Artificial nighttime lighting is suppressing moth activity

Moths move significantly less when exposed to artificial nighttime light, new research shows.

Moths’ attraction to artificial light, such as streetlights, is common knowledge and has been much studied. But, as many people will have observed, moths may also remain still if they land near a light, apparently “trapped”.

To understand this behaviour, University of Exeter researchers caught more than 800 moths from 23 species and exposed them to LED lights (of various colours and brightness) or to natural night conditions.

Moths were collected with light traps and butterfly nets on the Penryn Campus grounds, then placed outside in individual pots exposed to one of the light conditions, and filmed to record their movements overnight.

Under 10 lux – a level you might experience near residential streetlighting – moths moved 85% less on average than moths in natural nighttime conditions. This is a big impact on how moths behave, with implications for the time they have for essential activities like finding food or mates.

This could be because the artificial light confuses the moths’ sense of time, causing them to remain still as they do in the daytime, or because the light interferes with their vision.

The study also found that, at that same 10 lux intensity, amber lighting – commonly thought to be less harmful to insects – reduced moth movement just as much as typical white LEDs.

“Our results suggest that light pollution has widespread effects on nocturnal moth activity, irrespective of lighting type,” said Dr Emmanuelle Briolat, from the Centre for Ecology & Conservation at Exeter’s Penryn Campus in Cornwall.

“This could have important knock-on effects for moth populations and the ‘ecosystem services’ they provide; moths are important pollinators, and a key food source for many other species, from birds to bats.

“Across Europe, moths are in decline, part of a worrying picture of global insect losses, linked to threats like pollution and land-use change – and light pollution could be a significant contributing factor.

“We already know that artificial lighting has many negative impacts on nocturnal insects, and our findings further stress the importance of preserving dark skies.”

Some moth species were even affected by white LED lighting at low levels of 0.1 lux. This is equivalent to indirect ‘skyglow’, where light from towns and cities illuminates the entire sky across many kilometres, meaning the effects could be more far reaching.

The study also looked at how the activity of moths changed through the course of the night.

Dr Jolyon Troscianko said: “While we might think that most moths are simply ‘nocturnal’ and active through the whole night, we found huge variability among species. Some were most active at the start of the night, others at the end, and most were only ever active for short periods.

“Having such short windows of activity has implications for how artificial light could interfere with the moths’ ability to find food or mates before their short adult lives end. While sitting still under artificial lights won’t kill the moths immediately, these sub-lethal effects could easily add up and prevent them from breeding successfully.”

The study was funded by the Natural Environment Research Council.

The paper, published in the journal Proceedings of the Royal Society B, is entitled: “Severe and widespread reductions in nighttime activity of nocturnal moths under modern artificial lighting spectra.”