One of my favorite insects decked out in Halloween colors of black, orange, and white is the shaggy tussock moth caterpillar, which dines on members of the milkweed plant family. Every time I see one, it reminds me of the irascible creature Cousin Itt, of Adam’s Family fame. Hordes of these leaf-munchers are often seen in late summer and autumn consuming leaves of milkweeds. These caterpillars are the offspring of a species of moth known as the milkweed tussock moth or milkweed tiger moth. To understand why they present a scary meal to would-be predators, let’s review a little bit of the biology of milkweed plants and milkweed-feeding caterpillars.
Milkweed gets its name from the sticky white sap exuded from stems or 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 animals like horses that eat the plants containing 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 then store these noxious compounds in their bodies. Caterpillars of both the monarch butterfly and milkweed tussock moth obtain cardiac glycosides and retain them as they develop into a butterfly or moth, respectively.
Milkweed tussock moth caterpillars devour leaves of milkweeds. As they feed, toxic cardiac glycosides found in milkweeds are ingested and stored in their bodies. Caterpillars become a noxious meal. Their 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 tussock 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. Nevertheless, its impressive abdomen sports the Halloween colors orange and black. The fact that caterpillars of the milkweed tussock 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 tussock moths as prey. For many bugs in orange and black there are no treats for hungry predators.
Adult milkweed tussock moths sport Halloween colors and warn bats of their distastefulness with a spooky sound.
Acknowledgements
Bug of the Week thanks 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 star.