It has long been though that light attracted insects. However, new evidence suggests that is is not attraction, but disorientation. Photo credit: Johannes Plenio via Unsplash
“I am a moth who just wants to share your light”: these words from Thom Yorke of Radiohead suggest that insects gather around lights by attraction, but new research points to entrapment as a more likely explanation. From the dawn of fire to someone switching on a light in the summer, humans have watched insects perplexingly buzz around lights, and unsubstantiated theories to explain this behaviour have proliferated. New camera technology has unveiled the secrets of this behaviour; insects get stuck in confused circles around a light source due to the way they have evolved to orientate whilst flying.
From the dawn of fire to someone switching on a light in the summer, humans have watched insects perplexingly buzz around lights…
Previous proposals for explaining this phenomenon include that night-flying insects are inadvertently using artificial lights instead of the moon to navigate, or that it is the heat given out by lights that attracts them. These ideas have lacked support or been debunked over time.
Robust testing of these ideas is difficult. Getting the necessary data requires tracking very small and fast flying insects around a light: a difficult combination for a camera to capture, yet increasingly sensitive cameras have paved the way for this work.
…insects get stuck in confused circles around a light source due to the way they have evolved to orientate whilst flying.
By fitting test insects with tiny reflective markers, researchers followed changes in their location and body position in great depth using motion capture cameras. To ensure this artificial setup did not lead to interpreting unnatural behaviour, the researchers also filmed insects in the wild around lights, using multiple cameras to record their mysterious behaviour.
These two sets of recordings revealed that both day-flying and night-flying insects tilted their backs towards the light as they flew around it: a behaviour called the dorsal light response. Normally, this behaviour would help insects orientate as they fly. Keeping their backs pointing towards the light would make sure they were flying the right way up, as before human interference, the sky (both in the day, and through the stars at night) would have been the brightest features in their world.
Around a lightbulb, however, this orientation behaviour causes them to fly in circles, effectively trapping them in a loop. Furthermore, if the light is from underneath them in flight, the insect will flip itself upside down, leading to a crash landing.
Older ideas suggest that insects mistake the light for the moon, which may be used to navigate. This new research debunks this theory. If the insects were using the light to navigate, we’d expect them to be keeping a consistent direction towards it (using it like a compass) rather than the circling observed.
Despite the new data, this is not case closed. It remains unexplained why very small number of species didn’t have their flight affected by artificial lighting. This could be driven by species having different wavelengths of light to which they’re most sensitive.
Although this study explains what happens in our living rooms and around campfires, it doesn’t explain how artificial lights draw in insects from further away. In a previous study, only two out of 50 moths released 85 metres away from a streetlight ended up flying around the light. This suggests that artificial lights might only trap nearby insects rather than bringing in those from further afield.
Understanding this behaviour not only reveals the biology behind the phrase “like a moth to a flame”, but also highlights the impact that everyday actions have on animals that evolved in a world free from human disturbance.
