The phenomenon of insects congregating around artificial light is known as positive phototaxis, the innate movement of an organism toward a light source. Not all light attracts flying insects equally. The true determinant of a light’s attractiveness is not brightness but the specific spectrum of light it emits, measured by its wavelength. This difference depends on how insect eyes perceive colors in the electromagnetic spectrum.
The Biological Mechanism
The structure of an insect’s compound eye gives it a distinct visual range compared to humans, whose vision peaks in the green-yellow spectrum. Most insects are highly sensitive to shorter wavelengths, such as ultraviolet (UV) and blue light, spanning roughly 300 to 450 nanometers. This sensitivity is rooted in their evolutionary history, where natural light from the sky contains a significant amount of UV light.
For millions of years, nocturnal insects, such as moths, have used distant celestial light sources for navigational purposes. They instinctively maintain a constant angle relative to a distant, stable light source, allowing them to fly in a straight line. Artificial lights, however, are nearby and disrupt this ancient guidance system.
The erratic flight is often attributed to a confusion of the insect’s dorsal light response. Insects orient their bodies by keeping their back, or dorsal side, toward the brightest light source, which in nature is always the sky. An artificial light fixture shining below them scrambles this instinct, causing them to tilt their back toward the ground-level light source and resulting in a spiraling flight pattern.
Lights That Are Most Attractive
The light sources that attract the most bugs are those that produce high amounts of short-wavelength light. Mercury vapor lamps and standard fluorescent tubes are highly attractive because their operational mechanism generates significant ultraviolet emission. These bulbs were once common in outdoor settings due to their rich UV output.
Traditional incandescent bulbs, while not primarily UV emitters, still attract insects due to their broad-spectrum light output. They also generate heat, which, while secondary to wavelength, can attract some species. Modern lighting technology requires careful selection, as “cool white” LED bulbs, typically rated 4000 Kelvin (K) or higher, are particularly appealing to insects.
These high-Kelvin LEDs emit strongly in the blue spectrum, which falls within the insect’s most sensitive visual range. Therefore, a bright, cool white LED is more attractive than a warm-toned equivalent, even if the warm light is technically an incandescent bulb. This is why cool-colored lighting, including some metal halide lamps, can quickly create a swarm of flying insects around an entryway or patio.
Lights That Minimize Attraction
Minimizing bug attraction involves selecting light sources that emit primarily longer wavelengths, namely yellow, amber, and red. These colors fall outside the optimal sensitivity range of most flying insects, making them less visible. By reducing the light output below the 550 nanometer range, the visual cue that confuses their navigation is reduced.
The most practical residential solution is the “bug light,” which is often an incandescent or LED bulb with a yellow or amber coating. These lights are specifically engineered to filter out the insect-attracting blue and UV wavelengths. Warm-toned LED bulbs, generally those with a low color temperature of 2700K or less, are highly effective because they produce light that is richer in the yellow-orange spectrum.
In commercial or industrial settings, high-pressure sodium (HPS) lamps are often used because they emit a monochromatic yellow-orange light at a wavelength of about 589 nanometers. This specific output is largely invisible to most insects, making them a good option for large-area lighting. Switching to these longer-wavelength lights can drastically reduce the number of bugs congregating around outdoor fixtures.