One of the true delights of the steamy summer season in Maryland is the return of the monarch butterfly. This past weekend I had the pleasure of joining a jolly band of youngsters and their parents as we stalked the magical monarch in the meadows of the Howard Conservancy. We were rewarded with sightings of several monarchs nectaring on fragrant wildflowers and delighted with the discovery of almost a dozen ravenous monarch caterpillars devouring tasty leaves of milkweeds.
Serenaded by a cicada, a monarch tanks-up on sweet nectar of the Joe-Pye weed before seeking a milkweed plant on which to lay an egg.
Last autumn the great, great, grandparents of these beauties survived a dangerous and arduous migration from the eastern United States to their overwintering sites in central Mexico. During the long winter they bested predators and weather in their highland forest retreats. This spring the vagabonds flew several hundred miles from Mexico to the southern United States before finding suitable milkweed plants to serve as food for their young.
The female monarch lays her eggs on the undersurface of a leaf. After several days the egg hatches and the tiny monarch caterpillar begins to consume the nutritious leaves. From the milky sap of its host the larva obtains potent defensive chemicals known as cardiac glycosides. These compounds are stored by the caterpillar and passed along to the adult butterfly. Birds are important predators of many kinds of butterflies, including monarchs. Would-be predators such as blue jays foolish enough to have a go at the monarch retch and spew when they encounter the cardiac glycosides found in the butterfly. The conspicuous orange and black coloration of the monarch serves as a warning to the enlightened bird that eating this attractive morsel will have nasty gastric consequences.
The caterpillar becomes a leaf-eating machine and gains more than 2,000 times its birth weight during its development. Just before the transformation to the pupal stage the caterpillar suspends itself with silk from a leaf or stem and assumes the shape of a “J”. The last larval skin splits open to reveal the dazzling pupa, also called the chrysalis. In about a week the monarch butterfly emerges from the chrysalis and begins the tasks of finding nectar to eat and a worthy mate to carry on the royal lineage.
Monarchs continue their annual march northward to feeding grounds in the northern United States and southern Canada. Triggered by falling temperatures and shorter days that signal the approach of winter, monarchs undertake one of the most heroic adventures of any living creature. Monarchs from the central and eastern US make a beeline for the mountainous forests of Michoacán, Mexico, a trip that may exceed 3,000 miles. Monarchs in western North America make a similar journey from southern Canada and the northern US, but instead of heading for Mexico, they veer south and west to the coast of California. Here among the boughs of towering eucalyptus and Monterey pine, they find refuge from freezing temperatures, a habitat that offers high humidity with morning fog, and shelter from the wind.
On chilly days and at night, monarchs huddle in large clusters and conserve body heat. On warm days when temperatures exceed 55 degrees Fahrenheit, monarchs fly in search of nectar to replenish energy reserves. In addition to providing a convenient roost, some eucalyptus trees bloom during the monarch’s annual visit to California and provide food. Many predators, including several species of birds, take advantage of the bounty of so many insects in one place. At overwintering sites in Mexico, black-backed orioles and black-headed grosbeaks kill millions of monarchs each year. Clever birds learn to avoid more poisonous parts of the monarch’s anatomy, like the wings, and dine on less offensive parts like the thorax.
In late winter with moderating temperatures and increasing day length, monarchs become reproductively active. They mate, leave the refuge, and begin a journey that takes several generations and many months to complete as they follow the milkweed to the northern limit of its range. But this wondrous annual migration of monarchs is in peril. The overwintering habitat of monarchs in the mountains of Mexico has been declining for years and the winter of 2013 – 2014 witnessed the lowest number of overwintering butterflies since records have been kept. Monarch guru Lincoln Brower believes several factors conspire to reduce populations of monarchs. Illegal logging of trees in the mountains of Mexico has reduced critical overwintering habitat for monarchs. Without this refuge the monarchs cannot survive winter.
Here in the US, urban sprawl and the use of herbicides in agricultural production can greatly reduce populations of the milkweed plants vital for the survival of monarch caterpillars. Extreme weather events throughout the vast range of this vagabond may also reduce the monarch’s numbers. Fortunately, the monarch is widely distributed and the prospects for global extinction are slim. However, the magical migration of monarchs in the eastern United States is imperiled and the disappearance of this unique biological mystery would be a loss to us all. In the waning weeks of summer, go to the meadow and enjoy these beauties and next spring be sure to include milkweed in the plans for your perennial gardens.
By the time its development is complete, this very hungry caterpillar will have gained more than 2,000 times its birth weight.
Bug of the Week thanks Dr. Paula Shrewsbury and the wonderful staff of the Howard Conservancy and the participants in last weekend’s meadow safari for providing the inspiration for this episode. Two excellent references, “The Butterflies of North America” by James Scott, and “Foraging Dynamics of Bird Predators on Overwintering Monarch Butterflies in Mexico” by Lincoln Brower and William Calvert were consulted for this story.
To learn more about monarchs and their migrations, please visit the following web sites: