A waspy appearance and warning coloration may help this beautiful moth escape sharp beaks of hungry birds.
With the arrival of a new year and the Northeast firmly embraced in winter’s chill, Bug of the Week begins its annual sojourn to warmer places to meet fascinating insects from other parts of our country and the world. This week’s stop brings us to the eastern coast of southern Florida to meet one of the most spectacular moths in the insect realm, the polka-dot wasp moth. While many tasty insects like lantern flies depend on cryptic coloration to avoid the hungry jaws of predators, the spectacular iridescent colors of the polka-dot wasp moth shout, “Here I am, eat me if you dare.”
When colonists brought oleander to the New World, this plant of Mediterranean origin became an important larval food source for the polka-dot wasp moth.
How then does this scaly-winged harlequin escape the sharp beaks and pointy teeth of vertebrate predators? Two schemes are at play. First, the general resemblance of this non-stinging moth to a stinging wasp probably brings pause to hungry birds that have learned to avoid making meals of painful, stinging insects.
The second defense lies in the food consumed by the moth in its youth. Larvae of the polka-dot moth are known as oleander caterpillars. Their diet consists of leaves of members of the oleander family, the Apocynaceae. In its native range of Florida and Georgia, this orange and black beauty consumes leaves of its aboriginal host, devil’s-potato, a member of the Apocynaceae clan of plants that are well known for containing highly toxic compounds called cardiac glycosides.
A spotted oleander caterpillar prepares to join its siblings in the act of pupation beneath an overhang of an apartment building.
We were introduced to cardiac glycosides in Apocynaceae and related plants in previous episodes where we learned of the abilities of dogbane beetles and monarch butterflies to acquire these compounds and use them as a defense against predators. Like their cousins the monarchs, oleander caterpillars ingest and store cardiac glycosides that are passed along to the adult stage where they serve as dietary punishment for predators attempting to eat the glycoside-laced adults.
The contrasting pattern of black dots on a bright orange background conveys a strong visual image and a reminder that dining on this caterpillar can result in disagreeable digestive consequences. Striking warning colors and patterns deterrent to predators is called aposematic coloration, a clever strategy employed by many types of insects.
Oleander caterpillars are easily distinguished from spotted oleander caterpillars by spots and hair tufts lining their bodies. Oleander caterpillars have dark spots and black hair tufts.
The female polka-dot wasp moth truly is gorgeous and it is little wonder that males of this species find such a beauty irresistible. But finding a mate is a challenge for small organisms like insects in big world. Many female moths and other insects solve this problem by using chemical attractants called pheromones to signal their willingness to play the mating game. Most female moths release pheromones from specialized abdominal glands that help guide otherwise haplessly searching males to their connubial reward.
Whereas spotted oleander caterpillars have light colored spots and reddish-brown hair tufts.
However, the female polka-dot moth relies on yet another strategy to attract her mate. By vibrating plates called tymbals on the sides of her thorax, the female moth creates a rhythmic clicking sound, a kind of a mothy “yoo-hoo” to attract a suitor. In return, the male adds his own clicks to create an ultrasonic duet. Although their love songs are beyond the range of the human ear, if the lucky couple harmonizes successfully, the result of their union may be dozens of tiny orange caterpillars decorating oleander.
If travel brings you to the sunny climes of Florida or the Caribbean in the near future, be sure to visit an oleander or two and perhaps you will catch a glimpse of the wonderful polka-dot wasp moth or its offspring, the aposematic oleander caterpillar.
References
The web page listed below and two fascinating articles, “Cardiac glycosides (heart poisons) in the polka-dot moth Syntomeida epilais Walk. (Ctenuchidae: Lep.) with some observations on the toxic qualities of Amata (=Syntomis) phegea (L.)” by M. Rothschild and her colleagues, and “Courtship sounds of the polka-dot wasp moth, Syntomeida epilais” by M. Sanderford and W. Conner, were used to prepare this episode. To learn more about the polka-dot wasp moth, please visit the wonderful ‘Featured Creature’ article by H. McAuslane, “Oleander caterpillar, Syntomeida epilais Walker”, at the following website:
http://entnemdept.ufl.edu/creatures/orn/oleander_caterpillar.htm