Thursday, September 08, 2005

Speaking of Monarchs

Listen to the commentary
Real Audio : MP3 download

If you’re wondering whether you’ve been seeing a lot of monarch butterflies in the past couple of weeks, you probably have. Spurred on by cooler, shorter days, monarchs from southern Canada and the northern U.S. have already begun their southward fall migration. At our latitude, peak abundance for monarchs typically occurs between about September 10th and September 23rd, although variations in weather patterns and other factors shift the timing of this event from one year to the next.

If you are a native to Illinois, where the monarch is the state insect, you are probably familiar with some of the characteristics that make it one of the world’s favorite insects.

Chief among these is the fact that North American monarchs migrate, like no other butterflies in the world. Each year, the summer’s last generation of monarchs born east of the Rockies flies south to the mountains of central Mexico, a journey of more than fifteen hundred miles for some. Under normal conditions, migration advances at roughly fifty miles a day, although tagged individuals have been reported to cover close to eighty miles. Monarchs feed on nectar as they move south, and actually gain weight over the course of the journey.

At their overwintering sites, which were only discovered in 1975, monarchs congregate by the millions on Oyamel fir trees, resting quietly from mid November to mid April. As spring advances, the overwintering monarchs begin to mate, and they move north into the Gulf Coast states to lay eggs. The resulting young will become the year’s first generation of new adults. Members of this new generation will then continue the journey north, laying eggs as they go.

How the south-migrating monarchs of the fall locate the same clusters of trees their great great great grandparents left in the spring remains a mystery for scientists.

Monarchs are also memorable for the fact that as caterpillars they feed exclusively on plants of the milkweed family, which contain toxic compounds that make them unpalatable to birds and other would-be predators. Thus the showy bands of white, yellow, and black on the caterpillars and the vivid orange and black of the adult’s wings, are a warning to other animals: “Don’t eat me; you’ll get sick.”

This is not to say that monarchs have it easy. Monarch caterpillars are preyed upon heavily by ladybugs, as well as various other insects and arachnids, despite the toxic milkweed compounds that protect them from birds.

Monarchs are also quite frequently killed by cars. In fact, the first study to document systematically the magnitude of roadway mortality of butterflies and moths anywhere in the United States was conducted by a group including May Berenbaum of the U of I Department of Entomology. Extrapolating from counts of dead monarchs along roads near Champaign-Urbana, they estimated that, statewide, more than five hundred thousand monarchs became roadkill over one week during the course of their study.

Of course, the long-term well being of monarchs as a species depends on habitat. They are especially vulnerable at their overwintering sites, where logging and other disturbances can affect large portions of the population at once. But habitat alteration is also a factor here in the north, where suburban development and the expansion of agriculture leave less and less room for the plants that monarchs depend on.