In 1884, entomologist Mary Esther Murtfeldt noticed something odd. She’d captured butterflies at night in her sitting room that usually only came out during the day — they seemed attracted by her lamps.
The question of what brings insects toward lights of all kinds — from flames to porch lamps — is an ancient one.
“We’ve been observing this for thousands of years, and we can find writing on it for thousands of years,” Sam Fabian, a postdoctoral research associate at Imperial College London who set out to find the reason, told Business Insider.
Upside-down and crashing insects
Fabian, Yash Sondhi, and other biologists set up high-speed video cameras, both in the field in Costa Rica and in their lab, to see how light affected the movements of flying insects like butterflies, moths, and dragonflies, at night.
In the lab, they used motion capture by outfitting the insects with miniaturized markers similar to what actors often wear to film action scenes for films and video games.
“That actually gives us really phenomenal resolution of what’s going on,” Fabian said.
However, even with just the video footage, the researchers quickly saw a common trend across several species that exhibited what Fabian described as “these strange paths.”
The insects would turn their back toward the light, and if that light was below or horizontal to their flight path, it often caused them to fly in circles or crash.
The insects seemed to be using light to help them orient to the sky.
When bugs get too close to artificial light, they can become almost trapped by it, Peter Oboyski, executive director and collections manager at the Essig Museum of Entomology, who wasn’t involved in the study, told Business Insider.
“This idea that they want their dorsal surface, their top surface, to face the light makes a lot of sense,” Oboyski said.
Fish exhibit similar behavior, Fabian said, “but this is the first time that we were showing that this was happening to nocturnal insects around artificial light.”
The researchers published their results in the peer-reviewed journal Nature Communications.
Insects need to know which way is up
Many animals rely on gravity and visual cues to orient themselves, Oboyski said. One way some insects stay oriented is by using something called dorsal light response.
“For the insects to be able to fly level to the ground and bank left, they need to know which way is up,” he said. A reliable source of that information would be where the light is coming from — the sky.
“And throughout the billions of years of the planet, that would be up,” Oboyski said.
When confronted with a different light source — say a fire from below or a headlight to the side — the insects try to face the top of their body toward that light and can then become confused or crash, Oboyski said.
The study was focused on insects’ behavior when the light was at a close distance, less than 7 feet away. It’s still not clear what brings them there in the first place.
“I think there’s still the bigger question of why are they drawn to the light from further distances, which is a much harder question to answer,” Oboyski said.
The moon is not a compass
There are many theories about why insects gather at lights.
“The most common one is the lunar or celestial navigation,” Sondhi said. It is thought that insects who need to fly straight can use the location of the moon for a guide. The insects mistake the artificial light for the moon.
But Fabian explained that they had tested this theory, and discovered that certain insects began to travel in another direction after a light was switched on.
“They’d actually be going forwards and backwards, which doesn’t seem like the kind of thing that you would use a celestial compass for,” Fabian said.
A less-bright future for insects
There’s a reason scientists have long sought to answer the question of why insects are attracted to artificial light. “I think there are both practical reasons to want to understand this and also theoretical reasons,” Oboyski said.
First, it could someday help us prevent so much indiscriminate killing of insects. “There’s a lot of collateral damage going on with a bug zapper,” Oboyski said. “It’s not just the mosquitoes.” Figuring out what light attracts mosquitoes — and only mosquitoes — is a potential benefit.
Oboyski also worries that light pollution is impacting the way insects navigate through their environments.
“Maybe they’re not finding mates the way they should. Maybe they’re not finding the food resources the way they should,” he said. Instead, “they’re getting trapped in the realm of the light.”
More theoretically, he said, learning more about insect flight could also help engineers design flying cars one day.
For now, though, a simple mitigation technique would be to consider the direction of your outdoor bulbs, Sondhi said. If you point them downwards, “you can reduce the amount of insects that get attracted to these lights.”