An Obsession With Butterflies

IN LATE SEPTEMBER 1921, FOR EVERY MINUTE of eighteen days, an estimated 25 million Snout butterflies passed over a 248-mile front from San Marcos, Texas, south to the Rio Grande. The flight may have involved 6 billion insects.

The Snout butterfly has a long appendage on the front of its head; when the adult rests parallel to a twig, its closed wings look like a leaf and its extended “nose,” slightly angled, a leaf stem. In Texas, the upper wings of Snouts are brown or orange-brown with white spots.

A winter and spring drought in southern Texas had eliminated most of the parasitoid wasps that feed on Snout larvae. Good rains in May and June caused the butterfly population to explode in the next two or three generations. In July, the tropical storm Amelia produced even more rain. The larvae’s host plant, the desert hackberry, flourished. The Snout caterpillars, green and dotted with yellow, flourished. The caterpillars defoliated their host plants and pupated. A population of a half billion butterflies emerged, and the Snouts began to migrate.

The majority of the migrating insects were young males. Snout females are mated as soon as they emerge from their chrysalides, but at the original site, older males already waited by each pupa. So the younger males left, hoping to find unmated, available females elsewhere.

Some females also joined the migration, looking for a better place to lay their eggs. Most, however, stayed behind to exploit the dramatic response of the damaged hackberry—which was to sprout, as if it were spring, generous new leafy buds. Later, when the eggs hatched and the larvae began to feed on these leaves, many hackberries would die, preventing for some time future outbreaks of Snout butterflies.

When Larry Gilbert was a boy in south Texas, he also saw huge swarms of Snouts passing through after the summer rains followed drought, congregating in the hundreds on overripe dates in his grandmother’s yard. These migrations paralleled a burst of green leaves, flowers, and other insects. “The sounds and fragrances of life were everywhere,” he remembers, “where just days before, everything had been hot, dry, and bleak.”

In 1977, Gilbert had access to a large protected reserve and was able to find the source of the river of butterflies that crossed a highway several miles away. Thousands of green and tan pupae hung from the leafless desert hackberry. The air shimmered with that connection between childhood and the rest of your life. The physics of time, once again, proved exotic.

“I confess I do not believe in time,” Vladimir Nabokov wrote in his autobiography,

I like to fold my magic carpet, after use, in such a way as to superimpose one part of the pattern upon another. Let visitors trip. And the highest enjoyment of timelessness—in a landscape selected at random—is when I stand among rare butterflies and their food plants. This is ecstasy, and behind the ecstasy is something else, which is hard to explain. It is like a momentary vacuum into which rushes all that I love. A sense of oneness with sun and stone.

Butterflies on the move, migrating butterflies, may set off as individuals, or in pairs; as small groups, or in large groups. Most regular migrants live in areas of great seasonal differences, summer and winter, wet and dry. They follow a seasonal pattern. A few butterflies follow vegetation, host plants and nectaring plants, up and down mountains. Some migrations, like that of the Snouts, happen irregularly, caused by an explosion of population, overcrowding, and competition.

The Painted Lady is the world’s most common and abundant butterfly. Painted Ladies cannot survive extreme cold, so they often migrate south in winter, and north in spring and summer, their numbers estimated in the hundreds of millions, from Africa to Finland, from Mexico to Canada. In the summer of 1879, a migration of Painted Ladies through Europe was so great that it was called an invasion, one of nature’s odd military campaigns.

A few years later, an explorer recorded the start of a Painted Lady migration in a stretch of grass on the Sudanese Red Sea coast:

From my camel, I noticed that the whole mass of grass seemed violently agitated, although there was no wind. On dismounting, I found that the motion was caused by the contortions of pupae of V. cardui, which were so numerous that almost every blade of grass seemed to bear one. The effect of these wrigglings was most peculiar—as if each grass stem was shaken separately, as indeed was the case. . . . Presently the pupae began to burst and the red fluid fell like a rain of blood. Myriads of butterflies, limp and helpless, sprinkled the ground. Presently the sun shone forth, and the insects began to dry their wings, and about half an hour after birth of the first, the whole swarm rose as a dense cloud and flew away eastward towards the sea.

