Last winter disturbing news came from the mountains of central Mexico. Millions of dead monarch butterflies littered the forest floor at several sites where monarchs spend the winter. Eventually, entomologists determined that the die-off was natural, due to unusually severe winter weather.
How the winter monarch kill would affect this summer's butterfly populations weighed heavily on the minds of many butterfly lovers. But according to Dr. O.R. "Chip" Taylor, an insect ecologist at the University of Kansas and director of Monarch Watch, 2001 has turned out to be a banner year for these colorful butterflies.
"The key to summer monarch numbers," says Taylor, "are spring conditions in Texas. After leaving the wintering site, monarchs lay eggs in Texas and this year's wet spring was ideal for both monarchs and the milkweed plants they depend on. Monarchs numbers are up just about everywhere."
Seeing monarchs: That certainly has been my experience. I've been seeing monarchs since May, and the last few weeks they have been abundant. The monarchs we see over the next few weeks will be the ones that make the remarkable southward migration to the mountains of Mexico.
Like many birds which commute between temperate zones and the tropics, monarchs are one of just a few insects that also migrate south for the winter. Mark and recapture studies have shown that monarchs travel as far as 1,800 miles in just four months. They move only by day at a leisurely pace of five to 18 mph.
Not only do monarchs travel great distances, they do so with unerring accuracy. Year after year they return to the same winter areas, even the same fir trees in just a handful of isolated mountaintops in Mexico. So when millions died last winter, entomologists were rightfully concerned.
What makes the monarch migration so amazing is that each butterfly makes the trip only once. The monarchs that fly to Mexico are from the last brood of the summer, usually hatched in August. .
On the winter grounds monarchs are sluggish and inactive. They congregate on tree trunks by the millions. During the winter, monarchs burn little of their fat reserves. By the second week of March, they still have plenty of stored energy for the return trip north. Mating occurs before migration begins, and females lay eggs on milkweed plants as they move north through Texas.
Then they die: After reaching the southern states, these individuals, which have lived as long as eight months, die. But the eggs they leave behind ensure a new generation. This new generation continues northward, laying eggs as it goes, until three or four generations later adult monarchs reach the northern limits of milkweed distribution. Individuals from these summer broods live only three to five weeks, just long enough to reproduce. The final summer brood is the one that will return to the mountains of Mexico.
Due to their complex natural history, several generations of monarchs separate those that leave the winter roost from those that head south in the fall. Yet somehow, inexperienced monarchs return to their ancestors' traditional wintering areas. Taylor explains that monarchs use fall's shorter days and cooler nights to know when to migrate, and they use geomagnetic cues and the sun to navigate.
Though monarchs lead difficult lives, both caterpillars and adults enjoy one unusual measure of protection. They taste terrible to vertebrate predators because they are what they eat -- milkweed, which is toxic to most animals. Though monarchs are unaffected by the toxin, they incorporate the toxin into their exoskeletons. The poison is retained through metamorphosis so even adults have high concentrations of the foul-tasting chemicals.
A few enemies: Despite this natural protection, wintering monarchs do have a few enemies. According to Taylor, black-headed grosbeaks seem immune toxins, and black-backed orioles peel off the butterflies noxious exoskeleton and eat the soft, tasty inner tissue. These two birds may take as much as 15 percent of the wintering monarch population in some areas.
To learn more about monarchs and become part of a fascinating citizen science program (for a small fee), call Monarch Watch at 1-888-TAGGING or visit its web site at www.monarchwatch.org.