Very Crafty Caterpillars

Pollinator survival strategies

Butterflies are the charismatic megafauna of the insect world. Who doesn’t admire the stripes on an Eastern tiger swallowtail (Papilio glaucus), the eye spots on the Polyphemus moth (Antheraea polyphemus), or the amazing journey of the Monarch (Danaus plexippus), flying from Canada to Mexico to return to a place only known to its great-grandparents?

Their effortless flitting from flower to flower performs an essential function, pollination, without which many plants would be unable to form fruits and seeds. This is important to natural ecosystems as well as croplands — though bees and other insects also play a significant role in pollinating native plants and crops.

monarch-caterpillars
The bright coloration of Monarch caterpillars warn of their toxicity as they feed on their host plant, milkweed. Photo by Mike Weatherford, courtesy of Arkansas Native Plant Society.

But what about the less showy part of a butterfly’s life cycle, the larval stage? Caterpillars have crafty ways that enable them to survive to become beautiful butterflies.

At a glance, caterpillars seem like “sitting ducks” for, well, ducks, other birds, and a host of predators. A closer look reveals that these innovative creatures are anything but helpless, easy targets. Caterpillars have a number of strategies for evading predators, in both appearance and behavior.

One strategy is to hide. Many caterpillars feed on the undersides of leaves or at night when fewer predators are active. This explains why we don’t see them as often as we think we should, given how abundant they are, and provides insight to keep those tomato hornworms (five-spotted hawk-moth/Manduca quinquemaculata) from giving you the slip while they munch on your prized nightshade crops: eggplant, potatoes, and tomatoes.

Another strategy is to hide in plain sight with camouflage. Many loopers look just like twigs or plant parts, but a particularly ingenious one is the camouflaged looper (wavy-lined emerald moth/Synchlora aerata). David Wagner, author of Caterpillars of Eastern North America, calls it the “Mardi Gras caterpillar” because it attaches bits of flowers and plant parts to its back to blend in with the plant on which is resides.

A third strategy is to be showy but toxic, as exemplified by the Monarch. Monarch caterpillars feed exclusively on milkweed (Asclepias spp.), plants which contain toxic cardiac glycosides. The Monarchs are not poisoned by these toxins, but instead sequester them in their wings and exoskeletons and basically become poisonous.

The caterpillars’ bright yellow, black, and white stripes are a warning to birds and other predators: eat me and you will die – or at least vomit. Susan Halpern noted in her book Four Wings and a Prayer that even the dumbest bird figures it out by the second taste.

giant-swallowtail
Giant Swallowtail larvae resemble bird poop to hide in plain sight from predators. Photo by Virginia McDaniel, USFS.

The strategies go on and on, from harmless caterpillars who mimic the toxic ones, to those that have armor of stinging hairs, to others whose “eye” spots make them resemble a snake — such as the spicebush swallowtail (Papilio troilus).

All these survival strategies are for naught if caterpillars don’t have a place to live and food to eat. Habitat loss, pesticide use, and non-native invasive species are the three main threats to butterflies and other pollinators.

These threats decrease the area available for native plants to grow and the diversity of those plants. Why are native plants so important? Most butterflies are host-specific, meaning they have a particular plant genus or family that their larvae need in order to survive.

You may notice some butterflies or their caterpillar larvae have names that are associated with plants. This is not coincidental, as butterflies are often named after their host plant. For example, one of the spicebush swallowtail’s common host plants is — you guessed it — spicebush (Lindera benzoin). Some caterpillars are specialists in one genus, while others eat a variety of plants, but the main theme is native.

spicebush-swallowtail
Spicebush swallowtail lays eggs on its host plant, spicebush. Photo by Mike Weatherford, courtesy of Arkansas Native Plant Society.

Doug Tallamy, author of Bringing Nature Home, reported that native oak trees (Quercus spp.) host more than 500 caterpillar species, whereas the non-native ginkgo tree (Ginkgo biloba) hosts fewer than ten caterpillar species. In a time when pollinator numbers are in decline, the millennia-long relationships between native plants and insects are increasingly important.

The more habitat destroyed for development, the more pesticides used, and the more invasive plants planted, the less habitat there is for native pollinators.

As pollinators decrease, so does the food supply of the natural world. More than 75 percent of global food crops depend on pollinators — including almonds, apples, berries, chocolate, coffee, and grapes.

While the very hungry caterpillar in Eric Carle’s book ate everything from strawberries to lollipops, it felt best after eating a plain green leaf from its host plant — probably the moonseed (Menispermum canadense). Making sure our very hungry native caterpillars have lots of native host plants to feed on will allow them to grow up into brilliant butterflies that make food for the world.

For more information, email Virginia McDaniel at vmcdaniel@fs.fed.us.

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