Nightlife of Social Caterpillars

Natural History, Feb, 2001 by Terrence D. Fitzgerald

For cold-adapted larvae on the peaks of the Sierra Madre, the action begins after dark.

I began to suspect there might be trouble ahead when we had driven for many miles without seeing any cars in the oncoming lane. We had gotten a late start and were traveling at night along twisting and precipitous Highway 41, which leads from the Mexican coastal town of Mazatlan east to the city of Durango, some 180 miles from the Pacific in the Sierra Madre Occidental range. My colleague Dessie Underwood and I had come to this rugged region to study the madrone caterpillar, the larval stage of the pierid butterfly Eucheira socialis, a distant relative of the cabbage white butterfly familiar to North American gardeners. Living in colonies at elevations of approximately 8,000 feet, madrone caterpillars nest on the branches and feed on the leaves of the madrone tree, which grows on the fringes of pine forests on the mountain peaks and keeps its leaves all winter. Although snow is rare here, nights are cold and frost is common. But throughout the winter, the madrone caterpillars continue to feed and grow, foraging under frigid conditions that few other insects could tolerate. Dessie and I had teamed up to study their ability to survive deep winter in the mountains.

As our car approached a tortuous part of the highway aptly called Espinazo del Diablo, or "spine of the devil," we encountered a massive gridlock of vehicles stranded by black ice. Motorists huddled around makeshift fires dotting the sides of the highway. It wasn't until late the following morning, after the sun had melted the ice and a pathway was opened through the maze of stalled vehicles, that we were able to continue on our way. Arriving at our study site later that day, we realized we weren't the only victims of the cold. Some madrone trees and their caterpillar colonies had escaped damage, but throughout the forest we saw pockets of frost-damaged madrone trees with colonies of dead, frozen caterpillars hanging from the branches. Records we obtained from a local weather station indicated that the colonies had probably been killed shortly before our arrival, when temperatures fell to as low as 9 [degrees] F night after night for more than two weeks. Madrone caterpillars push the physiological limits of insects in dealing with cold temperatures, and although they can resist freezing during most winters, they are clearly susceptible to prolonged spells of severe cold.

The madrone caterpillar is one of some 300 species of caterpillars that live in social groups. (More than 140,000 Lepidoptera species exist worldwide.) According to Peter Kevan and Robert Bye, who studied the insect's general life history in the early 1990s, the Aztecs called it xiquipilchiuhpapatotl, or "butterfly that makes a pouch." The brilliant white, baglike nests that house the colonies are typically six to eight inches in diameter and are constructed of pure silk. Native peoples used these tightly woven shelters to make containers and fabric. The conspicuous nests, which can number twenty or thirty per tree, also attracted the attention of the explorer Alexander von Humboldt during his travels in Mexico in the early 1800s. Von Humboldt observed the caterpillars and used the nest material as parchment, signing his name in 1804 on a section cut from a nest.

While the bag nests are feats of caterpillar architecture, the insects' enduring family bonds and the recently discovered fact that the colonies are composed largely of males are what make the madrone caterpillar unique, so far, among social caterpillars. Each colony consists of up to several hundred siblings that remain together for nearly a year, pupating as a group and then transforming into butterflies while still sequestered in the nest. The female butterfly lays her cluster of eggs on a madrone leaf during the rainy season, in June or July. The young caterpillars hatch from these eggs in about three weeks and remain together to forage and to spin silk; eventually they cover a few leaves in a veil of silk, creating a primary nest within which they will rest between bouts of feeding. As fall approaches, the caterpillars quicken the pace of their spinning and create a secondary nest that completely engulfs the primary one. The walls of this outer nest are highly fortified, consisting of tens of thousands of strands of silk so tightly woven that the nest will hold water and a sharp instrument is needed to penetrate its walls. The caterpillars will spend the rest of their larval lives--some eight months--in the expanded nest, leaving it only to feed. (Some other lepidopterans have a lengthy larval stage, but most spend far less time as caterpillars than the madrones do. In the tent caterpillars of eastern North America, for example, the larval stage lasts eight weeks.) To feed, madrone caterpillars enter and exit the nest through a small opening at the bottom. Otherwise the nest is an impenetrable fortress, excluding birds, wasps, and most other predators that might find the caterpillars a tasty meal.