A hot day in mid-July and I am standing transfixed at the edge of a common milkweed patch, watching a bewildering number of colorful butterflies nectaring on the cluster of drooping, dusty-rose flowers. There are great-spangled fritillaries and tiger swallowtails, silver-spotted skippers and clouded sulphurs, gray hairstreaks and common sootywings, an American painted lady and a painted lady, a black swallowtail and the spectacular day-flying hummingbird clearwing sphinx moth. And, of course, there are monarch butterflies.
Without the 11 species of milkweeds in eastern North America (and Pennsylvania), our monarch butterflies would die, since they depend on them as egg-laying and larval and adult-eating sites. Even while I watched, one monarch deftly laid her eggs on the undersides of milkweed leaves, one egg per leaf, in order to give each of her offspring its own leaf to munch.
No other butterfly species is as dependent on milkweed, but at least one butterfly, the viceroy, mimics its orange and black coloration since it is a red flag for many predators. That’s because monarch caterpillars eat the foliage of milkweed, which contains the plant’s powerful alkaloids, and stores it in their tissues. When those caterpillars metamorphose into butterflies, their bodies leave some bird predators with a very bitter taste in their mouths so they avoid monarchs and the look-alike viceroys.
Even though the alkaloids found in the milky latex of the leaves, stems and other milkweed plant parts don’t affect monarchs, it does bother the yellow, black, and white tussock moth caterpillars. Once I found hundreds of them eating milkweed leaves and later learned that feeding in such numbers is a device they use to overwhelm the plant’s defenses even though the alkaloid-laced latex slows down their growth.
Moths of many other species, it turns out, also sup nectar at night from the milkweed, probably drawn by the cloyingly sweet smell of the flowers. These moths, butterflies, and a bewildering number of bumblebees, honeybees, wasps, and flies are extremely fond of the sugary nectar secreted by the flowers. While nectaring, some of them perform pollinating services for the plants, most notably the bumblebees, which seem to account for between 75 and 90 percent of daytime pollinations, according to milkweed researcher Douglass H. Morse. Working in Maine, he found that although most plants were pollinated during the day, between 5 and 25 percent were pollinated by moths at night.
An earlier study in central Pennsylvania of common and swamp milkweeds and butterfly-weeds (also a milkweed), by Stuart W. Frost of Penn State University in the summers of 1957, 58, and 59, also emphasized the importance of bees as pollinators. But Frost found that honeybees were even more numerous than bumblebees. I know honeybees are incurable addicts of milkweed nectar and frequently are trapped by the flowers and die because I have found their desiccated bodies still attached by their legs to the blossoms. Once, feeling charitable, I extricated a trapped and furiously buzzing honeybee from a milkweed flower. She immediately returned to nectaring at the blossoms, as if the danger of getting caught again was outweighed by the delicious nectar.
Honeybees are not the only insects trapped by one of the five narrow, stigmatic grooves that are part of a milkweed flower. Many others are similarly trapped but the heavier bumblebees, for instance, are able to pull free. In doing so, they pull up a pair of pollinia (many grains of pollen held together by a waxy coating), from the twin sacs on either side of the stigma and unwittingly carry it to the next milkweed flower. There the bumblebees go through the same routine and this time rub off the pollinia into the enlarged lower end of the stigmatic slit to finish the pollinating cycle. This mechanism is a great deal more complicated than I have described and it is utterly unique in the botanical world. Eager botanists have not only figured out the whole mechanism, but they have studying why and how it developed.
Other botanists have devoted their time to identifying the insects that nectar on common milkweed and whether or not they help to pollinate the flowers. One study found that 15 long-tongued bees, 6 short-tongued bees, 11 wasp species, 14 flies, 4 plant bugs, 1 beetle, 2 moths, one skipper (hoary edge), and 5 butterflies–monarch, mourning cloak, question mark, great-spangled fritillary and red admiral–had pollinia attached to them when nectaring. Another 4 long-tongued bees, 1 short- tongued bee, 2 wasps, 5 flies, 5 skippers (silver-spotted, common sootywing, Peck’s, clouded, and tawny-edged) and 5 butterflies (viceroy, regal fritillary, bronze copper, banded hairstreak, and pipevine swallowtail) did not have pollinia. Still another group of insects–4 flies, 3 moths, and honeybees–were found dead or entrapped by the flowers. This study, unlike Frost’s, recorded no honeybees with attached pollinia.
