Artificial light doesn’t actually attract insects but instead interferes with the control systems they use to orientate their body when flying

Why do insects seem attracted to light bulbs?

We finally have a good explanation for why insects gather around artificial lights, and it isn’t because they are attracted to them.

“Our results suggest artificial lights may only trap passing insects rather than attract them directly from farther away,” write Samuel Fabian at Imperial College London and his colleagues in a paper on the bioRxiv preprint server.

Until now, the leading scientific hypothesis has been that insects use the moon’s light to navigate at night and mistake artificial lights for the moon. But this celestial navigation idea doesn’t explain why insects that only fly during the day also gather around lights. It also predicts that insects will fly in a spiral towards lights, which isn’t what they do.

To find out what really happens, Fabian and his colleagues filmed insects in the wild around lights with a high-speed camera, and also used motion capture in an enclosure to trace the precise movements of other insects including dragonflies and moths.

This revealed three notable behaviours. First, when insects fly above lights, they often invert themselves and try to fly upside down, causing them to plummet.

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Second, 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.

Third, when insects approach a light from the side, they may circle or “orbit” the light.

The videos show that insects fly at right angles to lights rather than directly at them. However, because inversions and stalls sometimes result in insects falling on lights, it can appear to the naked eye as though they are deliberately flying at the lights.

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What is common to all three behaviours is that the insects are keeping their backs to the light, say Fabian and his colleagues, so they are proposing that lights trigger what is known as the dorsal light response.

This reflex, which exists in some fish as well as many insects, is 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 dorsal light response 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. Oleander hawkmoths and fruit flies, for example, didn’t invert or orbit near lights, the researchers found.

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

The dorsal-light response has been known about for decades but never proposed as an explanation for why lights trap insects until now.

“I think it is a totally plausible idea that the dorsal light response induces this rotational behaviour that traps insects around lights at night,” says Roman Goulard at Lund University in Sweden, who showed in 2018 that shining lights upwards from below made hoverflies more likely to crash.

The idea also seems a better fit than other hypotheses for why insects gather around lights. Besides celestial navigation, one is that insects fly towards light as an escape mechanism, because in enclosed spaces light sources show where gaps are. But the fact that insects rarely fly straight towards lights rules this out, say Fabian and his colleagues.

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Another idea is that insects are blinded by bright lights, but this doesn’t explain behaviours such as orbiting. Many studies have shown that insects aren’t attracted to the heat of lights.

The dorsal light response provides a credible answer to a question that has been around since at least the Roman era, says Roy van Grunsven at Dutch Butterfly Conservation in the Netherlands, who has studied the effect of artificial lights on insects. “I’ve never been convinced by the other theories.”

As Fabian and his colleagues studied only what happens within a couple of metres of a light, they can’t completely rule out a longer distance attraction to light. However, they don’t think it is likely.

They hope their research will help find ways to minimise the damaging effects of artificial lighting on insects, which might be contributing to their global decline. “Reducing bright, unshielded, and upward facing lights will mitigate the impact on flying insects at night,” write the researchers.

Reference: bioRxivDOI: 10.1101/2023.04.11.536486