Vol. 18 Issue 1, Spring 2013
by Kerry Bzdyk
Common Milkweed
Common Milkweed (Asclepias syriaca) – a fascinating plant with an unfortunate name. Often it is found growing in neglected sunny spots at the edge of farm fields. Rarely is this species cultivated in home gardens, but perhaps it should be! Monarch butterflies rely on it as a host plant and will lay their eggs exclusively on this plant where the caterpillars will feed and grow. As they do, they ingest an alkaloid (cardiac glycoside) that is toxic to humans, birds and other vertebrates. The adult Monarch stores these compounds in its wings and exoskeleton, rendering them unappetizing fare for birds and other animal predators. Predatory insects and spiders are unaffected by this toxin, so the caterpillars and butterflies are still vulnerable to these hunters.
Common Milkweed is found in the northeastern United States south to Virginia. It is a tall upright perennial which spreads by underground rhizomes or by seed. The mature plant can be one meter tall with thick strong stems that are covered with fine hairs. The stems and leaves exude a milky sap when broken (hence the common name).The leathery leaves are oblong to oval and opposite, and the flowers appear in rounded clusters (in June in our area), and are greenish purple to greenish white. The later fruits or pods are teardrop shaped and large (8 to 13 cm long). The pods, when dry, will open along one seam to release hundreds of seeds attached to silky hairs. The seeds are dispersed by the wind.
The flowers, on close inspection, are very intricate structures that rely on butterflies, bees and wasps for pollination. A myriad of these nectar-loving flyers can be found visiting a milkweed flower. The large bodied bees, such as carpenter bees and bumblebees, are especially fond of milkweed.
While the Monarch may be the best known insect to exclusively rely on milkweed, it is certainly not alone. If you examine a milkweed plant in the late summer, you might find another caterpillar feeding there. The Milkweed Tussock Moth also lays its eggs on milkweed, but unlike the Monarch this caterpillar lives in large groups that feed together. Two different strategies for survival are at play here. The Monarch will lay solitary eggs on many plants over a large area in hopes that many will find optimum conditions for survival, while the tussock moth will lay about fifty eggs on one plant, where they devastate the plant feeding on anything in their path (including unlucky Monarch caterpillars). They use a “strength in numbers” strategy to ensure that some will grow to adults and reproduce.
In addition to caterpillars there are: several species of milkweed beetles which live only on milkweed and feed on the stems and roots; milkweed bugs which eat milkweed plant matter, nectar and seeds; milkweed weevils, which bore a hole in the stem of plants to lay eggs; milkweed aphids, ants and many predator invertebrates, such as spiders, mantids and assassin bugs.
Visually, the Common Milkweed has something to offer in every season with its tall stature, bright leathery leaves, beautiful flowers and interesting seedpods. Give this common weed a space to thrive in your landscape, and then take the time to stop and observe the variety of life that calls it “home.”
Resources:
Rea, B., Oberhauser, K, and Quinn, M.A. 2010. Milkweed Monarchs and More, Bas Relief LLC
oak.ppws.vt.edu/~flessner/weedguide/ascsy.htm
www.monarchwatch.org/milkweed/index.htm
plants.usda.gov/plantguide/pdf/cs_assy.pdf