by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
The summer of 2012 so far has been hot and dry and that has been a real boon for many of our sun loving plants like the magnificent milkweeds dotting the landscape. Perhaps no other plant provides such a wide range of ecological functions for so many different creatures. Milkweeds belong to the genus Asclepias L. (1753), which describes a group of herbaceous, perennial, dicotyledonous (two embryonic seed leaves or cotyledons) plants that includes over 140 known species. Milkweed is named for its milky juice, which contains alkaloids, latex, and several other complex compounds including cardenolides (a bitter tasting steroid). Carl Linnaeus named the genus after Asclepius, the Greek god of healing, because of the many folk-medicinal uses for the milkweed plants. It previously belonged to the family Asclepiadaceae, but this is now classified as the subfamily Asclepiadoideae of the dogbane family Apocynaceae because it also shares the characteristic of milky (sometimes toxic) sap.
According to “The Ferns and Flowering Plants of Nantucket” by Eugene P. Bicknell (published in 1915; “Bulletin of the Torrey Botanical Club”, Volume 42; pp 27-54), the following species of milkweed were found on Nantucket between 1896 and 1915 and most if not all, can be found today): Asclepias tuberose L.(butterfly weed, somewhat rare on Nantucket but can be found in the sandplain grasslands, common on Martha’s Vineyard and abundant on Chappaquiddick), Asclepias purpurascens L. (purple milkweed; common on Nantucket), Asclepias incarnata var. pulchra Ehrh.(swamp milkweed, very common in low lying grounds), Asclepias amplexicaulis J. E. Smith (not abundant, but found in drier plains area), Asclepias exaltata (L.) Muhl. (rare, one plant at Almanac Pond in 1907 and a cluster in Squam near Eatfire Spring in 1912), Asclepias syriaca L. (very common, hence the name, common mikweed).
The Latin roots for Swamp Milkweed (Asclepias incarnata var. pulchra Ehrh.) translate to mean flesh colored (incarnate- in the flesh) and “beautiful” or “pulchra“. The flesh colored descriptor refers to their white fleshy looking roots. Swamp milkweed is an upright, 100- to 150-centimeter (39- to 59-inches) tall plant. Typically, its stems are branched and the clump forming plants emerge in late spring after most other plants have begun growth for the year. The oppositely arranged leaves are 7 to 15 centimeters (2.75 to 6 inches) long and are narrow and lance-shaped, with the ends tapering to a sharp point. The plants bloom in early to mid-summer, producing small, fragrant, pink to mauve (sometimes white) colored flowers in rounded umbels. Monarchs also love these milkweeds.
The bright orange flowers of Asclepias tuberose (butterfly weed) make this a standout beauty in the sandplain grasslands environment. According to the U.S. Department of Agriculture’s Plant Database this plant is listed as “threatened” in Vermont, “endangered “in New Hampshire, “possibly extirpated” in Maine, and “Special Concern” in Rhode Island http://plants.usda.gov/java/profile?symbol=ASTU; accessed July 8th, 2012. Please do not dig these up when you find them to transplant to your garden!
The common milkweed is thick-stemmed and upright. It grows to be 3-5 feet (sometimes even 6 feet!) tall. Its leaves are elliptical, and opposite; they are velvety on their upper surface, and downy underneath. They are 4-9 inches long and quite wide. They sometimes have red veins. The pinkish-purple flower buds look like loose broccoli; the flower itself is large and made up of individual florets gathered in an umbrella shaped globe that droops from the stem. The stem is hairy. This plant is found in fields, gardens, and along roads. There are several of these at the Nantucket Field Station near the stream and small bridge as you drive down the entrance road.
Common milkweed (and its relatives above) is able to reproduce both sexually and asexually. Asexual reproduction of common milkweed is rhizomatous spread. Common milkweed produces long trailers beneath the soil, which are referred to as rhizomes. The rhizomes travel away from the parent plant and produce small shoots, which break through the soil to form a new plant which is an independent duplicate of the parent. Common milkweed grows in dense, outward spreading clumps due to rhizomatous reproduction which often crowds out other plants.
