Published 12:00 am Friday, August 10, 2007
By Steve Coggin
For the Salisbury Post
One of my favorite groups of flowers is the milkweeds because these plants are robust, with colorful flowers and insidious poisons.
This combination of beauty and toxicity gives rise to a group of interesting animals. Break off a milkweed leaf and you see milky sap, the latex. The poison is in the latex and its job is to protect the plant from herbivores.
You can find milkweeds growing in fields, swamps and along roadsides. Our area hosts nine species of milkweed from the common butterfly weed, Asclepias tuberosa, with its bright orange flowers, to the rare purple milkweed, A. purpurascens, found in only a few counties.
Milkweed flowers are unique in the plant kingdom. The petals are colored and are peeled back from the flower and point down. Rising up above the petals is a structure found unique to milkweeds, the corona. Pollen is produced within the brightly-colored corona, in a two lobed sac called a pollinium.
Insects, attracted to the nectar, will pick up a pollinium and transfer it to another flower. Milkweeds don’t just make a single flower but a cluster called umbels that make an irresistible display to the insects that pollinate the plant. A milkweed plant can have many umbels and each umbel may have dozens of flowers.
Only a few flowers on any milkweed plant are pollinated but those that are develop large green seed pods called follicles.
These pods grow during the summer and by fall they turn brown, split along their length and release seeds that float away on silky white parachutes. The seeds can blow long distances and remain dormant through the winter. The following spring the seeds germinate and grow into a new milkweed plant. Milkweeds are perennials, so once established they will grow back each spring.
Watching a milkweed on a summer afternoon you may see bees, wasps, flies, moths and butterflies all drinking nectar and pollinating the plant. The most famous butterfly to visit milkweed plants is the monarch butterfly. The monarch, Danaus plexippus, is the iconic butterfly. It is large, boldly colored in orange, black and white and depends on milkweed for all stages of its life cycle. Monarchs feed on milkweed nectar and pay back the plant by transferring pollen.
A monarch butterfly will usually lay a single egg on a milkweed plant. The egg hatches into a larva after a few days. Monarch larvae are conspicuous with white, yellow and blank bands around their bodies. All butterflies and moths have this larval stage, the caterpillar. Caterpillars are eating machines and monarchs eat milkweed.
The poisons made by milkweed plants are called cardiac glycosides. When eaten by an animal, these compounds can cause vomiting and increased heart rate. Most animals eating milkweed experience nausea and a racing heart and quickly learn not to eat that plant again. But monarch larvae are unaffected by the cardiac glycosides and store the toxins in their body. The caterpillars eat and grow and molt four times.
The last larval stage becomes a pupa that makes the metamorphosis from a crawling, wormlike creature into a butterfly. Even through this complete transformation the butterfly still carries the milkweed toxins in its wings and other body parts.
Just as herbivores learn not to eat milkweed, predators learn not to eat monarch butterflies. The bright colors of the butterfly are an advertisement saying, “Don’t eat me, I am poisonous.” This warning signal to predators is used by animals ranging from lady bugs to coral snakes.
Most predatory birds avoid monarch butterflies, but sometimes a nave bird will catch a monarch, taste the poison, and let it go. You can find monarch butterflies with a bird-beak sized piece missing from its wing.
A few other herbivorous insects will also eat milkweed. The large milkweed bug, Oncopeltus fasciatus, can be abundant on milkweed plants. They are true bugs in the Order Hemiptera and show a bold orange and black pattern on their backs. The Hemiptera do not experience the complete metamorphosis like butterflies. Their larval stages are miniature versions of the adult.
You may see all the stages of the milkweed bugs’ life cycle on a single plant. Like the monarch butterfly, milkweed bugs use the toxins from the plant for protection and advertise this with their coloration.
The swamp milkweed beetle, Labidomera clivicollis, also spends its whole life on milkweed, concentrating cardiac glycosides that protect them from predators. The milkweed beetle shares the coloration of black and orange with the monarch and milkweed bug. Milkweed beetles can appear in huge numbers and eat all the leaves from a milkweed plant. I have several milkweeds in my yard and when I pull off the beetles to try and save some leaves my hands always have a nasty smell from the concentrated toxin.
So get out this summer, and enjoy the milkweeds and look for the hardy animals that have learned to live with these toxic beauties.
Steve Coggin is a professor of biology at Catawba College.