Insects attracted to artificial light, a death trap

Contrary to what one might think, insects do not point directly towards the artificial light source, but rather tilt their backs towards it, which randomly deflects and changes their trajectory. However, in natural light, similar behavior helps them maintain the correct position in the air. A team of biologists examined it and recorded it with a high-speed camera.

Who hasn't seen moths and other "bugs" flying around streetlights?

Artificial light has long been known to attract flying insects and has been used to trap them since Roman times. However, the reasons for this behavior are not completely known and various possibilities have been proposed, including: using light as an escape route, insects that are blinded by the light source, or confusion with the moon... and many others. theories.

To investigate this mystery, an international team of researchers led by Samuel Fabian of Imperial College London (UK) and Yasha Sondhi of Florida International University (USA) used high-speed cameras. Together they monitored the three-dimensional flight of many species of moths, butterflies, dragonflies and fruit flies, both in the laboratory and in the wild at the Monteverde Biological Station in Costa Rica.

Scientists studied its flight under a variety of lighting conditions, including point sources of ultraviolet rays and on surfaces with more diffuse light. In this way, they confirmed that insects exhibit a “dorsal response to light”, adjusting their orbits so that their back faces the light source. This limits your ability to navigate, according to a study published in the journal Nature Communications..

When natural light sources are used, such as the sun or a starry night, this response causes insects to maintain constant flight and precise orientation with respect to the horizon. However, artificial light causes its flight path to alter and adjust erratically often circling around its focal point.

"The main finding is that insects confuse light with the upward direction of the sky", explains Fabián and continues: "Knowing where the sky is is essential to fly because it is necessary to generate forces contrary to gravity"."This confusion then causes the insect to tilt its body towards the light and direct its flight against gravity. This results in its sinuous orbital path that we often see around streetlights. The most reliable evidence of this is when they fly over a bright light, turn around and fall from the air.".

The results also allowed us to rule out some hypotheses that had been considered but not tested: “We do not believe that this is a positioning in relation to the moon, and we also observe that the insects do not fly directly towards the light. Rather, it disorients them vertically and traps those who pass by by chance.”said the researcher.

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Occurs in many different species of insects

The team tested the same behavior on 10 different insect species. “This makes sense because everyone faces the same problem: figuring out where the gravity is. When they fly, they cannot use ground reaction forces (like we do when we walk), and when they turn around corners, they experience different types of acceleration (G-forces) that are often greater than gravity and undetectable. For example, direct detection by leg suspension is an imprecise mechanism.”explains Fabián. "But leaning your back toward the light, that is, toward what you think is the sky, is a good way to maintain an accurate position in the air.", he emphasizes. This is a reliable and fast mechanism that does not require additional sensors.

"Insects (including other animals like fish and marine invertebrates) constantly use this response around them to quickly lean in the right direction, and this mechanism remains very useful throughout the day."According to the authors, who used diurnal and nocturnal insects to test this, the dorsal response to light appears to be both diurnal and nocturnal.

Two exceptions

However, the researchers found two exceptions to this behavior. The first is the fruit fly (Drosophila) that flew through the UV light zone without leaning toward it or turning around. The reason for this different behavior is unknown, but researchers around the world are taking advantage of it, using these flies for aerial experiments where light or visual environments are not they interfere.

The second exception is the sphinx oleander (Daphnis nerii), which turned over when exposed to ultraviolet light and did not show altered flight patterns in the laboratory. “This is especially strange because this species approaches light traps in the field but not in the laboratory, and one reason may be that these moths have different flight modes.”Fabián said, as he glided at high speed. "It is clear that insects can sometimes suspend the response to dorsal light when it suits them, and we wanted to investigate this further.".

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The problem of artificial light and light pollution

In any case, this zoologist mentions that for 370 million years light was a good indicator of the direction of flight of insects, but recently humans have altered it with night lights: "When we started putting big, bright lights everywhere, this became a problem.".

"Suddenly, the brightest area visible at night is not the sky, which is extremely rare for insects. However, I want to emphasize that they are not stupid, they are wonderfully adapted to function in their natural environment. The reality is that we have changed the environment faster than they can adapt to it."

Fabián highlighted the impact "very negative"of night lighting in insect populations. “This not only attracts and traps them, but also disrupts their activity time. For example, nocturnal animals usually sleep when exposed to light.""Pesticides and land-use changes could have even more devastating impacts, but we still don't know exactly how much damage artificial light at night causes".

Next step: know the distance

The authors concluded that more research is needed to determine the effects of artificial light over long distances 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 starts to appear with different light sources"says Fabian, and concludes: "Our current data is about 2 meters around the maximum of the light source, but we don't know what happens at 20 m, 100 m or 1 km. Understanding this is key to conservation efforts and reducing the impact of Light pollution in our nocturnal wildlife".


Samuel Fabian et al. “Why flying insects gather at artificial light". Nature Communications., 2023.

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