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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Catching up with the Monarch butterfly, Danaus plexippus


Bending her abdomen beneath the leaf, the female monarch deposits an egg.


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 beetles and other fascinating insects of late summer. This week, on the evening of August 17, a sequel to this annual escapade will take place with a program called “Bugs & Bees…and Daiquiris!” Guests will have the opportunity to learn about some of the marvelous six-legged creatures that call the meadows of the Conservancy “home”. In addition to residents, we will search for intriguing vagabonds, including iconic monarch butterflies.


Whether on the east coast or the west, an early morning draught of nectar will power the monarch through the day.

One perennial question on my mind is the fate of these wondrous visitors. How did they fare last winter in their hibernal refuge in Mexico? I saw my first monarch of the season in the sleepy hamlet of St. James, Maryland, on May 15 at the Elms Environmental Education Center, an early arrival to be sure. At my home in Columbia, monarchs were a bit late with my first sighting of an adult in July. 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 Oyamel Forests of 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.

Eggs are laid singly by the female monarch, a process she repeats hundreds of times.

The female monarch lays several hundred eggs one-by-one on the undersurfaces of milkweed leaves. After a few days to a week, 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. The monarchs then continue their annual flight northward to summer feeding grounds in the northern United States and southern Canada.



With filaments flailing, this very hungry monarch caterpillar will have gained more than 2,000 times its birth weight by the time it is fully developed.

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 monarchs' annual visit to California and provide food.

The fully-grown caterpillar assumes the “J” position just before pupation. 

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 caterpillars, 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.

The monarch’s chrysalis is among the most beautiful in the entire realm of butterflies.

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 once again 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 according to records kept over the past two decades. The winter of 2013 – 2014 witnessed the lowest number of overwintering butterflies in Mexico. Populations rebounded in the winters of 2015 – 2016, and although population estimates last winter were lower than the previous two years, the numbers were up from the previous all-time low.  

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. Scientists suggest that more severe storms associated with climate change may also threaten monarchs. Last winter a late spring storm in the mountains of Mexico killed 5 - 10 % of resident butterflies. This translated into several million fewer monarchs making their way north last spring. 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. There is also concern that planting the exotic tropical milkweed Asclepias curassavica in southern states may interrupt the primal migration of monarchs and cause them to take up residence in the south, eschewing their age old migration northward.


Amidst the eucalyptus and pines, western monarchs brave the maritime chill on the Monterrey peninsula.

At present, the monarch is widely distributed and the prospects for global extinction are slim. Some monarchs are found year round in California, Florida, 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 plan to include milkweed in 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. The excellent references, “Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?” by Lincoln Brower and colleagues, and “Foraging Dynamics of Bird Predators on Overwintering Monarch Butterflies in Mexico” by Lincoln Brower, were consulted for this story as were several of the websites listed below. To learn more about monarchs, their migrations and perils, and how to conserve them, please visit the following websites:

To learn more about ‘Bugs & Bees…and Daiquiris!’ to be presented on August 17 by the Howard Conservancy please click on the following link: