Collecting sassafras sprays to feed the hungry cecropia caterpillars I was raising a couple of summers ago, I found a contingent of tiny caterpillars clustered on the underside of a leaf. They weren’t cecropias, or any other sassafras-eating caterpillar I knew. What struck me was the way they had arranged themselves: flank to flank, like pencils in a box, heads at the margin of the leaf, nibbling away.
I brought them home, put them in a container and kept them supplied with fresh leaves. As they passed through successive molts, they came to resemble the illustration of the caterpillar of the Io moth in my field guide. Adult Ios are stunningly beautiful: yellow or light brown, with midnight blue eye-spots on the hind wing, which they flash when startled, a defensive mechanism. The caterpillars were handsome too, their green bodies ornamented with lateral pink and cream stripes, backs bristling with clusters of branching spines.
“Watch out when you handle them,” my naturalist friend Harry Ellis warned me. “Those spines can give you a nasty sting.”
I heeded his warning, remembering an earlier experience with stinging caterpillars. Harvesting corn one year, I reached through the foliage and felt a jolt of pain, sharp as a wasp’s sting. I saw no wasps, but kept getting stung.
Finally, I noticed a few chocolate-brown caterpillars with conspicuous lime-green abdominal saddles and tufts of bristling spines. Nursing my hand, I retreated to the house to look the caterpillars up. “Sting intense and of considerable duration,” one guide noted of the saddleback caterpillar, the larval form of an inconspicuous brown moth.
Many caterpillars are hairy. Some have horns or spines that look dangerous but aren’t. But a few species — saddlebacks among them — have “sharp, fragile hairs that break off in the skin of an attacker and release a venom that stings like a nettle,” entomologist Gilbert Waldbauer writes in Millions of Monarchs, Bunches of Beetles.
“Under high magnification, a Io’s spines are really impressive,” monarch researcher Lincoln Brower told me. “They look like hypodermic needles.”
Knowing from experience the pain of a caterpillar’s sting, did I set the Ios back out in the wild? No. Why? Because Ios have an offsetting characteristic. In their larval stages, they’re social insects. Which meant that, when I was exchanging new sassafras sprigs for old in their cage, I needed only to find one caterpillar to locate the others. I raised the whole brood successfully and never once was stung.
When it comes to social insects, we automatically think of ants, bees, wasps and termites. These insect groups have long occupied the social-insect limelight by virtue of their “near-ubiquity, remarkable division of labor, sophisticated behavioral interactions and impact on human society,” writes Western Carolina University biology professor Jim Costa in a 1997 issue of American Scientist.
But there are other insects out there (Io caterpillars among them) that exhibit lesser degrees of sociality. “Ios stick together in early instars (stages between molts), and probably communicate chemically, but they haven’t been studied much,” Costa says.
Insects “who have something to say to each other” fascinate Costa. Introduced to social caterpillars as an undergraduate, he’s studied them ever since. Though he’s done some work with a Costa Rican relative of the Io, most of his research has involved Eastern tent caterpillars and sawflies (a large group of insects related to the bees, wasps and ants, but whose larval forms resemble the caterpillars of butterflies and moths).
Say it With Pheromones
The Eastern tent caterpillar is the most socially complex and best known of all social caterpillars “but its remarkable behavior is unlikely to be unique,” Costa thinks. As more social caterpillars are studied, the trail-marking behavior it exhibits may prove to be common. Still, tent caterpillars who build and expand their communal tent, bask in the sun, and feed together have taken trail marking to a high art.
Foraging caterpillars don’t follow one another as they leave the tent, but fan out, seeking tender foliage at twig tips. As they move along, they lay down silken strands, marking them with pheromones (chemical substances). A caterpillar who finds an abundance of food eats for awhile, then returns to the tent, re-marking its trail with a second “recruitment pheromone.”
Caterpillars who have been unsuccessful in finding food return periodically to the tent in search of one of these double-marked trails to follow. “In this way, the tent functions as a predictable information center — the essence of recruitment communication,” Costa says.
