The “fatal” attraction of insects to artificial light

Who hasn’t seen moths and other “bugs” fluttering around street lamps? It has long been known that the artificial light It attracts flying insects and was even used to catch these insects in the Roman Empire. However, the reason for this behavior is not exactly known, and various possibilities have been suggested: using the light as an escape route, the fact that the light source blinds the insects, or that they mistake it for the moon, among others. Many other theories.

To investigate this mystery, an international team of researchers led by Samuel Fabian from Imperial College London (United Kingdom) and Yash Sondhi from Florida International University (USA) used high-speed cameras. They used them to track the three-dimensional flight of various species Mothsother butterflies, Dragonflies and fruit fliesboth under laboratory conditions and in a natural environment: the Monteverde Biological Station in Costa Rica.

Insects have a “dorsal response to light,” correcting their flight path so that their backs are aligned with the light source.

The researchers examined its flight in different lighting conditions, including point sources of ultraviolet light and surfaces with more diffuse glows. In this way they have proven that insects have a “dorsal response to light“by correcting his trajectory so that his back is facing the light source. This reduces his ability to orient himself, says the study published in the journal Nature communication.

With natural light sourcesLike the sun or a starry night, this reaction causes the insect to maintain a stable and correctly aligned trajectory with its horizon. However, artificial light causes its flight to deviate and correct irregularly, often circling around the focus.

“The most important finding is that insects confuse light Direction to the top of the sky” Fabian tells SINC, explaining: “Knowing where the sky is is essential to flying, as you have to create the forces that counteract gravity.”

The key finding is that insects confuse light with the upward direction of the sky, which is important for flight because it requires generating forces that counteract gravity.

Samuel Fabian (Imperial College London)

“That’s why,” he continues, “this confusion This causes insects to tilt their bodies toward the light and direct their flight forces against gravity. This leads to their convoluted orbits, which we often observe near streetlights. The most reliable evidence for this is that when they fly over a bright light, they turn around and fall out of the air.”

Sam Fabian holds a butterfly during flight photography in the laboratory at Imperial College London. /Thomas Angus

The results also allowed it exclude some hypotheses which have been considered but not tested: “We don’t think it involves navigation across the moon, and we have also seen that insects do not fly directly towards the light.” Rather, it confuses them vertically and traps those “One who happens to pass by,” says the researcher.

It occurs in various insects

The team confirmed the same behavior in 10 different orders of insects. “This makes sense since everyone faces the same problem of figuring out where gravity is. When they fly, they can’t use ground reaction forces (like we do when walking), and when cornering, they experience all sorts of accelerations (G-forces), which are usually greater than that of gravity, not the difference. Direct detection, for example by dangling legs, is an imprecise mechanism,” explains Fabian.

“Leaning your back toward the light, which is where you think the sky is, is a great way to stay in the right position in the air,” he points out. It is a Robust and fast mechanism which does not require additional sensors. “Insects (among other animals such as fish and marine invertebrates) use this response constantly around us to quickly tilt in the right direction, a mechanism that remains very useful even during the day.”

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The dorsal response to light appears to be a response both during the day and at nightsay the authors, who used both diurnal and nocturnal insects for the test.

Two exceptions

However, researchers have found two exceptions to this behavior. The first was this fruit fly (Drosophila), which flew over the ultraviolet focus without leaning towards it or turning around. The reason for this different behavior is unknown, but researchers around the world take advantage of this when they use these flies for flight experiments where the lighting or visual environment does not appear to interfere.

The second exception was the oleander sphinx (Daphnis nerii), that did not turn around in ultraviolet light or show altered flight patterns in the laboratory. “This is particularly strange since this species achieves light traps in the field but not in the laboratory,” says Fabian, “and one possible reason is that these butterflies have different flight modes (gliding versus high speed, for example). “It’s clear that insects have the ability to sometimes turn off the light response on their backs when it suits them, something we want to study in more detail.”

Light pollution problem

In any case, the zoologist remembers that light has been a good indicator of the direction insects fly for 370 million years, but recently we ruined it with night lighting: “After humans started seeing big bright lights everywhere nearby “Our houses became a problem. Suddenly, the brightest visual regions at night are not necessarily the sky, which is extremely rare for them. But I want to make it clear that insects are not stupid, but that they have adapted fantastically to function in the natural environment. It’s just that we’ve changed the environment faster than they can adapt to it.”

Insects are not stupid: they are fantastically adapted to function in the natural environment. What’s happening is that we’ve used our lights to change the environment faster than it can adapt to it

Samuel Fabian (Imperial College London)

Fabian highlights the “very negative” impact that night light has on insect populations. “Not only does it attract and trap them, but it also interrupts their periods of activity (nocturnal animals often fall asleep after crashing into a light). Insecticides and changes in land use will probably be even more devastating, but we still don’t know exactly: “How much damage does this night light do?”

The moth Attacus lorquini flies with its back to the light source. / Sam Fabian

Next Steps

The authors conclude that further work is needed to investigate the long-distance effects of artificial light and that we can improve the habitats of these small animals by reducing unnecessary artificial light at night.

“Our next big question is to know at what distance this effect occurs in different light,” says Fabian, who concludes: “Our current data is around 2 meters around the maximum of the light source, but we don’t know .” What It occurs at 20 m, 100 m, 1 km away. Understanding this is key to conservation efforts and reducing the impact of light pollution on our nocturnal fauna.


Samuel Fabian et al. “Why Flying Insects Gather in Artificial Light.” Nature communication2023

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