I have also scrutinized list after list by scientists that are fascinated by all the insects that live, eat, nectar and/or die on common milkweed and each differs in species’ numbers and kinds. So even though scientists have been studying the common milkweed and its visitors for as long as 113 years–beginning with Illinois naturalist Charles Robertson’s pioneering, 25-year study–there is still much more to be learned about these intriguing wildflowers and their inhabitants.
The common milkweed, Asclepius syriaca, is a native of eastern and central North American and one of 107 North American species of Asclepius, a worldwide genus of plants. Through the centuries numerous medicinal values were attributed to milkweeds, hence its genus name which is the Greek god of medicine. And it turns out that these ancients were right. Cardenolides in the latex of stems, phloem and other plant parts are powerful heart poisons that can be used to treat congestive heart failure and atrial fibrillation.
A lover of open ground, the common milkweed thrives in fields, pastures, along roadsides, and in transitory forest clearings. Each patch is actually a clone because it is created by the underground growth of rhizomes. Our common milkweed patches seem to move around our First Field (actually an overgrown meadow). Crowded out or overshadowed in one area, they pop up in another.
Butterflies and moths are not the only colorful insects I find on common milkweed. Black-spotted adult red milkweed beetles (Tetraopes tetraophthalmus) feed and mate on the plant’s leaves and flowers, but they lay their eggs on nearby grass stems. When the larvae hatch, they drop to the ground and feed on milkweed roots. A recent study found that such feeding did not harm the roots.
Orange and black small (Lygaeus kalmii) and large (Oncopeltus fasciatus) milkweed bugs also mate on milkweed plants. Later in the season, after the pollinated flowers produce large green seedpods filled with between 100 and 200 seeds, the milkweed bugs eat both pods and seeds. Last October 5, I found several large clusters of large milkweed bugs eating pods in our biggest patch. As many as 25 were clustered together along with smaller, plumper orange bugs marked with black. Occasionally, one was all orange with white wings, a red head and legs. Other bugs had black heads and legs. The latter two were either immature large milkweed bugs or nymphs. It takes them 40 days to mature so they had probably hatched in late August.
Previously their parents had mated on the milkweed. Then the female had laid clusters of approximately 15 eggs on the undersides of milkweed leaves. The eggs hatched in three to six days and the nymphs had molted five times before reaching adulthood.
Large milkweed bugs migrate south in the autumn because they spend the winter as adults and are killed by freezing temperatures. Those same adults migrate north in the spring and generally produce two to three broods a summer.
Then there are the predators that feed on some of the inhabitants of a milkweed patch. Crab spiders change their colors to pink and white to blend in with the flowers. There they dine on the toxic aphids that have been dining on the plants and then these spiders on drugs weave asymmetrical webs. But they also eat other insects that come for the milkweed’s nectar. Some female crab spiders are so successful in their ambush hunting that they balloon to the size of a queen bumblebee in two weeks. Most of their added weight goes into producing a clutch of eggs. Each female crab spider then turns under the end of a milkweed leaf, lays her sac-enclosed egg clutch on the inner surface of the leaf, ties up the leaf with silk, and guards her eggs until they hatch. But often those egg sacs are parasitized by flies and small wasps.
Daddy longlegs (Phalangium opilio) or harvestman are also common inhabitants of milkweed plants. After a crab spider sucks out the insides of its prey, a harvestman is liable to scavenge it, along with the carcasses of other dead insects found in the leaf axils of milkweed stems. Or it may do its own hunting on milkweed flowers, where it also gathers nectar, and capture small prey including young crab spiders.
Whether you are a botanist, an entomologist, an ecologist or merely a curious naturalist, life in a common milkweed patch is never dull. From the time their tender, edible shoots appear in spring until late autumn days when their dried pods open to release brown seeds attached to silken streamers, common milkweeds and their inhabitants are anything but “common” in their life history.