Why Insects Are Lured to Lights in the Night

The enduring image of a moth frantically circling a nocturnal light source—whether candle, campfire or electric bulb—has long intrigued both scientists and literary types, including Shakespeare. (“Thus hath the candle singed the moth,” Portia quips in The Merchant of Venice.) Entomologists have mulled possible explanations for insects’ attraction to all forms of artificial light. The creatures, some have suggested, are drawn to a flame’s heat, or they mistake fire or electric light for the moon, which is assumed to act as a kind of celestial compass.

In perhaps the most intensive study to date attempting to answer this question, researchers claim to have come up with “the most plausible model for why insects gather at artificial lights,” as reported in Nature Communications. Investigators at Imperial College London, Florida International University and the Council on International Educational Exchange discovered through fieldwork and laboratory experiments that insects grow increasingly disoriented around artificial light—so much so that they lose all perspective about which way is up. It turns out that insects ordinarily maintain their up-down orientation by turning their backs toward the sky, the brightest thing they perceive even at night. That allows them to stay properly aligned on a steady flight path.

This evolutionary strategy sufficed for many millions of years, until humans came on the scene with their fire and electricity. When insects encounter an outdoor bulb after dusk, confusion reigns. They tilt their backs toward the bulb as if it were the sky and initiate endless, sometimes erratic loops around it. With light pollution increasing and with drastic declines in insect numbers worldwide, new forms of lighting may be needed to preserve these populations, which are integral to the health of global ecosystems.

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WITH THE SUN AT YOUR BACK

The tendency of insects to turn their backs toward the light is called the dorsal light response. Bigger animals such as humans can tell up from down based on the pull of gravity sensed directly by the inner ear, among other inputs. Insects’ minute sensory organs and their rapid aerial accelerations prevent a moth or a wasp from distinguishing immediately above from below. As a result, they use the sky’s brightness as a constant that allows them to self-orient by pointing their backs to the heavens—or at least they did before the arrival of human civilizations that always keep the lights on.

LIGHT AND FLIGHT PATHS

The sustained banking motion that a moth maintains to keep its back to the light results in a relatively unperturbed, orbiting flight path around the lightbulb. At times, though, the moth ends up flying under the bulb and begins a steep upward climb. The insect then begins to stall, losing speed as it climbs before crashing down. Similarly, when the moth flies over the bulb, its inverted orientation at the apex of its flight path can send it plummeting earthward.

WHICH WAY IS UP?

To probe the validity of their back-to-the-light thesis, researchers in the lab at Imperial College London created two opposing scenarios and tested them using high-speed video. In one scenario, ultraviolet light shining from above (simulating the sky) enabled the moths to fly along a stable, linear path. In the other, UV light emitted from the floor caused the insects to tilt, fully invert and come crashing down.