Last week my neighbor sent a fine image of a caterpillar resembling Cousin Itt of Addam’s Family fame. The shaggy caterpillar was comically cloaked in black, orange, and white tufts of hair. Hordes of these leaf-munchers have been discovered feeding on milkweed leaves over the last few weeks. With gardeners from coast to coast striving to help our imperiled monarch butterflies find food for their young by planting more milkweed plants, how concerned should we be about these gregarious interlopers horning-in on food for monarch caterpillars? Who are they and what should we do? These caterpillars are the offspring of a species of moth known as milkweed tussock moth or milkweed tiger moth. Before we rage on these rascals, let’s have a little course in milkweed plant and milkweed caterpillar biology.
Milkweed gets its name from the sticky white sap exuded from stems and leaves when their surface is broken by hungry insects or curious humans. Milky sap and cells within the leaves contain nasty chemicals called cardiac glycosides. As the name implies, these compounds have something to do with the heart. At higher concentrations, cardiac glycosides can be heart poisons, bringing death to humans and other animals foolish enough to eat them. However, many insects that eat milkweeds have evolved mechanisms to deal with these toxins and have the ability to consume leaves of milkweed without being poisoned. In fact, they obtain cardiac glycosides from their food and store these noxious compounds in their bodies. Caterpillars of both the monarch butterfly and milkweed tiger moth obtain cardiac glycosides and retain them as they develop into a butterfly or moth, respectively.
Milkweed tussock moth caterpillars devour leaves of milkweeds. Orange and black coloration warns predators not to mess with them.
What is all of this chemical chicanery about? Birds are important predators of many kinds of insects, including caterpillars and butterflies. Scientists discovered that cardiac glycosides found in monarch butterflies caused predators such as blue jays to vomit dramatically following an attempted monarch meal. Blue jays exposed to monarchs soon learned to recognize the monarch by sight and avoided eating these beautiful, but nasty tasting butterflies. Many of the insects that live on milkweed and consume its leaves display vivid patterns of orange and black as both juveniles and adults. This convergence on a similar, easily recognizable color pattern by two or more nasty-tasting insects is called Müllerian mimicry. Other milkweed feeders that participate in the milkweed mimicry ring include milkweed longhorned beetles, milkweed bugs, and milkweed leaf beetles we met in previous episodes. Like the larvae of the monarch, caterpillars of the milkweed tiger moth obtain cardiac glycosides from milkweeds and retain them as adults.
While the caterpillars of this tiger moth boldly wear the characteristic warning colors of orange and black as they feed during the day, the adult moth is comparatively drab at first glance with pale brown wings, but its impressive abdomen sports Halloween colors of orange and black. The fact that caterpillars of the milkweed tiger moth store cardiac glycosides for use as adults is somewhat perplexing. Primary predators of these night-flying moths are fearsome bats that hunt using sound rather than sight to locate prey. Orange and black coloration may have little value in defeating these night-hunting predators. However, the cardiac glycosides stored in the body of the moth are put to good use. The resourceful milkweed tiger moth evolved an organ that emits an ultrasonic signal easily detected by bats. The signal warns that an attack will be rewarded with a noxious distasteful meal and bats soon learn to avoid the tiger moth as prey.
What can you do to preserve your milkweeds as a food for monarchs? Well, you really don’t want to reach for pesticides to do away with any leaf-eaters that may have come to dine on your milkweed. Although we endeavor to help our monarchs, remember that these tussock moths are an important part of our natural ecosystems too. They have their own complement of predators and parasites that depend on them as a source of food. If you are trying to enjoy monarchs dining on the milkweed patch in your garden or landscape, perhaps the best strategy is to simply collect your tussock moth caterpillars in a container and relocate them to the nearest patch of milkweeds in a nearby meadow.
Bug of the Week thanks Randy Taylor, Sam Taylor, Chris Sargent, and several Bug of the week viewers for providing the inspiration for this week’s episode. Two delightful references “Sound strategy: acoustic aposematism in the bat–tiger moth arms race” by Nickolay I. Hristov and William E. Conner and “Secret Weapons” by Thomas Eisner, Maria Eisner, and Melody Siegler provided valuable insights into the mysterious ways of this week’s stars.