Why Are Insects Drawn to Light? A Perennial Question Gets a New Answer.

As reliably as the planets orbiting stars, moths and insects are attracted to lights in the dark.

This fact has been exploited by entomologists for years, who set up light traps to collect insects. The image of a moth caught in flames has been used by poets to represent self-destructive behaviors, from "The Merchant of Venice", to the Hindu epic Bhagavad Gita. Ecologists have recently begun to worry about the lure of artificial light across Earth's surface at night.

Scientists still don't understand why.

Avalon Owens is a Harvard biologist and said that entomologists are frequently asked about this.

She said that a popular, but incorrect, theory is that flying insects mistake our porch lights as the moon, or other celestial bodies, and this confuses their navigation. One theory is that the lights at night appear to be daylight filtered through thick vegetation. This causes insects to make a dash for what they perceive as open space.

A new answer is causing a stir in the world of entomology. The team, led by Samuel Fabian of Imperial College London and Yash sondhi of Florida International University, argues that many insects mistakenly believe they have found the direction to the sky when they see a bright night light and try to orientate themselves along an axis going up and down. This instinct causes them to turn their backs to the light. However, this is not the case when the light source is horizontal or on the ground. Instead, they will make endless banking turns or crash land.

The findings have been published in a bioRxiv paper, but it has not yet undergone peer review.

Entomologists have long known that such instincts can help flying insects maintain their level, by keeping the backs of their bodies pointing toward sunlight during aerial maneuvers. This new analysis shows, however, that the lights at night seem to hijack this instinct.

The scientists filmed dragonflies and butterflies flying around light bulbs in lowlight, both in the laboratory and in Costa Rica's cloud forest. The insects sometimes started making circular loops, like an orbiting lunar surface. Sometimes the insects would fly past the bulbs and then tilt up into a stall. Or, they might fly over the bulbs and then turn themselves upside-down before nose-diving toward the ground.

The team was able to confirm this result by attaching tiny motion trackers to the insects. The team then found that computerized flight simulations explained the looping and trapped flight paths of the bugs by these inadvertent rolls.

Tyson Hedrick is an expert on the aerodynamics of insect flights at the University of North Carolina. He said that he previously supported the idea of lights interfering with the celestial guidance of insects.

He said that "it's one of those things we all think we know about the nature, and it's being overturned."

The new study offers some suggestions on how to reduce the effects of artificial light. It can be poetic - like an insect burning in flames - or sinister - like being caught in a spider's web after it has discovered that artificial lights are prey. Some insects simply lose calories by flying around in circles.

Researchers found that insects are less affected by lights mounted horizontally or upwards, but more adversely affected by lights projected downward. Researchers have long recommended that light fixtures aimed downwards, which illuminate the ground only, be used to reduce light pollution.

Dr. Owens explained that lights not only attract insects but can also cause caterpillars to grow, stop fireflies from flashing and reduce the habitats of shyer species who venture out at night.

She said that turning off the light was always the most effective solution to this problem.