An international collaboration of scientists from the United States, the United Kingdom, and Costa Rica has revealed why moths and other flying insects circle street lamps and other nighttime light sources. The discovery was published in a prominent biology journal, Nature Communications.
Earlier ideas suggested moths were drawn to heat from flames. There is some truth to this, as a subset of insects is pyrophyllous and uses recently burned areas to its advantage. But this explanation does not account for the majority of species observed around artificial lights.
A second hypothesis proposed that insects are drawn to light as a basic need, a phenomenon known as phototaxis. In such a scenario, insects would try to avoid darkness and inevitably collide with lanterns rather than move in orderly patterns around them.
Another theory argued that artificial illumination might be mistaken for natural moonlight, a key orientation cue for many nocturnal flyers. Yet the observed flight paths near bright sources often do not align with a simple moonlight navigation pattern.
To test these ideas, the team recorded insects as they circled various light sources to trace their directions with precision. The analysis showed that moths frequently turn their backs toward the light, a behavior known as the dorsal light response. This finding indicates a deliberate orientation strategy rather than random diffusion.
Beyond circular orbits, researchers noted several dynamic responses. When an insect passed directly under a light, it tended to tilt upward as the glow moved behind it, keep its back facing the lamp, and then rise straight up before diving away. Conversely, when insects moved over a light source, they often flipped upside down, again exposing their backs to the light, before making a rapid descent. These nuanced movements reveal a consistent effort to manage perceived brightness while controlling altitude and trajectory.
Historically, the night sky was brighter than the ground in many places, offering a reliable reference frame for small, alert fliers seeking steady orientation. The advent of modern lighting has altered this balance, masking the natural cues insects rely on and prompting persistent, circular flight patterns near illuminated areas. This disruption in navigational cues appears relatively recent in human history and is now a focal point of ecological concern. Attribution: NatComms study
The study’s authors emphasize that reducing light pollution is essential to protect insect diversity and the broader ecological roles these creatures play. By limiting excessive illumination, communities can help preserve nocturnal ecosystems and support pollination, food webs, and nutrient cycling that rely on insect activity after dark.
Biologists have long warned about declines in insect populations worldwide, and the latest findings contribute a clearer understanding of one contributing factor. The research underscores the need for balanced lighting strategies that minimize disruption while maintaining safety and visibility in urban and rural environments. Attribution: NatComms study