My favorite time of the year has just opened its arms to launch a festival of colors and aerobatic displays. Birds are beginning to migrate, and so are the monarch butterflies. Most birds make their migratory trips south and back north all in one generation, but the fragile and beautiful monarch butterflies (Danaus plexippus) have a different scheme. Kind of like science fiction space travel taking several generations to make the trip.
Each year, four or more generations of monarchs are hatched, and each generation has a specific role to play in completing a multi-staged migration. The job of the first generation, the children of last year’s migrants, hatched in northern Mexico and the Gulf states, is to carry the baton partway back to Owl Acres, laying eggs along the way. The next generation, the second of the year, will finish the race back to Owl Acres. The third generation has a short, productive life on our milkweed, laying eggs that will hatch and turn into the fourth generation–the migrants. The fourth generation is the one getting ready now to leave for Mexico. They put reproduction on hold so they can fly south and spend the winter in the mountains of Mexico. In the spring, they will mate and produce the first generation of next year, laying their eggs in northern Mexico and the Gulf States. And the cycle repeats. So how does any given butterfly know which generation it belongs to and what its specific job is? And how do they know where to go?
Monarchs from the Eastern and Midwestern U.S. and Canada all fly to a place in the mountains of Mexico where millions congregate to blanket the tree-covered slopes in their brilliant colors. These congregation sites are all in a small area of high-altitude ecosystems containing oyamel fir trees. In 1983 the World Conservation Union designated these areas as “threatened Phenomenon” because of habitat degradation in Mexico and along their migration routes.
Monarchs will only lay their eggs on plants in the milkweed family. After mating, the female finds a milkweed plant and lays an egg on the underside of the leaf, gluing it there so it won’t fall off. She’ll lay 300 to 500 single eggs over the next few weeks. A few days after she lays an egg, it hatches. A tiny larva emerges and starts eating the fine hairs on the underside of the leaf. Over the next nine to fourteen days, the larva, or caterpillar, will go through five distinct stages or instars. Each time it sheds its outer skin and emerges bigger and more well-defined. The caterpillar body comprises a head, a thorax and an abdomen and is striped with black, white and yellow bands. The head sports six not very effective eyes, a pair of mandibles, and a pair of sensing organs that help it find its food. It also has a pair of short antennae that it uses to sense its environment. Six pairs of legs on the thorax give it mobility, and several appendages on the abdomen help it cling to various surfaces. At the end of its fifth instar, the caterpillar leaves its milkweed, finds a secure spot and spins a silk mat. It hangs from the mat and sheds its skin for the last time, exposing its bright green chrysalis. In eight to 15 days, the butterfly emerges fully formed. It pumps up its wings, dry’s off, and flies away.
The adult monarch is very recognizable. Its wings are bright orange with white-spotted black borders and prominent black veining. The head and body are black with white spots. The orange color serves as a warning to birds and other predators that the butterfly is poisonous and eating it can cause severe vomiting. The poison comes from the milkweed plant. During its time on the milkweed plant, the caterpillar ingests the sap containing poisonous glycosides. The poison doesn’t bother the caterpillar, but it remains in its tissues even after it becomes an adult living on nectar. The monarch’s wingspan can reach over four inches, supporting it as a slow, but powerful flier.
Another species, the milkweed tussock moth (Euchaetes egle) lays its eggs on the milkweed plant as well. The moth caterpillars also ingest the poisonous sap and wear bright orange, white and black tufts of hairs that make them look fuzzy but also warn predators to leave them alone. The adult moths are sometimes called tiger moths because they also have yellow or orange bodies with black spots along the sides and gray wings. The tussock moth also lays eggs on the underside of the milkweed leaves, but instead of laying one at a time, she deposits a clutch of dozens of eggs. When they hatch, they feed in a herd on the milkweed leaves. They all molt at the same time as well and then come back to devour more of the milkweed leaves. The moths don’t migrate. The adults don’t survive the winter. the moth caterpillars spin little gray cocoons and spend the winter inside, emerging as adults in the spring.
A caterpillar with large tufts of white, black and orange hairs feeds on a milkweed leaf. Milkweed tussock moth larva looks like a toothbrush. Photo by Katja Schulz
Another butterfly, the viceroy (Limenitis archippus), gets some of the benefit of the milkweed poison. But rather than ingesting the poison themselves, the viceroys only pretend to be poisonous by dressing themselves in monarch clothing.
Viceroy butterfly at rest. Similar pose as in feature photo, showing the animal’s remarkable visual similarity to the monarch. Photo by Judy Gallagher
Yet another insect, the large milkweed bug (Oncopeltus fasciatus), lives on the milkweed and sports orange and black markings to warn predators that they’re poisonous, too. The milkweed bug drinks the sap by piercing leaves, stems and seeds. Like the milkweed itself, the butterflies, moths and bugs that live on it are all native to North America and range from Canada to Mexico and from the eastern seaboard to the Rockies for the eastern versions, and west of the Rockies for the western versions.
Pill-shaped insect marked with transverse bands of orange and black sits on a milkweed leaf. Large milkweed bug. Photo by Katja Schulz
Loss of habitat and declining populations of monarchs in North America have raised concerns about the long-term health of the monarch population. Perhaps we should plant more milkweed on Owl Acres to help them along.
Feature photo by Derek Ramsey Alt text: Monarch butterfly rests, wings spread, on a milkweed. Colors are bold orange with prominent black veining, and white spots. This photo was chosen for its similarity to the photo of the Viceroy elsewhere in the post. There are differences in markings, but they’re easy to miss on casual observation.
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