The larvae of Painted Ladies migrate, too. In 1947, in the Saudi Arabian desert, a researcher observed an army of these caterpillars advance with the young hoppers of desert locust, eating the new spring growth.

In 1991, in California, a good year for butterfly eggs prompted another movement that entomologists could stand around and watch. By late May, hungry and overcrowded larvae had begun to crawl in straight lines in search of food. Compared to caterpillars raised alone, they showed increased activity, nervousness, cannibalism, and simultaneous pupation. The adults that emerged were also more active and gregarious. These adults had undeveloped reproductive organs and large reserves of fat. They didn’t mate. They flew north instead.

It is possible that food shortages and overcrowding alter the behavior and biology of Painted Lady larvae, who develop into adults that quickly migrate. When the migrating adults start to feed, their hormone levels rise, and they then reproduce. Later generations of these Painted Ladies may experience cold weather; these butterflies, too, will begin to move.

We watch the spectacle. We enjoy the abundance. We indulge in metaphor. We want armies and clouds. We want whites like snowflakes, sulphurs like buttered popcorn. My own greed seems palpable. My eyes gleam. A billion Snouts. Six billion.

But I am a child of my time, and I do not see much excess in nature. Passenger pigeons once darkened the sky. Caribou stretched horizon to horizon. Salmon were so thick you could walk across water. This is not the coin of the twenty-first century. We measure our wealth by different standards.

The Apache del Bosque in central New Mexico is a flyway for migrating water birds, which come every winter. As the sun rises, tens of thousands of birds rise from the artificial lake, choreographed, wheeling and crying, honking and hooting, before they leave to feed on nearby fields. I have a photograph of my nine-year-old daughter watching this scene, the horizon pink, her pigtails longer than they will ever be again. The sky is a map pinpointed with ducks, geese, coots, cranes, and terns.

The river that flows by my home in New Mexico goes dry in the summer. Early in our marriage, years before we had children, my husband and I watched a large pool shrink with every hour. The water contained an abundance of tadpoles waiting to die as the pool evaporated. The animals squirmed against each other in a carpet of flesh. My husband and I watched, fascinated. I remember it still. This is what happens when you cannot move.

I remember the Monarchs, too, the ones that drowse every fall in eucalyptus groves near the Pacific Coast. On a dirt path, my sister chased my niece with a coat, for it was cold, surprisingly cold. The mentholated trees fluttered, covered with butterflies. We looked up. We spoke in whispers. The church of the Monarchs.

Monarchs are the most famous migrating insect. Millions of these butterflies in Canada and the northern United States fly over 2,000 miles to overwinter in certain mountains in Mexico. (Some 5 percent of Monarchs, on the west side of the Continental Divide, fly to the Pacific Coast.) In the spring, these same individual butterflies start the return trip north.

Like Painted Ladies, Monarchs cannot survive an extreme winter. The Monarchs heading for Mexico take months to reach a roosting site that is generally above freezing but still cold enough to keep their metabolism and energy needs low for semihibernation. At this site, they have trees on which they can cluster. They have protection from wind and snow. They have water nearby. On warm days, they may rouse a little, fly a little, drink a little, and return to their somnolence, clinging to fir branches and to each other.

In March, they wake up. They feel the urge to mate. They move down the mountain and begin to fly north and east, on the lookout for milkweed plants. Before they die, females lay their eggs, recolonizing the southern United States. The generation that emerges now will continue to fly north, to mate and lay eggs, and to die in about a month. The next generation will do the same, and the next, until the last generation of butterflies reaches the most northern edge where Monarchs and milkweed plants can live.

The Monarchs that now pupate and emerge in late summer and early fall will be different from those of previous generations. In the larva and in the chrysalis, shorter days and cooler temperatures have triggered hormonal changes. The adult males and females are reproductively immature. At the first sign of cold weather, they stir collectively, a shiver of desire, and begin moving south—to a land they have never known.

