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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

The silk moth summer continues: Promethea silk moth, Callosamia promethea


Adult Promethea are magnificent. They can often be seen in the morning resting on buildings or vegetation near sources of nighttime illumination. Photo credit: Karin Burghardt


We recently met the gorgeous Cecropia silk moth, which has been quite abundant this year. This week after a serendipitous visit to the Oregon Ridge Nature Center in Cockeysville, MD, we have the good fortune to meet yet another member of this remarkable clan, the Promethea silk moth. As you will no doubt recall, Prometheus was a Titan who had a special fondness for humans whom he created from clay. He defied the Olympian gods by stealing fire and giving it to mankind. For this naughty deed, Zeus sentenced the immortal Prometheus to be chained to a rock where an eagle would pluck out and consume his liver on a daily basis. How’s that for a bad “Groundhog Day”? Fortunately, during my impromptu visit the Nature Center was short on liver-plucking eagles, but long on gorgeous, almost fully developed Promethea caterpillars.

And when all the spice bush leaves are gone, Promethea caterpillars use powerful jaws to eat the green tissues surrounding a woody twig. Video credit: Mike Raupp

Promethea is found in the eastern half of North America where it ranges from southern Canada to northern Florida. Like other giant silk moths, adults do not feed. All of the nutrients needed to grow and develop are obtained by the caterpillar. On the menu are a variety of plants including spicebush, sassafras, tulip tree, lilac, sweetbay, ash, and several others. As with Cecropia, male moths are attracted to the females by an irresistible sex pheromone that enables them to detect a mate among the leaves and vegetation of the forest. Once inseminated, the female deposits eggs in clusters of 4 – 10 on the top surface of a host plant’s leaves. Young caterpillars are a chummy lot and feed together. As they age and grow, caterpillars become solitary consumers. After completing larval development, the caterpillar strengthens the attachment of a leaf to a branch by adding silk to the leaf’s petiole. It then folds the leaf around itself and forms a cocoon, in which it pupates. In the northern part of its range Promethea spends the winter in this cocoon, emerging in spring when trees are flush with leaves. In the southern part of its range, the first generation of Promethea completes its life-cycle in late spring and a second generation of moths emerge, mate, and lay eggs which hatch into caterpillars that complete development just in time to pupate for the winter.

A caterpillar’s life is all about change, not only in size but in form and color. Video credit: Karin Burghardt and Paula Shrewsbury

As with Cecropia we met earlier, it appears that Promethea is declining in some parts of the country, including New England where it is thought that the gypsy moth parasite Compsilura concinnata attacks and kills caterpillars of several species of moths. Nonetheless, here in Maryland early summer continues to delight bug-lovers with a surprising bounty of silk moths. Hope you spot a few.


Bug of the Week thanks Jessica and the delightful staff of the Oregon Ridge Nature Center for providing the caterpillar paparazzi with an opportunity to film and photograph Promethea caterpillars. We thank Karin Burghardt and Paula Shrewsbury for cool images of adult and larval Promethea. The extraordinary website “Butterflies and Moths of North America: Collecting and sharing data about Lepidoptera” was used as the primary reference for this episode.