Probing Question: Why are moths attracted to light?

moth in green lava lamp
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What would a summer evening be without moths? They flutter around our campfires and beat their powdery wings against our lampshades. They congregate by streetlights and frequent torch-lit garden parties. But what is it about the lamp on your porch that moths find so irresistible? Is it the warmth? The pleasing glow? Why are moths attracted to light?

According to Mike Saunders, the answer is simple: They're not.

"Moths often use the moon to orient themselves during night flight," explains Saunders, a professor of entomology at Penn State. In visual terms, the moon appears at "optical infinity," i.e., far enough away that the rays of light it reflects toward Earth are parallel as they enter a moth's (or a human's) eye. This constant makes an excellent navigational tool. "Using the moon as a reference, moths can sustain linear flight in a given direction."

But technology has been unkind to the moth. "Artificial lights seem brighter than the moon," Saunders notes, "and moths end up orienting to them even though the artificial light is not at optical infinity." The moon remains safely out of reach, but a candle or lamp is a different story. As the moths get closer to the light, their ability to triangulate is thrown off.

Says Saunders, "Maintaining a constant frame of reference to the artificial light results in the moth circling the light over and over again." So the moth winging around your kitchen light is doing so more out of confusion than desire.

As to why moths tend to remain close to a light source once they have reached it, Saunders speculates that the poor creatures simply tire themselves out. "If they spend significant time circling the light, they may exhaust themselves, quit flying, and rest on nearby objects."

Some researchers have suggested that, having reached a brightly lighted spot, a nocturnal moth is tricked into thinking the sun is out, and settles in to sleep.

Another theory holds that candle flames emit wavelengths similar to those of female moth pheromones, attracting male moths intent on romance.

Yet another attributes the moth's seeming fondness for light to the search for food. Most moths are nocturnal, and many feed on the nectar of flowers, which often reflect ultraviolet light. They may mistake artificial ultraviolet light for a potential food source. "It is certainly possible that night-blooming flowers are detected by moths as a function of reflected moonlight," says Saunders. But nocturnal moths have other means of finding their nighttime meals, he adds. "Recent research indicates that the moth is capable of detecting high levels of CO2 being emitted by flowers. It is believed that these high CO2 levels signal increased metabolic activity in the flower, which may tip off the moth to the presence of nectar."

While these explanations may account for some light-seeking moth behavior, the vast majority are drawn to light due to navigational snafus, says Saunders. "One caveat," he notes, "is that there are many, many species of moth. Not all moths are nocturnal fliers, and maybe there are moths out there that are genuinely attracted to lights. But in general, it's just not true."

Mike Saunders, Ph. D., is professor of entomology in the College of Agricultural Sciences. You can reach him at mcs5@psu.edu.

Last Updated October 20, 2008