History of a giant silk moth in the city
Ailanthus silk moths (Samia cynthia) in the insect collection of the Academy of Natural Sciences of Drexel University (Courtesy of the Academy).
The ailanthus silk moth, also known as the cynthia moth, is a giant silk moth indigenous to China and introduced in 1860 into Philadelphia. Its wingspan may reach 15 centimeters (5.9 inches). Its favorite host plant is tree of heaven (Ailanthus altissima), native to Asia. This tree thrives in harsh urban landscapes.
The moth was introduced to establish a domestic silk industry, but this plan never materialized. The moth escaped cultivation and naturalized. It proliferated in Philadelphia and expanded its range to other cities in North America; however, by the end of the twentieth century, it had disappeared from Philadelphia and the other cities where it had once been abundant.
Cabbage white (Pieris rapae).
In a paper I wrote in 1986, I considered several causes for the disappearance of the moth in Philadelphia. Almost thirty years later, in my book Ecology of Center City, Philadelphia (2015), I focused on a singular cause: introduction of a European butterfly, the cabbage white, which hosts a parasite that attacks the moth. The cabbage white butterfly was introduced into North America about the same time as the moth. It has since become one of the most abundant butterflies on the continent, and it is the species of butterfly seen most often in downtown Philadelphia.
Many of us miss this beautiful giant silk moth. Old books on moths of North America present colorful illustrations showcasing this moth. But the moth had survived in cities only because its parasites had yet to establish themselves in them. For this giant silk moth, cities functioned transiently as sanctuaries. As these cities aged, parasites and their hosts moved in, eliminating the protection that cities had previously afforded.
In 2019 David Moskowitz reported a remnant population of this moth in North America in the New Jersey Meadowlands. (Moskowitz D. 2019. News of the Lepidopterists’ Society 61:200-204).
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