In 2021, researchers found 52% fewer moth caterpillars beneath LED; lit hedgerows than nearby unlit ones, an overlooked cost of brighter nights

A streetlight may make a dark road seem safer and easier to navigate; but for the insects living alongside that road, the same light could be affecting what takes place after sunset. When researchers looked at roadside habitats in England they came across something difficult to ignore: hedgerows located under LED streetlights had 52% fewer moth caterpillars than those nearby hedgerows which were not illuminated. However, a similar difference was observed with traditional sodium lighting, in this case, the effect was apparently smaller.

The observation adds to growing concern over light pollution and what happens when artificial light reaches habitats that would normally be dark at night. Since moth caterpillars stay close to where they live, they may provide scientists a much clearer picture of how streetlights are affecting local insect populations.

What did researchers find beneath LED lights?

For far too long, scientists have known that artificial light can interfere with nocturnal insects. Moths are particularly important because they serve as food for other animals and contribute to pollination. Yet much of the earlier research concentrated on adult insects, which can travel considerable distances.

This study took a different approach. Researchers counted moth caterpillars, which move only a few metres, making them useful for measuring what is happening in a particular stretch of habitat.

Douglas Boyes of the UK Centre for Ecology & Hydrology led the research, spending more than 400 hours sampling roadside habitats over three years. The team examined 55 lit and unlit sites across the Thames Valley and compared stretches of road that were similar except for whether they were illuminated at night.

Nearly 2,500 caterpillars were collected from roadside hedgerows and grasses.

The difference was striking. Hedgerows beneath bright LED streetlights contained 52% fewer caterpillars than comparable dark sections. Hedgerows illuminated by sodium lamps had 41% fewer.

The contrast became even more noticeable in grass margins. LED lighting was linked to a 33% reduction in caterpillar numbers, while sodium lighting had little effect on abundance in that habitat.

Why might LEDs affect moths more?

The researchers say the effects of street lighting were more pronounced with white LEDs than with traditional yellow sodium lamps. LEDs also tend to be brighter because they are energy-efficient and inexpensive to operate.

The exact reason for the difference is still uncertain. The researchers suspect that female moths may lay fewer eggs in places illuminated by streetlights, but they say more research is needed to establish whether that explains the lower numbers.

The caterpillars themselves also appeared different. According to Boyes, caterpillars were fatter in illuminated sections, which may indicate abnormal development.

Boyes said: “The effects observed – on local abundance, development and feeding behaviour – were more pronounced for white LEDs compared to traditional yellow sodium lamps.”

Artificial light can already interfere with moth behaviour. Many moths are attracted to light sources, and spending energy around artificial lighting may draw them away from food or shelter. Previous research has also suggested that artificial light can interfere with mating and pollination.

The new research therefore raises a different question: could streetlights affect not just individual insect behaviour, but the number of insects living in a particular area?

Could changing streetlights help?

The researchers say the growing use of LEDs also provides an opportunity to reduce their effects on wildlife.

Boyes said: “The rapidly increasing prevalence of LED lights, which are often much brighter as they are so energy-efficient and cheap to run, is likely to increase the negative impacts of light pollution on insects. This is expected to have knock-on effects on other species, including predatory insects, hedgehogs, and songbirds, which need to find hundreds of caterpillars a day to feed themselves and their young.”

He also pointed to relatively simple changes. LEDs can be dimmed, while filters can be used to block blue wavelengths considered particularly harmful to insects.

Professor Darren Evans of Newcastle University said: “Light pollution is one of the few causes of biodiversity loss that has easy solutions. Ultimately, 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.”

The wider decline in moths makes the finding particularly relevant. Dr Richard Fox of Butterfly Conservation said: “The total abundance of moths in Britain has decreased by one third over the past 50 years. Understanding, and reversing, the causes of this worrying decline is a top priority for us. This research provides a vital insight into the negative impacts of streetlights on moth populations.”

At the same time, the researchers caution against treating street lighting as the only explanation for falling moth numbers. Britain’s moth population has declined by around a third over 50 years, while habitat loss and climate change remain major drivers. Less than 3% of the UK is strongly illuminated by streetlights.

Still, the study points to something that could be changed without abandoning outdoor lighting altogether. Reducing brightness, filtering light and limiting when lights are switched on could offer insects darker spaces while maintaining public lighting where it is needed.



The research was carried out by the UK Centre for Ecology & Hydrology, Newcastle University and Butterfly Conservation, and was funded by the Natural Environment Research Council, part of UK Research and Innovation, and Butterfly Conservation. The findings were published in Science Advances.

FAQs

How many fewer caterpillars were found under LED lights?
Researchers found 52% fewer moth caterpillars in LED-lit hedgerows than in nearby unlit areas.

Are LED lights worse than sodium lamps for moths?
In this study, the effects were stronger under white LEDs than under traditional sodium lighting.