Last week we visited royalty as we met the magical monarch and its larval host the milkweed plant. This week, Bug of the Week was inundated with questions about another caterpillar munching on milkweed: the milkweed tussock moth, also known as the milkweed tiger moth.
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, including toxic 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.
What is all of this chemical trickery 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. Like the larvae of the monarch, caterpillars of the milkweed tiger moth obtain cardiac glycosides from milkweeds and retain them as adults.
Orange and black warns predators not to mess with milkweed tussock moth caterpillars.
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 a comparatively drab pale brown moth. The fact that caterpillars of the milkweed tiger moth store cardiac glycosides for use as adults is somewhat perplexing. Their primary predators are fearsome bats that hunt at night using sound rather than sight to locate prey. With few sighted predators, orange and black coloration has little value. 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.
Bug of the Week thanks several of Maryland’s keen Master Naturalists who sent images and questions about milkweed tussock moths that served as the inspiration for this 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.