In previous episodes, we learned about European hornet queens seeking hibernal refuge beneath bark, woolly bears using supercooling abilities to chill out, and overwintering mantises tucked away in frothy egg cases. However, many herbivorous insects have another strategy to deal with frosty winter temperatures and the attendant lack of leafy food in northern climes. They employ the snowbird strategy and head south.
Most folks are familiar with the eastern clan of monarch butterflies we met earlier this year, and learned of perils in their summer homes in the US and winter homes in the mountains of Mexico. Their heroic fall migration begins in Canada and northern reaches of the US and ends in wintering refuges in Mexico, a trip that may exceed 2,000 miles. This migration, triggered by falling temperatures and shorter days, is one of the most epic adventures of any living creature.
While the eastern migration of monarchs is often featured in the media, fewer folks know of the spectacular migration of monarchs in western North America. This annual journey west of the Rockies begins in autumn in southern Canada and the northern US, and ends in the welcoming coastal forests of California and other areas of the Southwest. To witness this wonderful phenomenon, Bug of the Week took a trip to the Monterey peninsula to visit one overwintering site of western monarchs in the lovely town of Pacific Grove, whimsically known as Butterfly Town, USA.
Here among the boughs of eucalyptus and Monterey pines, monarchs 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, many eucalyptus trees bloom during the monarch’s annual visit and replenish much needed carbohydrate reserves.
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. In California the California towhee is reported to eat large numbers of monarchs in their overwintering sites. Clever birds such as jays learn to avoid the more poisonous parts of the monarch’s anatomy, like the wings, and dine on less offensive parts like the thorax. The monarch refuge in Pacific Grove was filled with many birds in addition to more than 20,000 butterflies during my visit.
Gardens of nectar filled flowers provide welcomed carbohydrates for overwintering monarchs in the sanctuary.
In addition to vertebrate predators, monarchs are beset with diseases as well. While visiting the sanctuary in Pacific Grove, 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 milkweed, the food source for monarch caterpillars, to the northern limit of its range. I arrived at the Monarch Sanctuary in Pacific Grove between vicious coastal rainstorms with high expectations to witness thousands of monarchs hanging from trees and shrubs. I was not disappointed as several thousand monarchs were in the sanctuary. On a chilly afternoon under a leaden sky, monarchs hung motionless on branches of pines and eucalypti like clusters of so many dead leaves. Later on a sunny afternoon, hundreds of monarchs basked in the sun, floated through treetops, and visited clusters of flowering verbena and daises planted in the garden. What a treat to see monarch flying in December.
And how are the western monarchs doing? Information collected by the Xerces Society for Invertebrate Conservation annually surveys western monarchs. In their 2013 report, monarchs were believed to be holding their own, yet numbers were far below levels observed in the late 1990’s. Xerces attributes these declines to habitat loss, degradation, pesticides, and prolonged drought associated with global change. With national and international attention focused on pollinators and their plight, let’s hope we can find ways to preserve these fascinating creatures.
Bug of the Week thanks the docents at Pacific Grove Monarch Sanctuary 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. Bug of the Week wishes you all a very happy holiday season and a wonderful New Year!
To learn more about monarchs, their migrations, and Butterfly Town, USA, please visit the following web sites: