Giant Silk Moths in Cities
Polyphemus moth (Photo by Dustin Minialoff, iNaturalist.org, CC0)
Giant silk moths (Saturniidae) are among the world’s largest and most charismatic insects, with 28 species native to the eastern United States. Many species of giant silk moth here have been reported as declining, with one remarkable exception: the polyphemus moth (Antheraea polyphemus). It has been reported as increasing. One hypothesis to explain this surprising increase is that the polyphemus moth is uniquely tolerant of urbanization.
Compiling observations recorded on iNaturalist.org, I found that observations of the polyphemus moth exceeded those of all other giant silk moths combined. Observations of this moth inside cities exceeded those of this moth outside cities. For all other species of giant silk moth studied, the findings were the opposite: Observations inside cities were fewer than those outside. In addition, outside cities, the other giant silk moths as a group outnumbered the polyphemus moth. The cities included in this study were New York, Philadelphia, Baltimore, Washington and Chicago.
An exception to these findings occurred in the borough of Staten Island in New York City. In Staten Island, observations of the other giant silk moths as a group outnumbered those of the polyphemus moth. On Staten Island, the most common giant silk moth was not the polyphemus moth but rather the luna moth.
Luna moth (Photo by Lynn Harper, iNaturalist.org, CC0)
Human population density and impervious surface are lower on Statin Island than in every other borough of New York City. Staten Island has expansive natural forests. Boosted by high quality habitat, the other giant silk moths on Staten Island presumably flourished, increasing in abundance relative to the polyphemus moth. This finding demonstrates that conservation of suitable habitat can protect giant silk moths even in a megalopolis.
A unique combination of traits may explain the urban success of the polyphemus moth. They include:
Caterpillars that eat a wide range of plants.
Cocoons spun both in trees and on the ground.
A big wingspan––enabling long distance dispersal across hostile cityscapes.
Together, these traits may have prepared the polyphemus moth to cope with urban adversity. However, the success of the polyphemus moth compared to other giant silk moths in cities remains somewhat mysterious. Adaptive traits that account for the urban success of the polyphemus moth may yet be discovered.
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