An army of herbivores is chewing its way through our woodlands. But the soldiers in this plant-animal battle largely remain unseen. The combatants emerge under cover of darkness to wage war, or otherwise employ all manner of trickery to camouflage themselves. These ploys work: how many caterpillars do you see?

The quantity of caterpillars in Ohio’s forests and other habitats is stunning. Their collective biomass – total quantity – far exceeds that of a much larger, more conspicuous plant-eater, the white-tailed deer. That’s saying something when one considers that at least 500,000 deer roam our landscape, each at an average weight of 150 lbs. (adult female). It takes billions of caterpillars to outweigh 37,250 tons of white-tails!

Caterpillars are stage two of a moth or butterfly’s four part life cycle: egg, caterpillar, cocoon (moth) or chrysalis (butterfly), and winged adult. For most species, the caterpillar stage is the longest lasting. A caterpillar is essentially an eating machine. Some of the big ones may increase size several hundred fold through their various growth stages. In some moth species, the caterpillar may eat and grow for several months, while its adult form will be lucky to survive a week.

Ohio supports an estimated 2,500 species of moths, while less than 140 butterfly species have been found. Thus, moth caterpillars exceed those of butterflies by a staggering margin. While showy day-active butterflies are better known than most moths, one could argue that it is the often drab moths that do the ecological heavy lifting.

A caterpillar is a tubular bag of protein. Nearly all of our migratory songbirds, such as warblers, vireos, and tanagers are major consumers. The “cats” are so important to songbirds that our forests would fall silent were caterpillars to vanish. For instance, a red-eyed vireo – one of Ohio’s most common forest birds – consumes thousands of caterpillars during the five months it is in the Buckeye State. As nearly one million red-eyed vireos are thought to breed here, the numbers of caterpillars necessary to fuel them takes on staggering dimensions. Billions of caterpillars are eaten by birds alone.

It isn’t just songbirds that pursue caterpillars. Two of their grimmest enemies are flies and wasps. Tachinid (tak-in-id) flies are an enormous group, with an estimated 1,300 North American species, and there are even more braconid (bra-con-id) wasps. These insects are parasitoids, and that’s a lot worse than a parasite. A parasite is an annoyance, such as a mite or tick. Parasitoids typically kill their hosts, and in grisly sci-fi horror flick fashion.

When a female fly or wasp spots a suitable victim, it lays its eggs on the caterpillar, or inserts them into its tissues. Tiny grubs soon hatch, and begin feeding on the caterpillar’s innards. By the time the parasitoid larvae have matured, they’ve eaten the caterpillar alive, leaving little more than a hollowed out husk. When braconid wasp grubs burst from the victim, they create little white cylindrical cocoons that bristle from the victim’s body. Gardeners often encounter tobacco hornworm caterpillars on their tomato plants that have been hit by wasps.

All of these predators are vital to controlling caterpillar populations. One might think of our native plants as the fuel that grows a vast crop of caterpillars – so many that if they went unchecked our woodlands would be defoliated. But in an exquisite balancing act, predators cull the excess, which in turn gives life to myriad species higher up on the food chain. The tiny percentage of caterpillars that successfully run the gauntlet of predators morph into winged adults, reproduce, lay scores of eggs and begin the cycle anew.

Caterpillar diversity is amazing, and many species are as showy as exotic coral reef fish. Nearly everything about a caterpillar’s looks is an evolutionary adaption to help it avoid predators. The brightest and most colorful tend to be poisonous, their bodies permeated with toxins uploaded from the plants that they eat. Enemies quickly learn to shun them. Others look like twigs or leaves, and depend upon remarkable camouflage to hide from predators. Some caterpillars are heavily armed – they are covered with stinging spines and can deliver a mean punch if mishandled.

Turnabout is fair play, and plants have ways of dealing with caterpillars. The botanical crowd defends themselves chemically. Most plants produce compounds that prevent most species of caterpillars from consuming them. But there will always be some “cats” that have evolved the ability to digest even the foulest plant juice. A great example of caterpillar/plant chemical warfare involves milkweed plants. Their gooey white sap is full of nasty cardiac glycosides and highly unpalatable to nearly everything. But a few caterpillars, most notably those of the monarch butterfly, have learned to assimilate milkweed sap and thrive on these plants.

People are beneficiaries of plant/caterpillar battles. We find scores of uses for the chemicals that plants create to fend off their tubular foes. Many spices and herbs are tasty because of their chemical deterrents. Caffeine? It’s another anti-caterpillar compound developed by plants. Rubber? Now it’s mostly synthesized, but the stuff we all drive on was originally created from a thick latex designed to gum up caterpillar mandibles, courtesy of the rubber plant.

While caterpillars are largely out of sight and out of mind, their role in the environment is as enormous as their collective numbers. Were it not for these little bags of goo, and the plants that nourish them, the world as we know it would be dull indeed, minus the brilliant butterflies, cheerful birdsong, and delightful aromas and flavors of summer. - Jim McCormac, Ohio Division of Wildlife