LED streetlights decimating moth numbers in England
“Eco-friendly” LED streetlights produce even worse light pollution for insects than the traditional sodium bulbs they are replacing, a study has found.
The abundance of moth caterpillars in hedgerows by rural roads in England was 52% lower under LED lights and 41% lower under sodium lights when compared with nearby unlit areas.
In grass margins, moth caterpillar numbers near LEDs were a third lower than in unlit areas, whereas sodium lights had little effect on abundance. The white LED lights are more energy efficient but produce more blue light, say scientists, which is the colour predominantly seen by insects.
Moths are important pollinators and provide essential food for birds and animals, but the total abundance of moths in Britain has dropped by a third over the past 50 years.
Reports of plunging insect populations have alarmed scientists, with the destruction of wild places, pesticides and the climate crisis being major causes. Light pollution is increasing globally and was described by a recent review as an “important but often overlooked bringer of the insect apocalypse”, as it makes insects more visible to predators and disrupts feeding and reproduction.
The study is the first to examine the impact of LEDs in a real-world setting and the first to show the direct impact of light pollution on caterpillars. The caterpillars are less mobile than adult moths, and therefore show more precisely the local losses caused by light pollution.
The researchers said flying moths were attracted by light but may then be more vulnerable to predators, meaning they lay fewer eggs. They said the wide range of moth species they studied suggested their results would apply to other nocturnal insects.
“It’s a really striking result,” said Douglas Boyes, of the UK Centre for Ecology and Hydrology, who led the new research. “We found numbers that you’re not really used to ecology. You usually find maybe 5-10% changes here and there, but we found up to 50% drops in the number of caterpillars in the areas lit by streetlights.
“LEDs are the baddies in our story, if you like, because they’re worse in terms of their effect at the moment, but they also have the potential to be much better than sodium lighting.”
LEDs are dimmable, can be linked to motion sensors and can have cheap filters fitted to screen out blue light. LEDs are energy efficient, leading to reduced climate-warming emissions. They are often brighter than sodium lamps, although not at the locations in the study.
Prof Darren Evans, of Newcastle University, who was part of the study, said: “Light pollution is one of the few causes of biodiversity loss that has easy [and immediate] solutions. We need a balance between protecting both public safety and wildlife, by ensuring that lighting is well designed, away from important habitats and switched on for limited times.” Bat-friendly red lighting was installed on a road in Worcestershire in 2019.
Matt Shardlow, of the insect charity Buglife, said: “This new evidence demonstrates the massive impact that light pollution is having on local populations of insects, contributing to the terrible decline in insect abundance we have all observed.
“Given the harm artificial light causes, and the government commitment [in 2018] to reduce light pollution, it is unacceptable that it is refusing to commit to a national light pollution reduction target.”
Another study in Belgium showed that streetlights harmed the ability of female glow worms to attract a mate. It found that beetles in dark areas usually found a mate after one night of glowing but those in lit areas took up to 15 nights. In England, glow worm numbers have plunged by three-quarters since 2001.
Insect populations are suffering “death by a thousand cuts”, with many falling at “frightening” rates that are “tearing apart the tapestry of life”, according to scientists behind a volume of studies published earlier in 2021.
The latest research, published in the journal Science Advances, studied 26 pairs of roadside sites in Oxfordshire, Buckinghamshire and Berkshire, where lit and unlit sections were on average 100 metres apart. Boyes spent more than 400 hours sampling more than 2,000 caterpillars.
The scientists are using DNA analysis to see whether parasitic wasps lay their eggs more frequently in caterpillars in lit areas, which could be an additional explanation for the lower populations.
Boyes said better protecting moths was essential. “We’ve got 2,500 species in the UK. They’re really important as prey for birds, bats, hedgehogs and other predatory invertebrates. But they’re also really important pollinators. They do the night shift after the daytime pollinators have gone to bed.”
A spokesperson for the Department for Environment, Food and Rural Affairs said: “Insects are a vital part of our natural environment and protecting them is a priority. We have set a legally binding target for species abundance for 2030, which will drive the right mix of actions to address the loss of wildlife, including insects, and address the interacting pressures on biodiversity such as light pollution.”