At night, it’s not unusual to find a hoard of moths and other insects circling around a porch light or street lamp — but their reasons for being there are likely quite different from what most people assume, new research has found.
The insects are not actually drawn to the glow like “moths to a flame,” as the old saying suggests, but rather trapped in a disorienting orbit around the artificial light, scientists reported in a study published January 30 in the journal Nature Communications.
By using motion-capture cameras — and filming with infrared illumination so as not to disrupt the creatures’ vision — the researchers showed that when the insects flew around a light source, they were tilting their backs toward the light and keeping their bodies in that direction. By maintaining this orientation, the hapless critters created odd orbits and steering patterns, according to the study.
Gaining a better understanding of the impact of artificial light on these winged creatures is crucial as light pollution plays an increasing role in the decline of global insect populations, the researchers wrote.
Artificial light confuses nocturnal insects
When artificial light does not interfere, nocturnal insects keep their backs pointed toward whatever direction is brightest, which is typically the sky versus the ground.
This evolutionary trick has helped the critters know which way is up and keep them level during their night flights. However, when the insects pass by an artificial light source, they become disoriented, believing that the human-made lighting is the sky, said co-lead study author Samuel Fabian, an entomologist and postdoctoral researcher at Imperial College London’s department of bioengineering.
“Insects in the air don’t inherently know which way is up, they don’t have a very good way of measuring that. … It’s assuming the light is the direction of up, but it’s wrong. And if you tilt, that’s going to create sort of weird steering patterns, in the same way that if you were riding a bike and you tilt over to one side, you’re going to get to steer in a big circle, it’s all going to go a bit funky,” Fabian said.
Orbiting, stalling, inverting
The study team compiled hundreds of slow-motion videos capturing the behaviors of butterflies, moths, bees, wasps, dragonflies and damselflies, and found that the critters were not attracted to faraway lights. The insects only appeared to be drawn in when passing a light that was nearby. Consistently, the overwhelming majority of study subjects tilted their backs toward the light, even if doing so prevented sustained flight.
“Maybe when people notice it, like around their porchlights or a streetlamp, it looks like they are flying straight at it, but that’s not the case,” said co-lead study author Yash Sondhi, a postdoctoral researcher at the Florida Museum of Natural History, in a news release. Sondhi contributed to the research while a doctoral student of biology at Florida International University in Miami.
The team observed three common responses to the light source made by the insects, including orbiting the light, stalling — which caused the insect to steeply climb above the light — and inverting, in which the insect flipped over and crashed into the ground.
Some fast-flying insects, such as dragonflies, remained in orbit for minutes at a time, going swiftly round and round the light fixture, Fabian said.
In one experiment, the researchers emulated the night sky by shining a light on a white sheet oriented above and found the insects were able to navigate under it without issues. If the insects were inherently seeking the light, they would have crashed into the sheet, Fabian said.
“The behaviors of flying insects in the presence of artificial light close to the ground are non-uniform and surprisingly complex in a way that had not really been documented well previously,” said Floyd Shockley, the collections manager for the department of entomology at the Smithsonian National Museum of Natural History in Washington, DC.
“The insects aren’t directly flying towards the light but are orienting in such a way that they remain perpendicular to it is giving the illusion of attraction,” added Shockley, who is unaffiliated with the study, via email.
More theories on the odd behavior
Past theories on why many insects erratically fly around light sources have included the idea that they are drawn to heat and that the creatures — particularly those that ancestrally lived in caves and holes in trees — believe the light source is an escape to the outside.
The most common one has been that insects are confusing the light with the moon, which they use as a compass cue. Since the critters are not flying directly toward the light, and the behavior has also been observed in species that are not migratory and do not use compass cues, these old theories no longer seem likely, Fabian said.
“I think the biggest barrier to solving this for so long is dealing with low light conditions, small animals, and high speeds and unpredictable movement,” said entomologist Jason Dombroskie, manager for the Cornell University Insect Collection and the Insect Diagnostic Lab, who was not involved in the study. “The results speak for themselves. They make a pretty convincing argument that, you know, we can discard a lot of the other theories, at least in general.”
Light pollution and insect declines
The world has experienced a widespread “loss of the night” — scientists found light pollution rose at a rate of 2.2% a year in a November 2017 report that looked at the world’s radiance through the first calibrated satellite radiometer for night lights.
The increase in artificial lights has several harmful effects on wildlife, including habitat loss and fragmentation, according to a March 2022 paper cited by the National Wildlife Foundation.
The authors of the new study noted that light pollution is a growing cause of insect declines, referencing a September 2020 report that had found artificial light affected moth behaviors when it came to reproduction and larval development.
The new findings could help with conservation by fueling research on how to minimize the effects of light pollution on the insects, Dombroskie said. “I always advocate that if the light is not doing anything, turn it off.”
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