Gardeners from coast to coast include milkweeds in their landscapes to increase the supply of food for the imperiled monarch butterfly we met in previous episodes. Over the past week or two several viewers have commented on hordes of hairy black, orange, and white caterpillars munching on their milkweeds. They fear these very hungry caterpillars known as milkweed tussock moths or milkweed tiger moths will leave little food for larvae of monarchs, which are just beginning to make their presence known in the Washington metropolitan area. Who are these hairy caterpillars, why do they feed on milkweeds, and what can be done when they appear?
With the arrival of monarch butterflies in the Washington area, magnificent monarch caterpillars will soon appear in meadows and gardens to munch milkweeds.
First, let’s have a little course in milkweed 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 act as 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 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.
Milkweed tussock moth caterpillars devour leaves of milkweeds. Orange and black coloration warns predators not to mess with them.
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 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 visually-hunting 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.
While milkweed longhorned beetles and milkweed bugs do little to reduce the amount of foliage available for monarch caterpillars, milkweed tussock moth caterpillars and milkweed leaf beetle larvae often consume notable quantities of leaves.
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. This would harm monarch caterpillars as well. Instead, as rudimentary as it may seem, the best control is to simply don gloves, pick off the offending insects, and drop them into a container of soapy water. Not fancy, but it gets the job done! If the thought of destroying these marvelous but noisome creatures does not sit well with you, then simply collect them into a container and relocate them to the nearest patch of milkweed in a meadow or other uncultivated plot of land.
Bug of the Week thanks Chris Sargent for developing the concept and draft of this week’s episode. We thank several Bug of the Week viewers who sent queries 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” byNickolay 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.