Why do insects fly towards artificial lights?

Scientists get drawn to mystery like moths to flame. But why do moths fly towards flames?

Despite people knowing that insects are attracted to artificial light since at least the heyday of the Roman Empire, we still don’t have a great answer for why that occurs.

An international team of scientists has used 3D tracking to develop the “most plausible” theory yet. But first they had to discover that insects turn their backs to artificial light.

Their study, published in Nature Communications, suggests that lights are trapping insects by interfering with their ability to perceive the horizon.

In their paper, the researchers say that one of the reasons we don’t yet have a clear answer on this mystery is that it’s very difficult to track small flying objects in low light.

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An Automeris moth (Automeris dagmarae). Credit: Sam Fabian

“That did not stop researchers from attempting innovative experiments, such as attaching moths to polystyrene boats,” they write.

“However, in-flight 3D flight trajectory and orientation measurements have remained difficult. We leveraged advances in camera hardware and tracking software to consider the sensory requirements for insect flight control, and how artificial light may disrupt them.”

The researchers used high-speed infrared cameras to track insects’ movement around light in three dimensions.

Light shining in jungle next to building
A central UV actinic tube is separately lit by two infra-red illuminators, allowing high-speed cameras to record the flight patterns of insects trapped around the light. Monteverde, Costa Rica. Credit: Sam Fabian

They examined 10 different orders of insect, including moths, dragonflies and fruit flies – both captives in a lab, and wild insects at a field site in Costa Rica.

Co-lead author Dr Samuel Fabian, a postdoctoral researcher at Imperial College London, UK, says that the lab data took 2-3 months to collect. Dr Yash Sondhi, the other co-lead author and a postdoctoral researcher at the University of Florida, US, says the field data was taken over 4 weeks.

“Each experiment can be very short, with recordings being a mere few seconds,” Fabian tells Cosmos.

“However, in that time we collect thousands of data points which require all kinds of filtering and conversion until we have workable results.”

Person in dark lab holds yellow butterfly under uv light
Dr Samuel Fabian holds a brimstone butterfly (Gonepteryx rhamni) next to a UV actinic tube light inside the motion capture flight arena at Imperial College London. Credit: Thomas Angus

After analysing the movements of the insects in detail, the researchers were able to rule out several previous theories for insects’ light attraction. It’s not heat attraction, because they are also attracted to cold LEDs. Neither is it confused compasses or blinding: these theories don’t match the flight paths recorded.

It’s also probably not an escape response, because contrary to previous beliefs, insects don’t fly directly at light sources.

Instead, insects turn their upper sides – their dorsums – to the lights. This, believe the researchers, is the key to the real answer.

That dorsal turn is likely a natural response to the sun or sky, allowing the insects to navigate in normal conditions.

“Insects tend to assume the direction of light is ‘up’,” says Fabian.

Without artificial light, the sky is the brightest part of an insect’s vision – even at night. This means that insects have evolved to treat light sources as the sky.

But when faced with an artificial light, the insects end up steering continuously so that their backs are always to the light, trapping them in a circle.

This information can be used to lower the risk of trapping insects and help their conservation, according to the study.

Glasswing butterfly on leaf
A resting glasswing butterfly (Episcada salvinia), one of the day flying species entrapped by artificial light at night. Credit: Sam Fabian

The researchers emphasise removing unnecessary upward-facing lights in particular, or lights that reflect heavily off the ground.

“Upward-facing light can cause any insect flying above the light to get affected and invert or get trapped by light,” Sondhi tells Cosmos.

“Upward facing lights also have a longer reach, usually not facing any barrier for transmission, unlike a shielded light which gets more absorbed on the ground or gets scattered,” he adds.

“Any way in which we can reduce the spread of lights, and reduce direct shine and reflection upward into the air would be an advantage,” says Fabian.

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