In today’s column, the Monarch Butterfly will serve as an honorary bird. Certainly one of the most easily recognized butterflies, Monarchs occur broadly across the United States and southern Canada.
However, the chances are pretty good that you have seen very few Monarchs this year in Maine. The cool and wet weather may have been a contributing factor to their poor success this year. I recently surveyed volunteers in the Maine Butterfly Survey project. Not one of the 25 respondents reported seeing as many as ten Monarchs this year, although the season may be a bit delayed.
Other species of butterflies like Common Ringlets, Common Wood Nymphs and Silvery Blues that live their entire lives in Maine took a big hit this summer because of our inhospitable June and July weather. These species will likely take a few years to recover their former population sizes.
The Monarch, however, is a migrant. Poor reproduction in Maine will not necessarily translate into lower numbers of Monarchs next year.
The amazing southward migration of Monarchs has been appreciated for quite a while. We have long known that populations west of the Rocky Mountains moved south to winter in about 150 winter roost sites between San Francisco south to northern Baja California. These roost sites are usually within a couple of miles of the Pacific Ocean. Each site usually has between 10,000 and 40,000 butterflies. As you can well imagine, coastal development has threatened a number of these roost sites.
But, where do the eastern migrating Monarchs spend the winter? The answer was not known until 1975 when a researcher named Fred Urquhart announced a surprising discovery. Each fall all of the eastern Monarchs in Canada and the United States empty out, migrating to Oyamel fir forests in the mountains of central Mexico. The fall migration commences in August and continues into December. Urquhart amazed everyone with pictures of millions of Monarchs roosting in layers on trees, weighting down branches.
Ten overwintering sites are known in Mexico. All are within an area of about 500 square miles in a belt of volcanic mountains stretching across Mexico. Conservation biologists are working hard to preserve these habitats and ecotourism is flourishing at these sites. Nevertheless, logging pressure poses a major threat.
As winter gives way to spring, the Monarchs become more active and start to mate. After mating, northward migration begins. Once mated, the butterflies have only a month or so to live. Along the northward migration, the females lay eggs on milkweeds along the way. Milkweed leaves provide the nutrition for the caterpillars. The migrating butterflies continue north and east with some reaching the Gulf coast states before they die. Meanwhile, the eggs laid along the way have hatched and the gluttonous caterpillars grow rapidly. Nine to 14 days after hatching, the caterpillar enters the pupal or chrysalis stage. During the next eight to 11 days, the tissues of the caterpillar are transformed into the body of an adult butterfly.
These newly emerged Monarchs (the offspring of the overwintering generation) continue northward, laying eggs along the way and ultimately perishing. Each female can lay 500 eggs. Most are laid singly on a milkweed plant. In this leapfrog manner, all of the eastern United States and southern Canada are repopulated. The Monarchs that reach us may be the great-grandchildren of the overwintering population!
The Monarchs that will migrate back to central Mexico emerge in the fall. These butterflies do not become reproductive but rather go into reproductive diapause. They will not be able to mate until the following spring on the wintering grounds in Mexico. Unlike the other adults from other generations that have only a month or so to live, these overwintering Monarchs may live for seven months.
Perhaps the most remarkable aspect of this tale is the “hard-wiring” in the migrating Monarchs. Even though they have never made the journey to central Mexico, the migratory behavior and direction are genetically encoded.
Since 1990, Dick Walton and Lincoln Brower have been conducting a Monarch Monitoring Survey at Cape May, New Jersey. At times, more than 300 Monarchs per hour have streamed past the hawk watch platform at Cape May Point. You can see find year-by-year accounts of the Monarch migration there at: http://www.concord.org/~dick/mon.html Numbers for 2009 indicate average to above average counts, lightening the dismal season for Monarchs in northern New England.
I encourage anyone wishing to learn more about these remarkable butterflies to read Sue Halpern’s marvelous book, Four Wings and a Prayer. Her prose is lyrical and her accounts of field expeditions with established Monarch researchers are fascinating.
[First published October 3, 2009]