There are quite a few other social caterpillars in the southern Appalachians as well. The Baltimore checkerspot and some of its nymphalid (brush-footed butterflies) relatives “travel in little herds in their early instars,” he says.
Among the moths, the Eastern tent caterpillars and fall webworms are our most conspicuous social caterpillars. The two are easily differentiated by their web sites (at the ends of branches for webworms; in the crotches of trees for tent caterpillars), and the seasons in which they appear (spring for tent caterpillars; fall for webworms).
But the poplar tentmaker and red-marked tentmaker, the Datanas and several species of oakworms are also gregarious as caterpillars. Of the latter, the orange-striped and the pink-striped oakworms are especially common in our region.
Social behavior isn’t limited to caterpillars. “There is no question that thereís a whole range of insects and spiders that exist beyond the accepted definition of social, who live within family or larger groups,” Costa says. “They include representatives from the beetles, thrips, aphids, true bugs and sawflies.”
“From a biological point of view, these groups are as interesting as the more complex societies,” Waldbauer writes. “Group living at any level is important in the ecological scheme of things, because it enhances survival.”
Who Can Afford A Social Life?
Because of their lowly position on the food chain, insects have developed many strategies to keep from becoming someone else’s dinner. Cryptic coloration (blending into the scenery) helps some escape notice. Others — like early instar caterpillars of many swallowtail species, who are colored to look like bird droppings — mimic something unpalatable.
Some insects are armored with hard plates, spines, tubercles and hairs. Some, including sawflies, engage in simultaneous displays, lifting their heads and/or posteriors in unison and wave them back and forth to threaten predators or ward off parasitic flies and wasps. Others rely on chemical defenses to keep predators at bay, advertising their noxiousness through conspicuous coloration, distinctive odors or sounds. A species (insect or otherwise) who uses such warning signals is known as aposematic.
“There is ample evidence that these warning signals are heeded by predators and often save the lives of insects,” Waldbauer notes. “Coming together in groups can amplify and thus enhance the effectiveness of such warning signals.”
That’s because teaching predators a lesson “requires the sacrifice of individuals,” Brower says. “Avoidance is a conditioned response. If you’re an unpalatable insect, you’re safer in a group than on your own. If monarchs weren’t toxic (their bodies contain cardiac glycosides from the milkweeds they ate as caterpillars) they couldn’t afford to do what they do in Mexico, clustering by the millions in a few locations. They’re full of lipids (fats); they’d be a banquet for predators. If they were distributed all over Mexico instead of aggregating, birds would learn not to eat them more slowly, and more individuals would be killed.”
Contrasting patterns — orange and black, red and black, black and white, yellow and black — serve notice to enemies of bees, bald-faced hornets, monarchs, milkweed bugs and ladybugs, all of whom aggregate at least part of the time, and all of whom “mean what they say” (are toxic or equipped with stingers). Some palatable insects have evolved to resemble noxious species, gaining a measure of protection they wouldn’t otherwise enjoy.
But sociality can be costly, even for insects that are armed and dangerous. Living in close proximity can increase the incidence of disease, parasitism and predation. Two bird species — the black-headed grosbeak and the black-backed oriole — feast on overwintering monarchs in Mexico, to no ill effect. Nor are Eastern tent caterpillars — whose gut enzymes detoxify the toxins in cherry trees and produce hydrogen cyanide, a powerful poison — invulnerable to attack. Yellow- and black-billed cuckoos eat them with gusto.
“Cuckoos have a preference for hairy caterpillars that lots of other birds avoid,” Costa says. “Titmice are also known to eat, but not banquet, on tent caterpillars. But their most serious predators are fellow insects. Paper wasps and their relatives are fierce caterpillar predators and will repeatedly attack them. And there’s a species of stinkbug that will ravage them, sometimes even inside their tents.”
An ongoing, escalating “evolutionary arms race” is being waged by plants (that produce chemicals to repel attack), the insects that eat them, and the predatory species that eat toxic insects, Waldbauer says.
