The origins of Halloween began in the time of ancient Celts when summer ended and the time of harvest gave way to approaching winter. The start of the Celtic New Year on November 1 was a scary time as spirits of the dead returned to earth on the eve of October 31 to visit family and engage in frightening tricks. Orange and black became the colors of Halloween, orange representing the color of harvest and later the Jack O’ lantern, and black symbolizing the color of death. How fitting it is that we visit three beautiful but potentially deadly insects that dress in orange and black to send a warning to would-be predators, “eat me at the peril of death.” All of the insects in this Bug of the Week are common denizens of the remarkable milkweed plant.
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 such as 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 that eat them. However, insects that eat milkweeds are clever and all three amigos in this episode 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, butterflies, and bugs. 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. A second member of this mimicry ring is the large milkweed bug. Nymphs and adults of this bug cluster on developing leaves and seed heads in summer and fall. As members of the seed bug clan, milkweed bugs insert a long slender beak into the ripening seeds within the developing pod. After injecting digestive enzymes into the seed, they suck liquefied food through the straw-like beak into their gut where nutrients will be used for growth, development, and reproduction. During her lifetime, the female milkweed bug may lay up to 2000 eggs. Small orange and black nymphs hatch from the eggs and eat seeds of milkweed. Like other milkweed feeders milkweed bugs obtain poisonous compounds from milkweed that are used for defense.
Our next guests at the milkweed banquet are larvae of the milkweed tiger moth. 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 is somewhat less showy with brown wings and an orange abdomen. 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.
The lesson here for Halloween costumes is to dress in orange and black, make some ultrasonic clicks, and predatory birds and spooky bats will certainly leave you alone. Have a Happy Halloween.
References
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.
For more information on monarchs, milkweed bugs, and milkweed tiger moths, please visit the following web sites:
http://www.monarchwatch.org/
http://www.life.uiuc.edu/ib/109/Insect%20rearing/milkweedbug.html
http://www.texasento.net/Euchaetes.htm