Moths are strongly attracted to light, a phenomenon that is less a conscious attraction and more a catastrophic navigational failure. The exact reason for this behavior has been a long-standing scientific puzzle, one that has been largely clarified by recent high-resolution studies of insect flight mechanics. This seemingly illogical flight toward danger is a maladaptive response where modern artificial lighting interferes with an ancient, ingrained biological process. This miscalculation significantly disrupts the nocturnal environment, impacting moth populations and the wider ecosystems they support.
Why Moths Navigate Using Light
The most accepted scientific hypothesis suggests that moths navigate using transverse orientation, or celestial compass navigation. This method involves maintaining a constant, fixed angle relative to a distant celestial light source, such as the moon or stars, to ensure a straight flight path across long distances. Because the moon is so far away, its light rays are parallel, allowing the moth to fly great distances while keeping the light at the same angle to its body.
The problem arises when the moth encounters a much closer, powerful artificial light source, like a porch lamp or street light. Unlike the distant moon, the light rays from an artificial lamp radiate outward in all directions. When the moth attempts to maintain its constant angle to this nearby point source, the angle rapidly changes as the insect moves. To correct for this perceived drift, the moth instinctively turns inward toward the light.
This continuous course correction results in the characteristic spiraling flight path that brings the moth closer to the source. Studies also suggest that moths may mistakenly tilt their backs toward the nearest bright light, which they are naturally inclined to do to maintain a level flight horizon against the sky. If an artificial light is positioned above or behind them, this backward tilting causes them to stall or fly erratic, looping trajectories, effectively trapping them nearby.
Which Light Wavelengths Attract Moths Most
Moths are most sensitive to and attracted by shorter light wavelengths, specifically ultraviolet (UV) light and the blue end of the visible spectrum. This sensitivity exists because UV light is a significant component of natural celestial light, especially moonlight, which moths evolved to use for navigation. Their compound eyes contain photoreceptors finely tuned to these shorter wavelengths, making sources that emit them appear exceptionally bright.
Lighting types high in UV and blue light are the most disruptive. Traditional mercury vapor lamps, metal halide lamps, and modern cool-white LED bulbs—which can have a color temperature of 4000 Kelvin or higher—fall into this highly attractive category. These lamps mimic the spectral signature that the moths’ navigational system is primed to follow.
Conversely, longer wavelengths of light, such as yellow, orange, and red, are far less attractive. Lights like low-pressure sodium lamps, which emit a narrow band of yellow light, or warm-white LED bulbs (3000 Kelvin or lower) significantly reduce the number of moths drawn to them. Choosing lights that minimize blue and UV components, often called “moth-friendly” lighting, is a useful strategy to reduce the negative impact of night lighting.
Ecological Consequences of Light Attraction
The spiraling flight of moths around artificial lights represents a significant drain on the insect’s energy reserves. Circling a light for hours depletes the stored energy needed for foraging, mating, and egg-laying, leading to exhaustion and a reduced lifespan. This energy expenditure directly limits the reproductive success of the individual moth.
Artificial light sources also act as beacons for predators, creating an ecological trap. Nocturnal hunters, such as bats, spiders, and certain bird species, learn to congregate near lights to easily pick off the disoriented insects. Spiders frequently build webs directly onto light fixtures to maximize their catch.
On a broader ecosystem level, light pollution disrupts crucial nocturnal behaviors, leading to cascading effects. Moths are significant pollinators of night-blooming plants. When distracted by artificial lights, their feeding behavior is inhibited, limiting their ability to transport pollen. This disruption, coupled with increased mortality, contributes to the decline in moth populations, affecting the food web for animals that rely on them for prey.