“Milkweed” (the name refers to the milky sap that is produced when the stems are broken) is the common name for over thirty species of perennials, all members of the genus Asclepias, that are native to North America. Thirteen are native to northern New Jersey. Only one species, common milkweed, with huge clusters of mauve flowers, still seems to grow wild in our area, although it’s now pretty hard to find. Milkweed is one of those plants, like elderberries and serviceberries and asters, that should be everywhere, because it’s that crucial a part of our ecosystem. Sadly, many of these crucial plants have been crowded out by development and alien invasive plants, so the birds and butterflies that once depended on them have disappeared as well.
I grow two milkweed species, swamp milkweed, A. incarnata, and butterfly weed, A. tuberosa (shown above). All milkweeds have the unique flower structure you see in the photo, with clusters of small flowers whose parts are arranged in tiers of five; but very few plants have flowers with this wonderful intense natural orange color. Butterfly weed grows two to three feet tall and likes dry soil. It has brilliant orange flowers and blooms nonstop from the end of May throughout the summer. Swamp or red milkweed is about four feet tall, and as its name implies, it likes wet soil (although it is an adaptable plant that does fine in my dry, sandy soil). It has huge, deep-pink flower clusters and blooms in June. All milkweeds are magnets for butterflies and other pollinators and incredibly easy to care for—plant them in a sunny place and stand back. All produce seedpods that are a source of fascination to children. I can’t imagine why everyone doesn’t grow milkweed.
Most people know there’s a connection between milkweed and monarch butterflies, but there are a lot of misconceptions out there. The real story is quite complex, as is true of most things in nature. Monarchs, those gorgeous, large, black-and-orange butterflies that flutter lazily over the summer garden, are long-distance migrators. However, it’s not individual butterflies that migrate from Mexico to Canada and back again each season; it’s three or four different generations of individuals. After wintering in Mexico and south Texas (where their habitat is rapidly shrinking), the butterflies begin migrating north, some going up the west coast of the United States, some taking a Great Plains route, and some traveling along the eastern seaboard. Along the way, the first generation will mate, lay eggs, and die. The eggs that live become the second generation, and they continue on northwards.
I often see monarchs laying eggs in July in my garden, and sources differ on whether this is the second generation or the third of the year. Those eggs hatch into caterpillars with distinctive black and yellow stripes, and this is where the milkweed comes in. Monarch caterpillars eat milkweed and only milkweed. A female butterfly must find milkweed plants on which to lay her eggs. When she does, she perches on a leaf, curves her body under, and deposits her tiny eggs, one by one, on the undersides of the leaves. After laying eggs, she dies. If she doesn’t find milkweed—because someone mowed it down or used herbicides to kill it—she leaves behind no progeny to continue the journey north.
Suppose she manages to find a stand of milkweed—one that hasn’t been mowed down or sprayed with herbicides or pesticides (most pesticides kill all insects, including butterflies, ladybugs, lacewings, fireflies, and honeybees). She lays her eggs, and soon afterwards the tiny striped caterpillars hatch. They grow larger over several weeks, steadily eating the milkweed leaves. Then they pupate, or form pale green cocoons, or chrysalises; about two weeks later, adult butterflies emerge. The cycle continues as these adults fly north. In late summer, a final generation makes the long flight back to the southern wintering grounds. I often see monarchs nectaring on my New England asters in the fall. How do they manage to fly thousands of miles and find that place where their ancestors wintered in the past? It’s quite amazing.
But the milkweed is amazing as well. Notice that it’s the monarch caterpillars that exclusively depend on milkweed. Many insects sip milkweed nectar, and adult monarchs sip nectar from many different flowers—asters and liatris are favorites—but the caterpillars can eat only milkweed leaves. This is true of many butterfly species—the caterpillars can feed only on a single genus or species of plants, often, unsurprisingly, plants that are native, that used to be common, and that we consider to be weeds. Fritillaries need violets, skippers need grasses, red admirals need nettles. The list goes on and on.
The reason for this specialization is simple. Plants are the basis of the food chain, so animals eat them. To protect themselves, plants have evolved a variety of mechanisms, such as thorns, tough stems, hairy leaves, and chemical defenses. In response, specific animals have evolved the ability to withstand specific types of plant-produced poisons. That allows them to specialize in that one food source that other species ignore. Monarch caterpillars have evolved the ability to tolerate the deadly cardiac glycosides that milkweed plants produce. Female monarchs will lay their eggs only on milkweed plants. No milkweed, no monarchs.
Other animals have evolved the same strategy as well: small and large milkweed bugs, milkweed beetles, and milkweed aphids all feed exclusively on different parts of milkweed plants. All have bright orange or red markings, just like adult monarchs—small milkweed bugs are particularly handsome, with vivid orange X’s crisscrossing their black backs. Why the distinctive markings? All these creatures eat those deadly cardiac glycosides, and as a result, they themselves are rendered poisonous to other creatures that might prey on them. And there’s no point in developing a chemical defense unless your predators can recognize it, is there? A young bird that eats a monarch will taste something awful and get sick; if it survives, it will remember those orange markings and never eat another monarch.
The next time you see a monarch, observe its slow, smooth flight pattern. Other butterflies typically fly quickly and erratically to foil predators. Monarchs can take their time, because they have few predators. And guess what? Other butterflies have evolved to mimic the monarchs: they have look just monarchs to fool their predators.
Here’s a picture of swamp milkweed, which is just coming into bloom in my garden while butterfly weed is in full bloom:
Plan to get yourself some monarch magnets next summer.