Sexual reproduction involves insect pollination, followed by seed production and dispersal. Flowers are formed in dense clusters in shades of white to purple, which die back after insect pollination. Pollination in this genus is different from the pollination of most other plants. Pollen is grouped into complex structures called pollinia (or “pollen sacs”), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower visiting insects such as bees, wasps and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, pulling a pair of pollen sacs free when the pollinator flies off. Pollination is effected by the reverse procedure in which one of the pollinia becomes trapped within the anther slit. The pollen is like Velcro, going along for the ride whether the pollinator likes it or not. Fruits are green pods which turn brown before bursting open to let out fluffy seeds.
Asclepias species produce their seeds in follicles. The seeds, which are arranged in overlapping rows, have white silky filament-like hairs known as pappus, silk, or floss. The follicles ripen and split open and the seeds, each carried by several dried pappus, are blown by the wind. I am sure you’ll see these each fall because the little silken parachutes are everywhere. (http://en.wikipedia.org/wiki/Asclepias accessed July 8th, 2012).
Milkweeds are an important nectar source for bees and other nectar-seeking insects, and a larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects (including numerous beetles, moths, and true bugs) specialized to feed on the plants despite their chemical defenses. Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves, cardenolide (bitter tasting) toxins, and latex fluids. Monarch larvae and other caterpillars have developed feeding techniques to avoid the latex and cardenolides in the sap. Data from a DNA study indicate more recently evolved milkweed species use less of these preventative strategies, but grow faster than older species, potentially regrowing faster than caterpillars can consume them (Ramanujan, 2008).
I just love the description of the life cycle of a monarch found at http://monarchchaser.wordpress.com/about-monarchs/ accessed July 8th, 2012. There are extremely detailed pictures and drawings on the site explaining the various life stages of the monarch butterfly.
Two years ago I wrote an article called “The End of Summer Splendor” (http://www.yesterdaysisland.com/2010/science/17.php; accessed July 8th, 2012) about the monarch butterfly migration and their dependence of milkweed plants. The milkweeds are the only host plant for the larva of the monarch; the continent-wide decline in monarch numbers is due in large measure to a loss of milkweed habitat. Milkweeds, when broken, lets out a milky sap which has bitter tasting chemical toxin components called cardiac glycosides. These are used therapeutic ally to treat heart (cardiac) failure. These chemicals restrict the amount of sodium pumped out of the heart and can increase heart contractions. Some animals can eat the glycosides and not be harmed. When the Monarch butterfly’s caterpillar munches the leaves of milkweed, the glycosides go into its body, making the caterpillar poisonous to predators. Even after the caterpillar has changed into an adult butterfly, it keeps the glycosides in its body. The beautiful colors of both the larval and adult Monarch are a warning not to eat them; the term for that is aposematism or warning coloration. Certain predators have a high tolerance level to the chemical defenses produces by monarch butterflies. Species such as the black-beaked orioles and the black headed grosbeaks are common predators that can tolerate an increased level of cardenolides. Attack by these predators accounts for over 60% of monarch butterflymortalities.http://student.biology.arizona.edu/honors2002/group10/defense_.htm accessed July 9th 2012. There are a few monarch butterfly imitators like the viceroy butterfly (Limenitis archippus) who employ a defense mechanism called Batesian mimicry, in which a palatable (yummy) species attempts to mimic an unpalatable species in an attempt to increase its survival rate.
Milkweed flowers bloom from June to August, and are visited by many species of moths, butterflies, bees, and other insects. The flower nectar and pollen does not have glycosides in it, so these animals do not become poisonous. Milkweed is a shelter and hiding place for other species as well. Yellow Jackets eat bees and flies which get trapped in the flowers, and crab spiders ambush visiting insects. We already are seeing many common milkweed residents including monarch caterpillars and aphids on plants around the island. Common Milkweed is an important plant because so many species of insects depend on it. Monarch butterflies, milkweed bugs, and milkweed leaf Beetles only eat milkweed, and could not survive without it. Many other species of insects use milkweed as their primary food source, or as a major food source http://www.fcps.edu/islandcreekes/ecology/common_milkweed.htm; accessed July 8th, 2012.
