For the past several years, our friends at the Howard Conservancy in Woodstock, Maryland, have provided excellent educational experiences for local residents. Last summer families explored the Conservancy’s meadows in search of iconic monarch butterflies. This week, on the evening of August 27, the sequel to that adventure will transpire in a program entitled ‘Monarchs and Mojitos’. Visitors will have the opportunity to learn about the fascinating saga of monarch butterflies and enjoy other fascinating creatures, while partaking of a beverage which also has a colorful history.
In preparation for this event, Bug of the Week visited the Conservancy and was rewarded with sightings of several monarchs nectaring on fragrant wildflowers in the meadow and zesty zinnias in the garden. A quick search of some parched milkweeds revealed hungry caterpillars devouring leaves. In the exuberant late summer season, milkweeds host a bevy of orange and black insects, including milkweed tussock moths, large milkweed bugs, and milkweed leaf beetles in addition to monarch caterpillars. Why are so many denizens of milkweed dressed in orange and black? From the milky sap and tissues of the plant, many milkweed feeders obtain potent defensive chemicals known as cardiac glycosides. In the case of the monarch, these compounds are passed along from the leaf-eating caterpillar to the nectar-eating adult butterfly. Birds are important predators of many kinds of butterflies, including monarchs. Laboratory studies demonstrated that would-be predators such as blue jays foolish enough to have a go at the monarch retched and spewed when they encountered the cardiac glycosides found in the butterfly. The conspicuous orange and black coloration of the monarch and other milkweed residents serves as a warning to enlightened predators that eating this attractive morsel will have nasty consequences.
Early in the summer, Maryland’s meadows were devoid of monarchs. In my yard, monarchs were a bit tardy this year, arriving just a few weeks ago. 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 several hundred eggs one by one on the undersurfaces of several milkweed leaves. After several days, the eggs hatch and tiny monarch caterpillars begin to consume the nutritious leaves. Each 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 migrations 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 in 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 many 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 addition to vertebrate predators, monarchs are beset with diseases as well. While visiting the monarch sanctuary in Pacific Grove, California, helpful docents related stories of a pernicious protozoan attacking monarch caterpillars. Ophryocystis elektroscirrha, a.k.a. O.e., infects hypodermal tissues of the developing caterpillar causing reductions in the longevity and mating success of adult butterflies. Strategies are being developed now to reduce the impact of O.e. on populations of monarchs.
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 declined dramatically since records were first kept two decades ago. The winter of 2013–2014 witnessed the lowest number of overwintering butterflies in Mexico on record and last winter, 2014–2015, monarch populations were at their second lowest recorded levels. Monarch guru Lincoln Brower believes several factors conspire to reduce populations of monarchs. Illegal logging of trees in the mountains of Mexico has reduced the 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 greatly reduce populations of 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. There is also concern that planting the exotic tropical milkweed Asclepias curassavica in southern states may interrupt the primeval migration of monarchs and cause them to take up residence in the south and eschew their age old migration to the to north.
By the time its development is complete, this very hungry caterpillar will have gained more than 2,000 times its birth weight.
At present the monarch is widely distributed and the prospects for global extinction are slim. Some monarchs are found year round in California and on the Hawaiian Islands. 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. Be sure to consult a reference to learn what works well in your geographic region. Here in Maryland species including common milkweed, Asclepias syriaca, swamp milkweed, Asclepias incarnate, and butterfly weed, Asclepias tuberosa, are good choices. We all can play a role in conserving these remarkable vagabonds.
Bug of the Week thanks the wonderful staff of the Howard Conservancy for providing the inspiration for this week’s 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 as were several of the web sites listed below.
To learn more about monarchs, their migrations and perils, and how to conserve them, please visit the following web sites:
To learn more about ‘Monarchs and Mojitos’ at the Howard Conservancy please click on the following link: http://www.hcconservancy.org/upcoming-events.html