Insects flying erratically around artificial light is a common sight, a behavior known as positive phototaxis. This phenomenon is observed across many insect orders, including moths, beetles, and various flies. For millions of years, insects relied on natural light sources for navigation. However, artificial light at night profoundly disrupts the insects’ innate orientation systems. This article explores the current understanding of why insects gather at light sources.
Celestial Navigation Gone Wrong
The primary scientific explanation for this behavior is not that insects are actively drawn to the light source, but rather that the artificial light corrupts their ancient system for controlling flight attitude. Most flying insects use a deeply ingrained instinct called the dorsal light response to determine which way is “up.” This instinct causes them to automatically orient their body so that their dorsal side, or back, is facing the brightest visual region.
For the vast majority of their evolutionary history, the brightest source of light at night was the sky, illuminated by the moon or stars, which served as a reliable indicator of the horizon and gravity. By keeping their back to this distant, fixed light source, an insect can maintain a straight and level flight path. When a small, nearby artificial light source, like a porch light or streetlamp, is introduced, this navigational reflex misfires.
Instead of a fixed point, the insect encounters a close, bright source whose position changes as the insect flies past. To obey the dorsal light response, the insect continuously attempts to turn its back toward this nearby light. This constant, reflexive correction throws the insect into erratic movements, resulting in the familiar circling, or “orbiting,” flight path.
These flight patterns are not a sign of attraction but of confusion. If an insect flies directly over a light, the dorsal light response can cause it to flip upside down and nosedive, leading to a crash. When the light is behind it, the insect may tilt backward and stall, unable to maintain controlled flight. The resulting erratic motion is the visible manifestation of the insect’s attitude control system failing in the presence of an unnatural light source.
The Invisible Light Spectrum
While the navigational error explains the mechanism of entrapment, the likelihood of an insect being trapped depends significantly on the light source’s properties. Many nocturnal insects, especially moths, possess vision highly sensitive to the shorter wavelengths of the electromagnetic spectrum. They are acutely attuned to ultraviolet (UV) light and blue light, which humans see poorly.
This high sensitivity to UV light explains why certain light sources are dramatically more attractive than others. Light bulbs that emit a high amount of UV or blue light, such as mercury vapor lamps, fluorescent bulbs, and some types of cool-white LEDs, are particularly effective at drawing insects. In nature, UV light is a component of moonlight and is also reflected by flowers, leading some insects to mistake artificial UV sources for feeding or navigation cues.
Conversely, light sources that produce longer wavelengths—toward the orange and red end of the spectrum—are far less disruptive to many insects. Warm-colored lights, such as yellow or amber LEDs, are significantly less attractive because they emit minimal UV or blue light. Researchers found that switching from traditional incandescent or fluorescent bulbs to warm-colored LEDs can reduce the number of insects attracted by as much as four times. This difference is based on the biological limitations of the insects’ compound eyes, which perceive these longer wavelengths poorly.
Minimizing Attraction and Ecological Impact
The disorientation caused by artificial light at night leads to a significant ecological problem known as light pollution. Insects trapped by lights spend hours circling, exhausting their energy reserves, which should be used for foraging or reproduction. This continuous, fruitless flight increases their vulnerability, as they become easy targets for predators like bats and spiders that gather around the light source.
The resulting disruption contributes to the decline of insect populations, which are fundamental to pollination and the food chain. To mitigate this impact, several actionable strategies can be implemented based on the science of light and insect vision. One of the most effective methods is switching outdoor lighting to warmer-colored bulbs, specifically those emitting light with a wavelength longer than 550 nanometers, such as amber or yellow LEDs.
Another simple solution involves physically controlling the light’s direction and duration. Using shielded light fixtures that direct illumination downward prevents light from spilling upward into the sky, reducing the amount of light that can trigger the dorsal light response. Turning off unnecessary outdoor lights when they are not in use is a straightforward way to eliminate the problem instantly, as the negative effects of light pollution cease the moment the source is removed.