By Will Kuhn, Director of Science and Research with Discover Life in America
Monarch butterflies have begun their yearly northward migration and are due to arrive in the Smokies any day now.
This time of year, you may start to see Danaus plexippus flying around the region. These attractive orange and black butterflies have made the long journey from their overwintering sites in central Mexico and are looking for mates and milkweed plants.
“Each of a male monarch’s hindwings has specialized groups of scales that together look like a small black spot on its top surface,” said Todd Witcher, executive director of Discover Life in America (DLiA), a park partner organization. “These spots are helpful for distinguishing the sexes.”
After mating, the female lays a single egg on the underside of a young milkweed leaf. She may travel far and wide to find enough satisfactory plants on which to lay her 300 to 400 eggs. After that, her job is done, and she will die.
|During its eating frenzy, the monarch caterpillar will grow and shed its skin five times over about two weeks before it pupates into a green chrysalis accented with golden spots. Photo courtesy of Warren Lynn|
The tiny monarch caterpillar emerges from its egg about 10 days later. It will devour its own eggshell and then begin to eat the milkweed plant that its mother carefully selected, chewing through the thick leaves with its powerful mandibles. Commencing an eating frenzy, the caterpillar will grow and shed its skin five times over about two weeks before it pupates into a green chrysalis accented with golden spots. It emerges as a new monarch butterfly after 7 to 10 days.
In North America, east of the Rocky Mountains, the northward migration in monarchs takes place in stages. The first wave flies from Mexico to the southern US states in the spring. There, the butterflies reproduce on milkweed and die. When their progeny (the second wave) emerges in the summer, they fly further north into the northeastern and north-central US and eastern Canada to repeat the process for another generation or so. Amazingly, their descendants will travel thousands of miles to Mexico in one single trip in the fall.
Monarch caterpillars will only feed on milkweed and require it to reach adulthood. The plant’s name comes from its thick white sap, which oozes out when a leaf is pierced, or a stem is broken. The sap is distasteful to most herbivores, including insects, but monarchs love it.
This “milk” contains a class of defensive chemicals called cardiac glycosides, which provide additional protection to the plant from would-be herbivores. These chemicals become concentrated in the caterpillars as they feed and grow, so that monarch larvae and subsequent adults become distasteful to birds and other predators. The contrasting black, white, and yellow stripes on the caterpillars and orange and black pattern on the adults warn predators to not eat them.
“Eight different milkweed species have been documented so far in the Smokies and we hope to document more as part of the All Taxa Biodiversity Inventory,” said Witcher, referring to a project managed by DLiA to catalog every species living in Great Smoky Mountains National Park. “They take on a variety of shapes, sizes, and flower colors.”
According to the US Forest Service, which curates a list of milkweed species that monarchs will use in the continental U.S., at least seven of the park’s milkweed species can serve as hostplants for monarchs.
|The monarch butterfly on its foodplant, milkweed, the sap of which contains chemicals that become concentrated in the caterpillars as they feed and grow, so that monarch larvae and subsequent adults are distasteful to birds and other predators. Photo courtesy of Warren Lynn|
“In the spring, monarchs can be seen in Cades Cove,” said Becky Nichols, entomologist at Great Smoky Mountains National Park, “as well as in Cataloochee Valley and other sunny meadows in the park where milkweed is present.”
The park has mapped the milkweed in Cades Cove and works to schedule management activities, such as mowing and prescribed burning, to accommodate the life cycle of the many species that live there, including monarchs.
“Timing is critical so that plants are ready and available for the monarchs,” said Nichols.
Despite efforts to protect them, monarch populations have been declining in recent years. The Xerces Society, a Portland-based nonprofit devoted to insect conservation, has reported an 80% decline in monarch numbers east of the Rocky Mountains. The organization attributes this dramatic drop in monarchs to three main causes: habitat loss from development and agriculture, increased usage of pesticides and herbicides, and climate change. Researchers have documented a similar trend in many other pollinating insects, as well.
How can you help? One of the best ways is to plant native milkweed in your yard or other greenspaces.
“Ask your local nursery for native milkweed that has not been treated with insecticides,” said Witcher. “Most places sell non-native milkweed species, but these can carry diseases that can harm monarchs.”
Milkweed is not just good for monarchs, it also provides nectar for a wide variety of other insect pollinators, including bees, other butterflies, and even some fireflies, and serves as the hostplant for a few other insect species that can tolerate its defensive chemicals.
“Milkweed is a wonderful addition to any native garden in Southern Appalachia,” said Witcher, “it looks nice, it smells nice, and it’s a great source of nourishment for lots of species.”