We have all seen groups of insects flying around porch lights and streetlamps. Scientists have some explanations for this phenomenon, including insects being drawn to light because they think it is a gap in the foliage, or perhaps the light is blinding, causing the insects to crash into it. Regardless of which idea you are drawn to, the fact that insects are attracted to light is a growing problem, especially as increasing urbanization is causing increased light pollution across the globe. Using high-speed, motion-tracking videos to study the flight patterns of insects flying near lights, Samuel Fabian and Huai-Ti Lin at Imperial College, UK, Pablo Allen of the Council of International Educational Exchange, Costa Rica, and Yash Sondhi and Jamie Theobald of Florida International University, USA, set out to discover why insects are drawn to artificial lights and why they seem to get stuck once there.

First, the researchers traveled to the Monteverde Field Station in Costa Rica to record the general flying patterns of 10 different orders of nocturnal insects in response to artificial light. Using two high-speed cameras with extra infrared lighting, the researchers filmed insects flying around an outdoor UV light that shone either horizontally, upwards or downwards. Fabian and colleagues found that insects generally flew at a right angle to the light source rather than towards it. This resulted, on average, with insects flying in a circle around the light. The team found that the night-time flight behaviors they recorded fell into three broad categories: orbit, stall and invert. The researchers described ‘orbit’ flight as the insect flying in an arcing pattern around the light, ‘stall’ as the insect flying upward, towards the light, and slowing down without swooping back downward, and ‘invert’ as the opposite of stall, where the insect goes up and then takes an abrupt nosedive. Knowing what these flight behaviors look like helped the team focus on the finer details of how these insects fly around light when flying in a controlled laboratory setting.

Focusing on four species, two day-time flying dragonflies and two night-time flying moths, the scientists filmed these insects flying around a light source in the laboratory. Attaching small markers near the wings of the insects, the team recorded their position and orientation during flight using four pairs of synchronised cameras arranged in a ring. Using this high-resolution, motion-tracking footage, Fabian and colleagues found that insects do not fly directly towards the light, but rather turn their backs toward it, causing them to fly in an arcing shape around the light source. The authors concluded that the insects use light to work out which way is up, even at night. Insects tilt their backs to where they perceive ‘up’ to be based on where light is coming from. In nature, this light comes from celestial sources in the sky and is generally upward. However, artificial light at night can be from the side or below. This causes insects to tilt their backs away from pointing up, toward the light resulting in the insects getting stuck circling the lights.

The results of this study, though satisfying to our curiosity, are troubling given that night skies across the world have never been more polluted by artificial light. Although Fabian and colleagues did not study how the light's distance away from the insect, its brightness and color affect this behavior, our brighter night skies are trapping insects around artificial light sources, undoubtedly keeping them from living their lives and making these animals easy prey.

Fabian
,
S. T.
,
Sondhi
,
Y.
,
Allen
,
P. E.
,
Theobald
,
J. C.
and
Lin
,
H.
(
2024
).
Why flying insects gather at artificial light
.
Nat. Commun
.
15
,
689
.