How Light Pollution May Be Harming Insects

Head down nearly any street at night and it’s likely to be well-lit. This artificial light at night can have an impact on wildlife migration, as well as animals’ breeding, hunting, and sleeping patterns. New research finds nighttime light also may play a role in the declines in insect populations.

“Light pollution may be discussed a lot but it's only relatively recently that we've begun to understand just how harmful it can be for wildlife. A growing number of studies are showing it can be detrimental in so many ways—to plants, birds, bats, insects, etc.,” Douglas Boyes of the UK Centre for Ecology & Hydrology (UKCEH), who led the study, tells Treehugger.

To study the impact of artificial light on the insect population, Boyes and his colleagues spent three years studying moth caterpillars in southern England.

“We focus on caterpillars as these typically don't move very far in their lifetime, so when sampling at a given point, we can be confident we are precisely measuring local effects (whereas adults are very mobile and may move several kilometers in lifetime),” Boyes explains.

“Moths are highly diverse evolutionarily and ecologically (several thousand species native to Europe), meaning they should be fairly representative of nocturnal insects and are also relatively well-studied. This makes them uniquely placed for understanding the effects of lighting on nocturnal insects more generally.”

Counting Caterpillars

For the study, Boyes spent more than 400 hours along roadsides, studying and counting wild caterpillars. Dressed in high-visibility clothing because he often gathered data at night, he visited 27 pairs of sites that were home to two different groups of caterpillars that were easy to sample.

Each pair of sites consisted of a hedgerow or grass margin along the roadside that was lit by streetlights and an identical but unlit habitat. The lit sites included 14 that were illuminated by high-pressure sodium (HPS) lamps, 11 with light-emitting diodes (LED) lamps, and two with older low-pressure sodium (LPS) lamps.

In order to count the insects, Boyes beat hedges in spring and summer to count flying caterpillars and swept the grass with netting to count those that only come out at night to climb on grass to feed.

Of the total 2,478 caterpillars that Boyes counted, the majority of them came from unlit areas.

Artificial lighting reduced the number of caterpillars by somewhere between a half and one-third, the researchers found. Nearly all the lit areas, which had been illuminated for a minimum of five years, had fewer caterpillars.

Boyes weighed the caterpillars and found they were in general heavier in lit areas, which the researchers suspect is due to stress and is a result of rushed development. “This will lead to smaller adults, which are less evolutionary fit (lay fewer eggs, etc.),” he says.

In almost all situations, the results were worse under white LED lighting compared to traditional yellow sodium lighting. Boyes points out, “This is concerning given the ubiquitous transition toward white LED street lighting.”

They also did an experiment where they put up temporary LED lighting in rural grass margins that had never been illuminated before. They found that the feeding behavior of nocturnal caterpillars was disturbed.

“Our separate experiment showed that white LEDs disrupt the normal behavior of nocturnal caterpillars—possibly because white LEDs are quite similar to daylight, so the caterpillars 'think' it's still daytime,” Boyes says.

The findings were published in the journal Science Advances.

The Bigger Insect Picture

Researchers examined how their study results might translate into the larger landscape and found that just 1.1% of the land area in the study site is directly lit by streetlights. Suburban areas are frequently illuminated (15.5%) but only 0.23% of arable land and 0.68% of broad-leaved wooded land is lit.

“The evidence suggests lighting is probably not the main cause of insect declines, but clearly can contribute,” Boyes says. “The main factors are climate change, habitat loss, agricultural intensification, and chemical pollution (including pesticides, nitrogen deposition), but lighting we expect will certainly be important in some contexts.”

The areas that are impacted by lighting keep growing, he points out. Streetlights are not the only cause of light pollution, but the study results can help call attention to the connection of artificial light and potential issues with wildlife.

“They highlight that lighting is a hugely important local influence but one that's perhaps quite overlooked/underappreciated. One of the nice things about working in this field is that there are tractable solutions (compared to climate change which is a much more difficult problem to solve),” Boyes says.

He suggests that LEDs can be modified more easily than sodium lamps, through dimming and using filters to reduce blue wavelengths that are most harmful to insects.

“An 'insect-friendly' streetlight would have brightness, perhaps red in color (or at least few blue wavelengths), motion sensors, or dimming when fewest people are around. If possible, though, the best solution that evidence tells us to minimize the harms on insects is to avoid lighting where possible—but of course this is easier said than done.”