Although the sap is poisonous, the leaves and stems and roots and even flowers can be eaten when prepared properly. Indigenous North American people have been eating milkweeds for many hundreds, if not thousands of years. The Wild Foods site (http://www.wildfoods.info/wildfoods/milkweed.html accessed July 8, 2012) very clearly details how to prepare the various parts of the milkweed plant, which milkweed species are tastier, and when to pick them.
The ecology of common milkweed and its relatives is intricately woven in into the life histories of a huge variety of insects, birds, spiders, and harvestmen in an excellent example of a microhabitat (http://www.fcps.edu/islandcreekes/ecology/common_milkweed.htm; accessed July 8th, 2012). Milkweed is a shelter and hiding place for many species as well. Yellow Jackets eat bees and flies which get trapped in the flowers, and crab spiders ambush visiting insects and lay their eggs underneath the leaves. These eggs are then sometimes parasitized by small flies and wasps. When the beautiful purple flowers start to blossom in late spring and early summer, a menagerie of bees, moths, caterpillars, aphids, spiders, wasps and other critters are attracted to the leaves and flowers. Some ecologists believe that up to two hundred or more species of insects, spiders, birds, and other plants depend at least in part on the milkweed plant. They seem to me to be like a perfect community. Milkweed is beneficial to nearby plants, repelling some pests, especially wireworms (click beetle larvae).
The milkweed filaments from the follicles are hollow and coated with wax, and have good insulation qualities. Tests have shown them to be superior to down feathers for insulation. During World War II, over 5,000 tons of milkweed floss was collected in the United States as a substitute for kapok (Java cotton or ceiba). As of 2007, milkweed is grown commercially as a hypoallergenic filling for pillows. (Evanglista 2007). Today, you can buy pillows, jackets, and comforters stuffed with this material, which is wonderfully soft and has a higher insulative value than goose down, from a company called Ogallala Down, in Ogallala, Nebraska (they also sell milkweed balm for arthritis). In the past, the high dextrose content of the nectar led to milkweed’s use as a source of sweetener for Native Americans and voyagers. Milkweed latex contains about 1 to 2% latex, and was attempted as a source of natural rubber by both Germany and the United States during World War II. No record has been found of large-scale success.
Milkweed is a common folk remedy used for the clotting of small wounds and the removal of warts. The built in toxins may have been utilized by natives of South America and Africa to make arrows poisoned with these glycosides to fight and hunt more effectively. Many colleges today feature excellent ethnobotany classes in the curricula where one can learn about the other medicinal uses of milkweed (http://www.woodrow.org/teachers/bi/2000/Ethnobotany/milkweed.html accessed July 9th, 2012). They include its use as a laxative, as a diuretic, for contraception (not sure I would place all my bets on that one), to kill parasites (that is pretty believable), as a cancer cure, for chewing gum, and to treat asthma and bronchitis as well as induce sweating. Milkweed also causes mild dermatitis in some who come in contact with it which is why it can be effective to remove warts. Milkweed sap is applied directly to the wart several times daily until the wart falls off. Milkweed sap is also externally used as a natural remedy for poison ivy. I could have used that last week! (http://en.wikipedia.org/wiki/Asclepias; accessed July 8th, 2012.) Who would have thought that one kind of weedy looking plant could serve so many functions? I think it would even give bamboo a run for its money in the useful plant Olympics, and for the many creatures it shelters and supports, there are few of its caliber. So get outside and find some of these before they all go to seed.
Additional Resources:
Evangelista, R.L. (2007). “Milkweed seed wing removal to improve oil extraction”. Industrial Crops and Products 25 (2): 210–217. DOI:10.1016/j.indcrop.2006.10.002.
Ramanujan, Krishna (Winter 2008). “Discoveries: Milkweed evolves to shrug off predation”. Northern Woodlands (Center for Northern Woodlands Education) 15 (4): 56.
