Centuries ago, curious observers established a simple yet fascinating paradigm: moths are helplessly attracted to light. This was further corroborated with the advent of electricity and today we take for granted that patio lights will call insects to gather. But like many insects, moths are increasingly threatened by artificial light at night, which causes them to crash and fall, and puts them at risk from predators. In recent years, researchers have tackled the question of why moths are drawn to light. We now know that, when flying by an LED lamp, many insects will position their back to the light source. They then fly erratically and become entrapped by the light, which leads to exhaustion and often death. But does that mean that only those unlucky enough to fly by a lightbulb will be affected? In a recent study, Jacqueline Degen (University of Würzburg, Germany) and colleagues from Germany explored this question.
The researchers found a dark field close to the small German village of Großseelheim and placed six identical streetlights in it, 85 m apart, forming a circle. In addition, they trapped lappet moths (Euthrix potatoria) and hawk moths from the local surroundings before attaching tracking devices to 94 of the insects. Then, they released the moths one at a time from the centre of the light circle and tracked them with radar for up to 1 km as they flew, until the radar signal was lost or they stopped moving for longer than 10 min. After analysing the moths’ flight paths the researchers were surprised to discover that only 4% of the insects ended up being attracted to, and flying right at, one of the streetlights. In fact, ‘attraction’ might not be a fair word to describe how moths feel about light. Instead, they were rather disoriented and confused, becoming entrapped in the ring of light encircling the six lights, almost as if they were enclosed by a barrier.
The scientists also studied how much the insects’ flight paths began meandering when the lights were on, compared to when they were turned off. As the lappet moths were local to the field, the researchers suspected that their flight paths would naturally be windy, as they wander around their home searching for mates. In contrast, the hawk moths, which resided in the surrounding areas, should fly in a straight path out of the field as they aim to return home. Knowing that both moths’ flight plans should be quite different when there was no streetlight, the team wondered how the flight paths would change when they switched on the lights and whether any other factors might impact the routes the moths took.
Analysing the flight routes recorded by radar, the team noticed that on nights when the moon was visible, the streetlights caused the hawk moths to take more winding flights. They speculated that since hawk moths use moonlight as a navigation guide, streetlights mislead them because the brightest region in their world view is no longer the sky. Meanwhile, the lappet moths meandered more without the moon. This suggests that, compared to the hawk moths, this species relied less on the moon for navigation. But without it, the illumination in the lit area was too drastic relative to its dark surroundings, confusing the moths and keeping them trapped within its range.
These findings challenge the early, widespread notion that insects are simply attracted to light. So next time you see a swarm of nocturnal critters partying under a light bulb, remember they probably only represent a tiny fraction of the population; the majority might be stuck elsewhere in an invisible light trap.
