With the arrival of a major winter storm along the eastern seaboard this week, Bug of the Week took a trip to sunny California to visit the overwintering
site of the western monarchs in the lovely town of Pacific Grove whimsically known as Butterfly Town, USA. Previous episodes of Bug of the Week focused on other aspects of monarch biology such as their potent chemical defenses made possible by the monarch caterpillar’s ability to eat milkweeds and store nasty chemicals found in the leaves. Most folks are familiar with tales of the fall migration of monarchs from their summer feeding grounds in northern reaches of the US to wintering refuges in the south. This migration, triggered by falling temperatures and shorter days that signal the approach of winter, is one of the most heroic adventures of any living creature.
Monarchs from central and eastern US, head for the mountainous forests of Michoacán, Mexico, a trip that may exceed 2,000 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 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 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 such as jays 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.
I arrived at the Monarch Sanctuary in Pacific Grove with high expectations to witness thousands of monarchs hanging from trees and shrubs. A friendly and helpful denizen of the town directed me to several locations where monarchs had been spotted in years past, but luck escaped me and not a single cluster of butterflies was seen. The monarchs apparently decided to spend that day or perhaps the season in a more hospitable location. My acquaintance offered that the monarchs had been scarce in recent winters. Too many visitors, declining health of the trees, and other problems related to urban sprawl might contribute to the local decline of monarchs in Butterfly Town. “Perhaps, the monarchs are in Santa Cruz or Saint Luis Obispo,” she said. A bit disappointed, I left the sanctuary to catch a plane to the chilly northeast. On my way out, I spotted a single monarch perched on a shrub by the trail. Whether this was the vanguard of mobs of monarchs returning to the roost or just a singular misguided traveler, I do not know. Yet, spotting a monarch in December is very cool indeed.
Bug of the Week thanks an unknown resident of Pacific Grove 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, their migrations, and Butterfly Town, USA, please visit the following web sites. Bug of the Week wishes you all a very happy holiday season and a wonderful New Year.