Orange and black colors warn predators not to mess with this beauty.
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 four 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 four 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. What is all of this chemical trickery about?
A monarch caterpillar tanks up on cardiac glycosides with every bite.
Caterpillars of the monarch butterfly, Danaus plexippus, obtain cardiac glycosides from feeding on milkweed and retain them as they develop into a butterfly. 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 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. Like monarchs, many other 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 insect species that share a common predator is called Müllerian mimicry.
After a hearty meal of milkweed leaves, this milkweed leaf beetle is ready to lay some eggs.
A second member of this mimicry ring is a delightful insect called the milkweed leaf beetle, Labidomera clivicollis. Adults and larvae of this gorgeous insect eat leaves of milkweeds. Beetles first appeared in mid-summer and removed large slices of the leaves of my milkweed. After dining for a few days, females laid eggs that hatched into rather handsome larvae. These tiny orange cows grazed on my plants until late summer, then dropped to the earth to form pupae in the soil. By September a fresh batch of adult beetles had emerged and colonized the milkweed to fatten up on tasty leaves before seeking a protected refuge to survive winter somewhere in my garden.
Another member of the milkweed gang is the large milkweed bug, Oncopeltus fasciatus. In late summer my milkweeds were colonized by milkweed bugs. 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.
Milkweed bug adults and nymphs jostle for feeding sites on a milkweed pod.
Caterpillars of the milkweed tiger moth resemble furry dust mops as they wander about the plant.
Our final guests at the milkweed banquet are larvae of the milkweed tiger moth, Euchaetes egle. Like the larvae of the monarch, caterpillars of the milkweed tiger moth obtain cardiac glycosides from milkweeds and retain them as adult moths. 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 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.
This Halloween, dress in orange and black, make some ultrasonic clicks, and predatory birds and spooky bats will certainly leave you alone!
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. This week we salute our web editor Chris Sargent and web designer Kris Keochinda who assist in bringing Bug of the Week into your home, office, notebook, or cell phone.
To learn more about this week’s cast of creepy creatures, please visit the following websites:
http://bugguide.net/node/view/2970
http://lhsfoss.org/fossweb/teachers/materials/plantanimal/milkweedbugs.html