These butterflies have unusually long life spans, as long as nine months, time for them to fly to their overwintering grounds, to dream through the winter, to mate in the spring, and to start the journey back north. Unlike their parents, they are not solitary but huddle together, roosting at night as they hurry south. During the day, they fly in clouds, as high as 1,000 feet and as fast and far as fifty miles. As they go, they stop and feed. They even gain weight.

For navigation, they use the sun. In one experiment in the American Midwest, researcher Sandra Perez netted a random group of migrating Monarchs and kept them in her lab for two weeks. There she changed their light and dark cycle so that the butterflies became accustomed to a different time zone. When the Monarchs were released, one by one, they flew in the wrong direction, based on where the sun should have been, based on what time they thought it was.

On cloudy days, the Monarch relies on a magnetic compass: tiny bits of magnetite in the thorax. When Sandra and her colleagues exposed migrating fall Monarchs to normal magnetic fields, the butterflies flew normally toward the southwest, on to Mexico. When the butterflies were exposed to a reversed magnetic field, they flew in the reverse direction, northeast. When they were exposed to nothing, when they had no magnetic field, they flew hither and thither.

Monarchs, like other migrants, probably also use visual landmarks. Butterflies commonly compensate for crosswind drift; in experiments across open bodies of water, butterflies able to see landmarks on the horizon compensated better. Sulphurs and skippers flying over ocean areas without landmarks also seemed to use, with moderate success, nonstationary objects such as clouds or rippling waves.

Most butterflies get through life by flapping their wings, the basic flight pattern, up, down, up, down. But a Canadian Monarch needs to reach its Mexican wintering site in less than ten weeks. The most earnest flapping is not good enough. Instead, Monarchs use thermal updrafts to glide and soar like eagles. In the late afternoon, they may stop migrating for the day because the earth has cooled and the thermals have gone. Monarchs also take advantage of the wind if it is blowing their way. If it is not, they adjust by flying closer to the ground, where there is less wind. They may choose to rest, eat, drink, and wait.

Other migrating butterflies have different patterns. Many fly in a straight line, purposeful and low. One scientist noted that “even when a migrating butterfly is trapped on a porch, it appears to be trying to batter down the house, and will persist in a chosen course rather than retreat a few yards or deviate from it.” Cloudless Sulphurs, Gulf Fritillaries, and Long-tailed Skippers all tend to travel in the less windy “boundary layer,” a few yards above the surface of the earth. Great Southern Whites, moving up and down the coast of Florida, fly in streams forty-five feet wide, rarely more than twelve feet above the ground. On breezy days, they use sand dunes as a buffer. On still days, they fly directly over the dunes. Solitary Red Admirals, migrating from northern Europe to a warmer place in Spain, are often seen at about waist level, set on a determined track. Put something in that butterfly’s way and it will swerve around, or over, reorient, and continue, straight ahead.

The Monarch is at one end of a scale. At the other end is the Colorado Hairstreak, which commonly strays a few yards in its lifetime. The stability of the host plant may help determine how much a butterfly will travel. Stay-at-home butterflies usually lay their eggs on dependable perennials such as trees. Peripatetic butterflies tend to lay eggs on less dependable plants such as weeds and annuals.

Butterflies everywhere are on the move, and butterflies are not alone. Each year, freshwater eels slither through dewy grass to reach the ocean. Seabirds clock in 20,000 miles. Walrus cover a tenth of that. Mexican free-tail bats fly across the desert. Microscopic flatworms wiggle eight inches, twice a day. In one month, in April 2002, a group watching migrations tracked whooping cranes, humpback whales, Monarchs, hummingbirds, caribou, bald eagles, and robins.

I like the numbers, the big numbers. More is better. More butterflies are better than fewer butterflies. A river of butterflies is a wonderful thing. Millions of butterflies are the jackpot. I like the largesse, the almost casual gesture, as if a generous earth were whispering into my ear, “See how I replenish myself, see how I birth and birth and birth and darken the skies and fill the waters and cover the ground and still I have more to give.”