Scientists have a new theory about what attracts insects to lights

Scientists conducted field experiments in Costa Rica to find out how different insects respond to artificial lights at night.
Yash Sondhi

  • Scientists have long wondered why insects are attracted to artificial light, but they have lacked a good answer until now.
  • Researchers have observed unusual flight patterns in insects that fly toward artificial lights at night.
  • The patterns indicate that the insects use light to determine the appropriate direction to help them fly straight.

In 1884, entomologist Mary Esther Mortfeldt I noticed Something strange. She took pictures of the butterflies at night in her living room that usually only come out during the day – they seemed attracted to her lamps.

The question of what drives insects toward lights of all kinds — from fires to porch lights — is an old one.

“We've been observing this for thousands of years, and we can find writing about it for thousands of years,” Sam Fabian, a postdoctoral research associate at Imperial College London who sought to find out why, told Business Insider.

Upside down and smashed insects

The researchers observed “strange paths” in the flight patterns of insects attracted to lights at night.
Chris Robbins/500px via Getty Images

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 affects the movements of flying insects such as butterflies, moths and dragonflies at night.

In the laboratory they used Motion capture By outfitting the insects with miniature tags similar to what actors often wear to film action scenes in movies and video games.

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“This actually gives us an extraordinary solution to what is happening,” Fabian said.

However, even with just video footage, researchers soon saw a common trend among many species that showed what Fabian described as “these strange tracks.”

Insects will turn their backs toward the light, and if that light is below or horizontal to their flight path, this will often cause them to fly in circles or collide.

It was as if they were using the light source as a means to guide themselves towards the sky.

When insects get too close to artificial light, they can become almost trapped by it, Peter Oboyski, executive director and collections director at the Essig Museum of Entomology, who was not involved in the study, told Business Insider.

“The idea that they want their dorsal surface, their upper surface, to face the light makes a lot of sense,” Oboyski said.

The researchers attached markings to the backs of moths and dragonflies to capture data on how they fly near lights.
Sam Fabian

Fish show similar behavior, “but this is the first time we've shown that this happens to nocturnal insects around artificial light,” Fabian said.

The researchers published their findings in the peer-reviewed journal Nature Communications.

Insects need to know which direction to reach them

Many animals rely on gravity and visual cues to orient themselves, Oboyski said. One way some insects maintain their orientation is by using what is called the dorsal light response.

“In order for insects to fly at ground level and on the left side, they need to know which direction they are going,” he said. The reliable source of this information is the place where the light comes from, the sky.

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“And over the course of billions of planet years, that will increase,” Oboyski said.

When insects encounter a different light source — such as a fire from below or a headlight off to the side — the insects try to face their upper body toward that light and can then become disoriented or struck, Oboyski said.

The study focused on insect behavior when light is at a distance of less than 7 feet. It's still unclear what brought them there in the first place.

“I think there is still a bigger question about why they are attracted to light from greater 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 is lunar or celestial navigation,” Sondhi said. The idea is that insects that need to fly in a straight line can use the moon's position as a signal. They mistake artificial lights for the moon.

The direction of light affects insects in different ways. Lights directed upward or placed horizontally were the most annoying.
Sam Fabian

But Fabian said they tested this theory and found that some insects started traveling in a different direction when a new light source was turned on.

“They're actually moving back and forth, which doesn't seem like the kind of thing you would use a celestial compass for,” Fabian said.

A less bright future for insects

There's a reason why scientists have long sought to answer the question of why insects are attracted to artificial light. “I think there are practical reasons for wanting to understand this and also theoretical reasons,” Oboyski said.

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First, it could one day help us prevent much of the indiscriminate killing of insects. “There's a lot of collateral damage that happens with a bug zapper,” Oboyski said. “It's not just about mosquitoes.” Knowing what light attracts mosquitoes — and only mosquitoes — is a potential benefit.

Oboyski is also concerned that light pollution affects the way insects move through their environments.

“Maybe they're not finding mates the way they should. Maybe they're not finding food resources the way they should,” he said. Instead, “they are trapped in the world of light.”

In theory, he said, learning more about insect flight could also help engineers design flying cars one day.

A simple mitigation technique for now is to look in the direction of outdoor lights, Sondhi said. If you direct them downward, “you can reduce the amount of insects that are attracted to these lights.”

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