From the Bhagavad Gita to the Merchant of Venice, storytellers have warned of the unattractive fate that awaits those who are drawn like a moth to a flame.
Despite the rich history of the startling behavior, the science of why insects congregate around lights at night has never been nailed down. Popular theories suggest that moths navigate the moon and mistake lamps for moonlight, or that the insects fly to light to escape impending danger.
Now researchers think they have a more convincing answer: rather than being attracted by light at night, moths and other flying insects are caught in their glow. This is the unfortunate result of a neat trick developed over millions of years that has failed in the modern world.
According to Dr Sam Fabian, an entomologist at Imperial College London, moths and many other night-flying insects have evolved to tilt their backs to wherever is the brightest. For hundreds of millions of years it was the sky rather than the ground. The trick told insects which way was up and ensured they flew level.
But then came artificial lighting. With fresh sources of illumination to contend with, moths found themselves turning their backs on street lamps. This sent them in endless loops around the lamps, the insects trapped by their evolutionary instincts.
Fabian and his colleagues captured insect flight paths to light using high-resolution, infrared motion capture in the laboratory and high-speed infrared video recordings in Costa Rica. The footage reveals that moths and dragonflies repeatedly turned their backs on artificial lights and played havoc with their flight paths.
“If the light is above them, they might start to spin, but if it’s behind them, they start to tilt backwards and that can cause them to climb up and up until they stop,” Fabian said. “More dramatic is when they fly directly over a light. They turn themselves upside down and this can lead to accidents. It really indicates that the moth is confused about which way to go.”
The work, published in Nature communication, suggests that artificial lighting may not draw flying insects out of the dark, but simply trap those that fly by. “It’s almost like having a net,” Fabian said.
Researchers have long warned that light pollution is a major driver in the dramatic decline in insect populations. Moths and other insects trapped around lamps become easy prey for bats, but the lighting can also trick them into thinking it’s daytime, causing them to go to sleep and skip a night’s feeding.
“The effect on their flight is really only a small component of how artificial light can mess up the lives of these nocturnal insects,” Fabian said.
There are, he believes, useful lessons from the research. “What I think this tells us is that the direction of artificial light matters. If you’re going to have lights at night, you really want them to be shrouded and not throw loads of light sideways, and especially not up into the atmosphere,” he said.
Prof Gareth Jones at the University of Bristol called the work “fascinating”. “It is remarkable how an innate and adaptive behavior, whereby an insect positions itself so that its back faces the light and thus maintains a steady flight path, becomes maladaptive near sources of strength such as lamps,” he said. “The findings indicate that the large numbers of insects that congregate at street lights are trapped there by circling the lamps.
“Reducing attraction to – and containment of – lamps will be very important to reduce impact on insects. This can be achieved by using lights that reduce the use of short wavelengths, such as blue and especially UV, and possibly even produce metameric light that appears white to humans but contains spectra that are less attractive to insects.
