Scientists discover why insects are drawn to artificial light

By Mark Waghorn via SWNS

The saying "like a moth to a flame" may have to be revised as artificial light does not attract insects but 'captures' them, according to new research.

It interferes with the systems they use to orientate their body when flying, say scientists.

Lead author Dr. Samuel Fabian, of Imperial College London, said: "Our results suggest artificial lights may only trap passing insects rather than attract them directly from farther away."

The centuries-old adage indicates sometimes things that seem irresistibly inviting might hurt you.

It was first used to emphasize a type of attraction that might cause someone's destruction.

In Shakespeare's 1596 work "The Merchant of Venice", one of the main characters, Portia, compares men to annoying pests.

She says: "Thus hath the candle singed the moth."

Until now, the leading theory has been nocturnal insects use the moon's light to navigate - and mistake artificial lights for it.

But the idea doesn't explain why diurnal counterparts also gather around lights, reports New Scientist.

It also predicts that insects will fly in a spiral toward lights, which isn't what they do.

To get to the bottom of the mystery Dr. Fabian and colleagues filmed insects in the wild around lights using a high-speed camera.

They also used motion capture in an enclosure to trace the precise movements of other insects including dragonflies and moths.

The study showed when insects fly above lights, they often invert themselves and try to fly upside down, causing them to plummet.

Just after insects pass under a light, they start doing a loop-the-loop. As their climb angle becomes too steep, they stall and start to fall.

When they approach light from the side, they also may circle or 'orbit' the light.

The videos revealed insects fly at right angles to lights rather than directly at them.

But inversions and stalls sometimes result in individuals falling on lights - making it appear deliberate to the naked eye.

Dr. Fabian explained: "What is common to all three behaviors is the insects are keeping their backs to the light - triggering what is known as the dorsal light response."

This reflex exists in some fish as well as many insects. It's a shortcut for animals to work out which way is up - and keep their bodies upright.

It relies on the fact that, even at night, the brightest hemisphere in the visual field is usually up.

There are circumstances in which the phenomenon might lead an insect awry - such as at dawn or dusk.

But insects use a combination of methods to tell which way is up - and different species rely on the dorsal light response to different
degrees.

The researchers discovered oleander hawkmoths and fruit flies, for example, didn't invert or orbit near lights.

However, in many species, the dorsal light response appears hard-wired. And in a simple computer simulation, virtual insects gave a dorsal light response also inverted, stalled and orbited - just like they did in the videos.

Dr. Roman Goulard, of Lund University in Sweden who was not involved in the study, showed in 2018 that shining lights upwards from below made hoverflies more likely to crash.

He said: "I think it's a totally plausible idea the dorsal light response induces this rotational behavior that traps insects around lights at night.

The dorsal light response provides a credible answer to a question that has been around for at least 2,000 years - going back to the Roman era.

Dr. Roy van Grunsven at Dutch Butterfly Conservation in the Netherlands, who has studied the effect of artificial lights on insects, said: "I've never been convinced by the other theories."

Dr. Fabian hopes the research will help find ways to minimize the damaging effects of artificial lighting on insects - which might be contributing to their global decline.

He said: "Reducing bright, unshielded, and upward facing lights will mitigate the impact on flying insects at night."