Hanging Out With The Crowd
There are other reasons besides defense against predators for insects to aggregate. Scientists try to distinguish between truly social insects (those who communicate) and those who come together “but aren’t paying attention to each other,” Costa says. Because they haven’t been studied extensively, many species “fall into a gray area between the two.”
Some insects appear in groups because they feed on a single plant species that isn’t very common. “That’s aggregating for resources,” he says.
Others aggregate to facilitate mating. This behavior is particularly important to short-lived species like mayflies, whose adult lives last a matter of hours, or to periodical cicadas, who only appear in adult form every 13 or 17 years. Male periodical cicadas form singing choruses to recruit females. (Non-periodical cicadas also sing, but not in choruses.) Interestingly, the sheer volume may afford the highly palatable periodical cicadas some protection against birds, both because it’s painfully loud and because it interferes with the birds’ songs and calls.
Insects sometimes aggregate to fight a plant’s defenses. The pine bark beetles currently decimating pine stands in the southern Appalachians are an example. The trees secrete resin that would drown the beetles in their tunnels under the bark, so the beetles launch a two-pronged attack. They release aggregation pheromones to recruit additional beetles, and inoculate the trees with fungal spores that further weaken the trees’ defenses.
Others feed cooperatively, among them milkweed bugs, who pierce the milkweeds’ thick seed pods to suck juices from the seeds. To break the seeds’ contents down, they inject them with saliva. Because a single bug can’t secrete enough saliva to break down the contents of a large dry seed, several bugs pool their salivary secretions and feed together on a single seed.
Insects also aggregate to protect themselves from the elements, and for climate control. Tent caterpillars emerge from their egg masses just as the first cherry leaves do and are vulnerable to frost. Temperatures inside the tent are higher than the ambient air, and the caterpillars orient tents so their broadest walls face the southeast, catching the morning sun. Basking on outer walls, they “can raise their body temperatures by about 30 degrees Celsius, greatly facilitating metabolism and growth,” Costa says.
Monarchs derive a measure of protection by overwintering in dense clusters. “They can survive sub-freezing temperatures if they don’t get wet,” Brower says. Studies he and associates conducted showed that the butterflies clinging to the trunks of the largest trees had the highest survival rates after winter storms. “Those trees are like hot water bottles. They’re warmer than the surrounding air. Their stored heat radiates through the butterflies’ bodies” — a tiny boost that may spell the difference between life and death.
Insects also aggregate to conserve moisture. That’s probably what those ladybugs are doing, packed tightly together in our attics all winter. They’re not eating (they live off stored fats and don’t have to feed), or mating. They may be keeping warm. But Waldbauer suggests that they’re protecting themselves from desiccation. Daddy longlegs, an eight-legged insect relative, conserve body moisture by packing together in huge, dense clusters, bodies together, legs out, until they resemble “a thickly-haired pelt.”
It’s Not All In The Family
“Funny you should call about tent caterpillars today,” Costa said, when I telephoned him in late March. “I just saw my first caterpillar tent on campus, and my students and I are about to set up an experiment.” He sounded happy, a rare reaction to the appearance of tent caterpillars.
Female tent caterpillars lay all their eggs in a single egg mass. So when researchers noticed that egg masses are often located in close proximity to one another, sometimes on the same twig, they knew that unrelated tent caterpillars were mixing together in tent colonies. “Usually, cooperation evolves in family groups,” Costs says. “So when we find non-relatives cooperating, as we do with tent caterpillars, it’s interesting.”
His current experiments involve collecting egg masses and creating colonies of 100 caterpillars each in the lab, using pairs of egg masses, then planting the colonies in the field. Each colony contains 100 caterpillars: one with all the caterpillars from one egg mass; one with all caterpillars from the other; and one with 50 caterpillars from each egg mass.
“We’re controlling for colony size, comparing single and mixed families, then watching them in the field to see whether mixing non-relative caterpillars has any effect on growth rate or survivorship. This is the second year of this experiment. We got some interesting results last year. It looked as though mixed family groups outperformed single family groups,” he says.
“The last couple of years have been exciting, a whole new range of experiments,” he says.