End of summer marks monarch migration

It’s been a banner year for hummingbirds and butterflies here on the ridge. I’ve already written about hummers several times, so suffice to say that the clouds of nectar sippers I described earlier in the summer have become thunderheads of hummingbirds. They’re draining the nectar feeders twice a day.

Butterfly numbers have shown a similar pattern this summer. Tiger swallowtails, fritillaries, and skippers have decorated the yard since June. In July pipevine swallowtail caterpillars emerged en masse. Over the course of four weeks, they consumed half the biomass of the volleyball-sized pipevine leaves. For the last two-weeks the black fleshy caterpillars, distinguished by red dots and fleshy appendages, have been on the move in search of safe havens to form the chrysalises that will get them through the winter.

Monarch butterflies

But it’s monarch butterflies that have really captured my attention. Though I received many reports of monarchs and their caterpillars as early as June, I didn’t see one until mid-July. Since then, however, their numbers have mushroomed. I’ve found caterpillars of all sizes on milkweed plants, and adults have become so common, I’ve seen them splattered on auto grilles and windshields.

Though everyone can appreciate colorful butterflies dancing through a bed of flowers, it is the miraculous monarch migration that grabs my attention as summer winds down. Like many birds which commute between temperate zones and the tropics, colorful orange and black monarchs migrate south for the winter. Mark and recapture studies have revealed 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.

One-time migration

What makes the monarch migration even more amazing is that each butterfly makes the trip just once. The monarchs that fly to Mexico are from the last brood of the summer, usually hatched in late August or early September.

On the winter grounds monarchs congregate on fir tree trunks by the millions, where they rest and burn little of their fat reserves. By mid-March, they still have plenty of stored energy for the return trip north. Mating occurs before the journey north begins, and females lay eggs on milkweed plants as they make the return trip.

After reaching northern Mexico, the wintering butterflies, which have lived as long as eight months, die. But the eggs they leave behind ensures the next generation. This new brood 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. Unlike the final summer brood, which devotes most of its energy to migration, spring and early summer broods invest their energy in reproduction.

What we know

Because of 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. Dr. O.R. “Chip” Taylor, an insect ecologist at the University of Kansas and director of Monarch Watch, says monarchs, “use fall’s shorter days and cooler nights to know when to migrate, and they use the sun as a compass to navigate.”

Furthermore, Taylor reports that, “strong magnets disorient monarchs, so it’s likely they also use the earth’s geo-magnetic forces to navigate.”

Monarch caterpillars and adults enjoy one unusual measure of protection: They are distasteful 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 poison into their own body tissues. The poison is retained through metamorphosis so even adults have high concentrations of the toxins. (Pipevine leaves confer a similar advantage to pipevine swallowtails.)

To learn more about monarchs, contact Monarch Watch at (888) TAGGING or visit www.monarchwatch.org.

XSend questions and comments to Dr. Scott Shalaway, R.D. 5, Cameron, W.Va. 26033 or via e-mail to sshalaway@aol.com