Light pollution imperils Imperial moths, Eacles Imperialis

A few years ago, we met the remarkable caterpillar of the Imperial moth as it meandered along the scenic C & O canal trail near Sharpsburg, Maryland. We learned about the dramatic, disturbing decline of Imperial moths and other members of the silk moth clan including Royal Walnut moths and Cecropia moths in New England where populations of these gorgeous giants have been extirpated in several locations. There is growing evidence from around the world that several human related activities, including habitat destruction, agricultural intensification, pesticides, invasive alien species, climate change, and other factors acting independently and in concert contribute to these declines. Disappearances of several species of moths in New England are linked to parasitoid flies imported decades ago to help control dastardly gypsy moth caterpillars. Sadly, they also attack and kill several species of native moths, including some of our giants.

In late summer and early autumn, Imperial moth caterpillars depart their lofty feeding grounds in the canopies of trees. After a perilous journey across human-made structures like pathways and roads, they reach the safety of soil where they will disappear underground to form pupal chambers. With the return of warm weather and foliage on trees, beautiful adult moths will emerge from the earth to mate and deposit eggs on leaves.

Another factor linked to the decline of large, nocturnally active moths is sky brightness caused by artificial lights. High intensity street lamps and security lights on the outside of buildings are extremely attractive to night-flying moths. Many insects use distant light sources such as the moon and stars to navigate the nighttime skies. “Near” light sources, lights illuminating buildings and streets, disrupt the ancient navigation systems evolved over eons by nocturnal flyers. Rather than flying in one direction, they spiral inward to near light sources. As insects collect around these artificial illuminators, they may be exposed to predators, which pick-off moths and other night fliers. Next time you visit a nighttime sporting event in the summertime at a field illuminated by high intensity lights, check out the massive swarms of insects attracted to the lights. Don’t be surprised to see bats swooping in and out of the milieu for easy meals. Artificial lights also may disrupt other vital activities of nocturnal insects such as locating mates and finding correct places to deposit eggs.

Circling back to the raison d’etre for this episode was an encounter with a lovely but imperiled Imperial moth.  While stopping at a local coffee shop on a recent trip to Selbyville, Delaware, I spied a very cool Imperial moth woefully out of place. Beneath a high intensity light, clinging to the leg of a black plastic chair, an Imperial moth (gender undetermined) quaked in a morning breeze. While photographing the moth, a pair of caffeinated tourists stopped to see what all the fuss was about. Having pointed out the moth, out came the iPhones for a photoshoot and moth love fest. After agreeing on the moth’s beauty, they departed with the comment of how lucky they were to have seen the moth. Well, perhaps bringing nature to a parking lot where it can by witnessed by humans is reason enough for high intensity lights on buildings, but maybe this is not so good for moths.

High intensity security lights like one above this door attract many night-flying insects. Beneath the light, on the leg of a plastic lawn chair, a beautiful Imperial moth quivered in the morning breeze. Is it likely that a mate will be found in this alien environment? Not so much, but an inattentive footfall could bring an end to this pretty creature. The moth was rescued and released in a sweet gum grove far from artificial lights.  

How can we reduce problems associated with artificial light pollution? Scientists list a few things we all can do to help. Use motion detectors to activate outdoor lights whenever possible, instead of using lights that burn all night. Use directional covers above or around lights to illuminate only places that need light. Yellow colored lights attract fewer moths than white or blue lights (other species of insects may differ in their responses), so use longer wavelengths if you can. Scientists also recommend reducing the amount of artificial light in rural areas to help reverse losses of night-flying moths in these less human-altered landscapes. But the best piece of advice is to simply shut off lights at times and in places when they are not necessary.

What of the Imperial moth marooned on a chair near a coffee shop? Well, the chance of finding a mate on the leg of a plastic chair seemed infinitesimally smaller than that of being inadvertently trampled by a caffeinated tourist. Yes, we did violate the Non-Interference Directive of Starfleet. The moth was gently collected, placed in an elegant bug cage, one fit for royalty, transported to a grove of sweet gum trees far, far from a streetlight, and released. Good luck majestic moth.

Acknowledgements

Bug of the Week thanks Peter and Whitney of the Moth Project for inspiring this episode and Dr. Shrewsbury for photographing and wrangling the star of this story. Several great reads on giant silk moths and artificial light pollution include the following: “Light pollution is a driver of insect declines” by Avalon C.S. Owens,  Précillia Cochard, Joanna Durrant, Bridgette Farnworth, Elizabeth K. Perkin, and Brett Seymoure, “A window to the world of global insect declines: Moth biodiversity trends are complex and heterogeneous” by David L. Wagner, Richard Fox, Danielle M. Salcido, and Lee A. Dyer, “Declines in moth populations stress the need for conserving dark nights” by Frank van Langevelde, Marijke Braamburg-Annegarn, Martinus E. Huigens, Rob Groendijk, Olivier Poitevin, Jurriën R. van Deijk, Willem N. Ellis, Roy H. A. van Grunsven, Rob de Vos, Rutger A. Vos, Markus Franzén, and Michiel F. Wallis DeVries, and “The Devastating Role of Light Pollution in the ‘Insect Apocalypse’” by Jason Daley.

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