the evening and throughout the nighttime hours in abundant amounts.

^{Common milkweed flowers, Asclepias syriaca, by Jason Hollinger Aug 2007; Common milkweed, Asclepias syriaca, by Amos Oliver Doyle Jun 2018; Common milkweed leaves, Asclepias syriaca, by Paul Henjum Oct 2004}

At the base of each flower there are two slits that easily fit a visiting pollinator’s feet, making it easy to stand and drink nectar.  Thread-like strands of pollen found in the slits become entangled around pollinators’ feet when they leave.  However, only a few flowers are successfully pollinated each year since pollen transfer is good only if the strand is delivered to another plant, as milkweed is incapable of self-pollinating.  Getting feet caught in the entangled threads and slits is a real danger to pollinators, and about five percent of insects become trapped and perish this way each season.

^{Common milkweed seeds, Asclepias syriaca, by Kingturtle Nov 2007; Common milkweed burst seed pod, Asclepias syriaca, by Brocken Inaglory Oct 2010; Common milkweed seed pod, Asclepias syriaca, by H Zell Aug 2009}

In an average year, a milkweed plant will develop eight to twelve seed pods, with each pod containing dozens of seeds.  Seeds are attached to long silken threads that act as parachutes and after a pod pops open, seeds are pulled out by the wind.  Seeds typically travel only a few hundred yards, at most, but can remain viable in the soil for four to ten years.  Plants also spread from buds grown on roots running out from each plant, producing colonies that cover small areas.

Milky sap flows through the plant, and it is a defense against herbivores.  It contains toxic chemicals identified as cardiac glycosides which affect the functions of the heart, brain, and gut, and can be lethal depending on the dosage.  Insects that breed and feed on the plant have developed physiological adaptations so they can ingest and absorb the toxins.  Many species retain these glycosides to use in their own defense, making them taste terrible to potential predators.

^{Monarch butterfly, Danaus plexippus, by Kenneth Dwain Harrelson May 2007; Monarch butterfly, Danaus plexippus, drinking nectar by Lynn M. Kimmerle 2006; Monarch butterfly, Danaus plexippus, caterpillar by Rhododendrites Sep 2024}

Monarch butterflies, Danaus plexippus, are just one species that lay their eggs on milkweeds.  Monarch caterpillars eat only milkweed foliage, although adults feed on nectar from a variety of plants.  Monarchs migrate from Mexico northward into the Unites States and Canada to lay eggs in the early spring.  These hatch into the current generation of adults seen at this time of year.  Today’s adults will lay another round of eggs which will metamorphose into adults late in the year and make the migration back to Mexico for the coming winter.

Other insect species that breed on milkweeds include a rare tiger moth called the unexpected cycnia moth, Cycnia collaris; the milkweed leaf beetle, Labidomera clivicollis; the red milkweed beetle, Tetraopes tetrophthalmus; the dogbane beetle, Chrysochus auratus; and the milkweed stem weevil, Rhyssomatus lineaticollis.  Additional beetle, moth, and butterfly species plus earwigs, plant bugs, and aphids regularly visit the plants for nectar.  Wilhelm and Rericha, authors of Flora of the Chicago Region, report over three dozen bee species in northern Illinois alone visit milkweed regularly.  By day, crab spiders hide in the flowers and ambush any insects that come close.  At night they stretch webs between plants to trap flying insects.  Ants and daddy longlegs scavenge the remains of trapped insects, providing a cleanup of the area.

^{Clockwise from top left: Dogbane beetle, Chrysochus auratus, by WanderingMogwai Aug 2017; Milkweed leaf beetle, Labidomera clivicollis, by Smidon33 Nov 2011; Milkweed stem weevil, Rhyssomatus lineaticollis, by Beatriz Moisset Jun 2006; Unexpected cycnia moth, Cycnia inopinatus, by Andy Reago & Chrissy McClarren Jul 2014; Red Milkweed Beetle, Tetraopes tetrophthalmus, by Rbreidbrown Jul 2020}

Milkweed is being studied for medicinal uses, although there is only limited use at this time.  During World War II, millions of seed pods were collected by children in Michigan.  The silken thread was separated from the seed and used to stuff life preservers found aboard warships.  Twenty-six ounces could keep a 150-pound person afloat for 48 hours.  Thousands of life preservers were made, but production tailed off after the war was over and new synthetics could be produced more cheaply.

Milkweed populations have been drastically reduced in the last few decades by urban development and agricultural crop spraying to reduce weed species.  Climate change, including timing of hot and cold periods and changing amounts of rainfall repeatedly disrupts the growing cycle and does not give the population of plants time to re-establish itself in any given area.  Given the number of species that use and depend on milkweed, there are no longer enough plants to support healthy insect populations.  Growing plants in your communities will help to maintain healthy and diverse habitats.  To learn more, visit the Jurica-Suchy Nature Museum or any nature area near you.