Why Are Flying Insects ‘Attracted’ to Lights? Scientists May Finally Have an Answer

Moths and other insects might turn their backs toward the brightest source of light around—which has historically been the sky—to determine which way is up and which is down, according to a new paper

Large moth flying next to bright white light
When researchers reviewed footage of flying insects, they saw that the bugs tilted their backs toward the source of artificial light. Imperial College London

Flip on a porch light on a summer evening and, within minutes, bugs will begin to congregate near the bulb. This behavior is so commonplace, it’s given rise to a popular simile that describes attraction: “like a moth to a flame.”

Though insects have been gravitating toward bright light sources for millennia, scientists never truly understood why. Now, they may finally have an answer.

Insects aren’t attracted to artificial lights, according to a new paper published Tuesday in the journal Nature Communications. Rather, the glow might be interfering with their natural sense of direction. The bugs you see encircling street lights, in other words, are likely lost.

This can be a hazard, because insects circling around artificial lights are “unable to secure food, easily spotted by predators and prone to exhaustion,” the study’s authors write for the Conversation.

“Many die before the morning comes,” they add.

The reason insects fly around light will surprise you

The new finding hinges on a theory about navigation. When insects fly, they can experience whole body accelerations up to five times greater than the acceleration caused by gravity. As such, it can be difficult for them to know which way is up and which way is down as they zip through the air.

The study authors suspect insects rely on the glow emanating from stars, planets and the moon to reorient themselves. For most of Earth’s history, the sky was the brightest source of light at night, so flying insects turned their bodies to make their backs face skyward. But then, artificial light entered the picture.

“Insects have been flying around for 370 million years,” says study co-author Samuel Fabian, an entomologist at Imperial College London, to Scientific American’s Rachel Nuwer. “It’s just in the last 150 years that it’s really gone wrong for them.”

'Like a moth to a flame' - this strange insect behaviour is finally explained

To reach these conclusions, scientists used high-resolution cameras to film insects flying around artificial lights in Costa Rica. They also attached miniature sensors to dragonflies and moths, then filmed infrared motion-capture videos of them in flight. Together, these methods allowed researchers to slow down the insects’ normally frenetic pace and study their movements in greater detail.

The footage showed insects turning their backs toward sources of artificial light—even at the price of flipping over, tumbling or crashing.

“If the light’s above them, they might start orbiting it, but if it’s behind them, they start tilting backwards, and that can cause them to climb up and up until they stall,” Fabian tells the Guardian’s Ian Sample. “More dramatic is when they fly directly over a light. They flip themselves upside down and that can lead to crashes. It really suggests that the moth is confused as to which way is up.”

Nocturnal Insects Orbiting Around Artificial Lights

As light pollution continues to worsen, the researchers wondered what type of artificial light might be the least harmful for flying insects. After running some experiments with an illuminated sheet, they determined that diffuse light shining straight down provided the least disruption to insects’ flight. Lights that pointed straight up, on the other hand, caused the most difficulties—“you’ve got insects raining down out of the sky,” Fabian says to NPR’s Nell Greenfieldboyce.

In the future, the team hopes to explore why some bugs seem impervious to artificial light sources—such as oleander hawk-moths and some species of fruit flies—while others can’t seem to escape it.

Scientists have long known that artificial light affects wildlife—often to the animals’ detriment. Sea turtle hatchlings, which use light from the stars and the moon to guide them toward the ocean, can be misled by artificial light and accidentally end up on roads and in parking lots rather than in the water. Artificial lighting can cause night-migrating birds to veer off course and deplete their already scant energy stores. It can affect the breeding periods of toads and frogs and disrupt the reproduction of glowing insects, such as fireflies and glow-worms.

The new research adds more nuance to this picture by attempting to explain how and why flying insects become ensnared by artificial light. The findings also offer one plausible explanation to satisfy an age-old human curiosity in one of Earth’s most enduring natural mysteries.

“We’ve probably been wondering why moths fly to lights since the invention of fire,” says Avalon Owens, an entomologist at Harvard University who was not involved in the new research, to NPR.

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