Sky Island Fireflies

By Stephen Hummel, Board Member

Fireflies are not an insect species commonly associated with the desert. They are typically found in humid, dark, wooded environments typically found in the eastern half of the United States and elsewhere. Relatively few fireflies can be found in the dry conditions of the western United States. One exception is the Sky Island Firefly, only known to live in the Davis Mountains of West Texas.

The Sky Island Firefly, or Photuris Flavicollis, may have once been seen as far as New Mexico according to reports dating back over one hundred years ago. Since that time its range has steadily shrunk due to oil and gas development in the Permian Basin, extended periods of drought, loss of suitable ground cover due to overgrazing, and spread of light pollution. Today they are only known to survive in the thicker vegetation and cooler temperatures the Davis Mountains provides. The combination of numerous threats and reduced range have resulted in the species being listed on the International Union for the Conservation of Nature (IUCN) Red List for Threatened Species. Despite the status, small populations of fireflies can still be found relatively easily in the Davis Mountains along Highway 118 in June through August. The dark conditions, spring-fed water sources, and mostly untouched habitat provide a refuge for the fireflies.

Fireflies are one of the insect species most vulnerable to light pollution. While an alarming number of insects worldwide are in sharp decline, with light pollution being a critical factor, fireflies are in a unique group. Many insect species are attracted to artificial light, particularly blue light, to the point that they may become disoriented or even exhaust themselves to death attempting to get closer to the source. Fireflies, however, do not simply fly towards any source of artificial light like many moths do, and seem to largely ignore blue lights or are not able to perceive them. Instead, they are only attracted to lights if they blink in a unique pattern and are a green or yellow color, similar to a potential mate. Many firefly species have unique flash patterns in an attempt to only attract other fireflies of their same species. Fireflies of the genus photuris are nicknamed “femme fatale” lighting bugs because the females mimic the flash patterns of other firefly species in order to lure them in and eat them! The Sky Island Firefly is one of these femme fatale species, although the likely prey more upon smaller insects more than other fireflies.

The phosphorescent flash of a firefly’s abdomen is quite dim, coming in a just a fraction of a single lumen. By contrast, even a modest night-light found in a bedroom may be 50 or more lumens, and a household security light may emit 1500 or more lumens. It doesn’t take much ambient light to overpower a firefly’s flash and make it difficult for them to detect each other. A recent study in Brazil found that the closer you get to light sources, the fewer fireflies are present. Worse, another study concluded that because most fireflies are nocturnal and only flash at night, if conditions are too bright they may be fooled into thinking it is still daylight and refrain from flashing at all, or reduce the rate of their flashes.

The habitat, behavior, and population of Sky Island Fireflies remains poorly understood, but scientists from the Xerces Society are working to learn more about this species. You can help by reporting observations of fireflies to FireflyAtlas.org. Observations of Sky Island Fireflies are highly valuable, but any firefly sighting can be reported.

You can also help protect nocturnal animals and insects by shielding outdoor lights, aiming them down, using amber or red lights instead of white or blue colored lights, and turning lights off when they are not needed. You can learn more at BigBendDarkSkyReserve.org/lighting.