Like a moth to a flame? A new study debunks an age-old theory

Chances are you've heard the phrase "like a moth to a flame"—and probably seen it in action at a nighttime bonfire or barbecue, too.

It turns out, however, this age-old saying may not be quite right. Rather than being attracted to light, moths' tendency to fly toward brightness is a behavioral response to help them determine which way is up, suggests a new study published today in the journal Nature Communications.

In a series of experiments using artificial lights, researchers discovered that moths—and many other insects—turn their top sides toward illumination while they are flying, instead of heading directly into the glow, which would be expected if they were simply attracted to it.

To avoid crashing to the ground, insects need to know which way is up, says study co-author Yash Sondhi, an entomologist at the University of Florida and a National Geographic Explorer. The brightness of the sky, even at night, has always been a strong indicator of “up”—until artificial lights came along.

Unlike humans, most insects don’t have vestibular systems to help them maintain spatial orientation. But they do have a “system that allows them to really quickly tell what's bright versus what's dim, and give them feedback about the sky so that they can fly stably,” Sondhi says. “If they mess that up, they start crashing or they go too high and then they stall.”

Which way is up?

Throughout history, a number of theories have emerged to explain why insects flock to light, a leading one being that they rely on the moon for navigation and commonly mistake artificial light sources for this celestial compass.

But scientists have long found this explanation lacking because not all winged insects spiral around the light in the way that they would if they were using it as a navigational cue.

Other theories have suggested that insects are blinded and disoriented by artificial light, are attracted to the thermal radiation of lights, or think the light is an escape route, like a gap in a bush.

Sondhi and a team of researchers put this question to the test in the Monteverde Cloud Forest of Costa Rica, home to one of the most biodiverse arrays of insects on the planet.

They started by recording the insects' movements in complete darkness, using infrared, high-speed cameras, to get a baseline for how the bugs typically fly. Then, they set up experiments to investigate how different lights influence flight behavior. First, the researchers suspended a light bulb from a rope between two trees—resembling a typical downward-pointing light you might see on a porch—and in a separate test, they flipped the light upward by mounting it on a tripod.

In both cases, insects tilted their winged backs to the bulb. From dragonflies to butterflies, they were constantly chasing this angle, trapping them in a perpetual orbit around the bulb. But when the light was pointed up, the insects often stalled or crashed.

What if the light is diffused over a wide area rather than coming from a singular point? The researchers stretched a large white sheet across the tree canopy and projected a UV light on it. If the insects were truly “attracted” to the light, they should fly up toward it, says Sam Fabian, a study co-author and insect researcher at Imperial College in London.

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Instead, the insects cruised straight through this illuminated corridor, flying how they would during the day.

The team then placed the sheet on the ground and shined the light on it, which is when something truly peculiar happened: "All these moths flying over it turned upside down and crashed out of the sky," Fabian says. These behaviors suggest that artificial light can cause a mismatch between the insects' sense of which way is up and the true direction of gravity.

The scientists also performed several of these light experiments in a lab and found similar results, though they can’t yet rule out that other factors also may be contributing to insects’ tendency to swarm the light.

The growing problem of light pollution

One of the most important factors in flying is knowing which way is up. Flying insects seem to depend on light to do this, the new study finds, keeping the brighter backdrop overhead and darker settings below them.

"It's not that they just are attracted to light. It's that they're balancing light and dark and expect it to be balanced in a certain way," says Simon Sponberg, a physicist studying animal motion at the Georgia Institute of Technology who was not involved in the study. "But then you get an artificial environment that flips it on its head—literally."

The bad news: This behavioral instinct could spell trouble for insects populations, which are already facing widespread declines around the world due to climate change and habitat loss driven by human activities. Satellite data has revealed that the use of artificial light has skyrocketed over the past decade, with the world getting 2 percent brighter each year. Depending on how lights are used, their glow could capture insects in an exhausting orbit—or send them plummeting to their death.

"One of the fundamental aspects of this paper is it tells us that light pointing upwards is really, really bad for insects," Sondhi says. Along with disrupting flight, light pollution can adversely affect insects' ability to find potential mates, forage food, and defend themselves against predators.

People can help mitigate these impacts by switching off their lights whenever possible, or ensure the light is pointing directly downward. Most streetlights in cities already face downward, but they use bulbs that send light in all directions. Governments can update fixtures to include a metal shielding that shines the beam of light to just one spot, Sondhi says.

“Directionality of light is a huge factor in the pattern we have seen, not just brightness or color,” he says.

Some states have gone a step further by establishing large dark sky areas in which artificial brightness must be minimized to a specific level. These efforts are recognition that light pollution is a serious problem for many more animals than just insects: Artificial light misdirects baby sea turtles trying to reach the ocean; blinds birds flying between tall buildings of gleaming windows; and tricks corals into spawning at the wrong time. Designated “Dark Sky Places,” such as those in Big Bend National Park, in Texas, and Buffalo National River, in Arkansas, are intended to safeguard the night for animals, plants, ecosystems, and stargazers alike.