Why Insects Are Attracted to Light at Night

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The first humans to sit around campfires probably observed a strange example of animal behavior that continues to perplex people today. They would have seen insects that emerge from the night and begin to endlessly circle the artificially produced light source, with sometimes fatal results.

Experts and lay observers alike have come up with all sorts of reasons to try to explain this behavior, but none of the descriptions have offered any hard proof. Now research published this week in Nature Communications might have finally solved the mystery: artificial light confuses insects’ ability to orient themselves to the horizon, scrambling their sense of what is up and down and causing them to confusedly fly in circles.

“Lots of animals turn their back on the light to orient themselves in space and work out what’s up and down,” says Samuel Fabian, co-lead author of the study and a postdoctoral researcher in entomology at Imperial College London. “What we’re bringing here is just linking this behavior to an insect behavior that was considered a puzzle until now.”

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Scientists have raised a number of hypotheses over the years to explain why light essentially acts as a trap for insects after dark, and these ideas have varied. One held that nocturnal insects were following an instinct to fly toward the brightest spot in their field of vision, which they mistook for the sky. Another suggested that insects were trying to warm themselves with the heat emitted by the light. Still one more posited that insects might have been getting blinded by the glare of artificial light.

The most popular theory, though, was that insects were confusing lights with the moon or other celestial bodies that they normally use to navigate with. Although some insects do navigate in this way, the new findings indicate that this ability has nothing to do with the explanation of why they are attracted to artificial lights, says co-lead author Yash Sondhi, a postdoctoral fellow at the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History.

To probe for the real reason, the team carried out a first set of experiments in an insect flight arena at Imperial College London. The researchers used eight high-speed infrared cameras equipped with motion-capture technologies to track 30 insects from three moth and two dragonfly species. They also flew captive insects from six different insect taxonomic orders that were too small to use motion-capture technology with, including fruit flies and honeybees, to make sure different insects all showed similar responses to light. They used these data to create models that simulated the insects’ three-dimensional flight trajectories through space.

The more challenging part of the research, in the Costa Rican jungle, came next. Working in collaboration with co-author Pablo Allen of the Council on International Educational Exchange in Monteverde, Costa Rica, the researchers lugged heavy cameras, lights and tripods to two field sites to gather behavioral data from insects in the wild.

It quickly became apparent from a quantitative and qualitative perspective that none of the existing hypotheses explained what the team was seeing, Fabian says. “We used quite advanced tracking technologies, but if you just look at the videos, you see the insects constantly flipping upside down with their backs to the light,” he says.

The team was able to verify that insects were not beelining for the light but rather circling it as they tilted in an attempt to orient their back toward it. This behavior, known as a “dorsal light response,” normally helps insects to remain in an unchanging path of flight that is properly aligned to the horizon. Artificial illumination that arrives from a point source causes them to fly in erratic patterns as they try to turn their backs to what they are mistaking as the sky.

“They are likely also using additional conflicting sources of information for which way is up; however, it looks like the direction of light overrules and dominates other cues,” Fabian says. “The visual system is saying, ‘No, you really need to keep that light over your back.’”

Researchers also found that insects in the lab and field flew normally when the light was diffuse and came from overhead rather than from a single point source. “That very concretely showed that it’s not a light attraction issue but an orientation issue,” Sondhi says.

“I was very happy when I read this paper, as it gave for the first time a satisfying answer to a long-standing phenomenon,” says Florian Altermatt, an ecologist at the University of Zurich, who was not involved in the research. “From a scientific perspective, it was also interesting to see that it was an actually a rather simple explanation, defying the previous more complex ones.”

Avalon Owens, an entomologist at Harvard University, who also did not participate in the research, agrees that “it’s exciting to have a new observation” on a phenomenon that humans have likely been observing and wondering about for millennia. “By taking advantage of advances in high-speed camera technology, the authors have discovered something completely undescribed and frankly unexpected,” she says.

In future studies, she adds, she would like to see researchers investigate how universal this particular type of flight response is among insect species.

Indeed, in the new study, there were two intriguing exceptions to the rule. Unlike all the other species tested in the lab experiments, oleander hawk moths and Drosophila fruit flies flew normally in the presence of light. Oleander hawk moths are frequently caught in traps in the field that use light to attract insects, however, so it might be that certain species can suppress or increase their orientation behavior depending on the circumstances, Fabian says. “We’d like to pick apart some of these exceptions in future research.”

For most flying nocturnal insect species, though, the findings only reiterate the fact that artificial light is bad news for their survival. “Insects have been flying around for 370 million years,” Fabian says. “It’s just in the last 150 years that it’s really gone wrong for them.”

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