Why are bugs attracted to light?
There are many theories that attempt to explain why the proverbial moth, among other insects, is drawn to a flame, particularly at night. The theory of thermal radiation suggests that insects are attracted by the heat of artificial light. On the other hand, the escape mechanism theory says that insects respond to their instinct to escape, and fly toward a light source because they mistake it for a gap or escape path in a dark environment, such as in foliage. The popular celestial compass theory proposes that insects use the Moon for navigation and mistake artificial light from a steady source for the light from the Moon.
Critics of these theories have put forth evidence that calls into question their validity and universality:
- Although attraction to light might seem natural for pyrophilic (fire-loving) insects—those that depend on fire and the heat of post-burn environments for their survival and reproduction, such as fire beetles (Melanophila species)—the presence of this behavior in non-pyrophilic insects challenges the assumption that insects are drawn to light sources for heat. Moreover, many insects have been observed fluttering around ultraviolet lamps and other light sources that emit low quantities of heat, as well as seeking out light sources when the ambient temperature is high, contradicting the premise of the thermal radiation theory.
- For the escape mechanism theory to be valid, insects would have to head straight toward the light source in an attempt to escape the surrounding darkness. However, laboratory studies have shown that insects tend to fly at right angles to the light source rather than directly toward it.
- Although it has remained the most widely accepted theory since it was first proposed in the 1970s, the celestial compass theory cannot explain the flight paths actually followed by insects around artificial light sources, which research from 2024 has shown to be complex and even a bit frenzied.
Confusion, not attraction
Based on laboratory experiments and field studies, researchers in 2024 found that insects orient their backs toward an artificial light source while flying, causing them to loop endlessly around it. Researchers also found that repositioning the light causes insects to stall mid-air and turn their bodies so that their backs face the light perpendicularly at all times. This reflexive behavior—reorienting themselves in response to the brightest object in their field of vision, natural or artificial—is called the dorsal light response. Under natural conditions, with light from the sky, this behavior helps certain animals, such as fish and insects, sense which way is up and maintain stability and balance during movement. Flying insects, in particular, are likely too small to rely on gravity and their sensory receptors alone to stay aligned with the horizon. So they use light from the sky to ascertain which way is up and adjust their orientation. Artificial light, however, interferes with this behavior.
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The findings of the 2024 study add to the growing concerns about the threats posed by artificial light at night (ALAN). By interfering with their natural reflexes and interrupting their normal flying behavior, ALAN compromises insects’ ability to find food and mate, and makes them vulnerable to predation and death by exhaustion. To mitigate the harmful effects of ALAN, experts suggest reducing nighttime outdoor light usage; avoiding floodlights and upward-facing lights; using LED lights on warmer settings; and shielding light sources to reduce glare and point light downwards. (See also light pollution.)
Before humans lit fires and created other artificial light sources, the Moon and the stars provided the required cue for insects to adjust their flight attitude (orientation relative to gravity) at night. Because these celestial objects are very far away, they act as an almost fixed point of reference in the sky, visible overhead as an insect flies. However, an artificial light source at a lower elevation than the Moon appears to move past a flying insect, and the insect must reposition itself to keep the light visible overhead. This interferes with its straight flight path, causing it to fly in circles, halt mid-air, fly upward along the light, flip upside down, and eventually crash to the ground. The swarming, swirling behavior of flying insects around our porch and street lights is therefore the result of their disorientation because of a source of artificial light, rather than their attraction to it for heat, navigational cues, or escape direction.
The researchers suggest that artificial light likely draws in and traps insects in a frenzy of erratic flight patterns only when the insects are close by. And not every flying insect is equally affected by it. The oleander hawkmoth (Daphnis nerii) and the vinegar fly (Drosophila species) did not show change in movement patterns when exposed to artificial light placed in various ways during experiments, suggesting that some species may not depend strongly on dorsal light response to orient themselves during flight.
