Streetlights, especially super bright LEDs, may harm insect populations

Moths flock to streetlights, bewitched by their luminous brilliance. But bathing in brightness all night seems to have consequences for the grounded forms of these fliers. Illuminated stretches of English roads housed up to 52 percent fewer moth caterpillars than adjacent dark patches, researchers report August 25 in Science Advances. Streetlights could be contributing to declining insect populations in developed areas, the researchers say.

Artificial light is generally not good for nocturnal insects. Recent work hints the glow can mess with mating or disrupt pollination (SN: 5/13/15; SN: 8/2/17). But whether night lights contribute to population decline is understudied, says Douglas Boyes, an entomologist at the UK Centre for Ecology and Hydrology in Wallingford, England. 

Boyes and colleagues compared 27 stretches of road that appeared identical except some parts were lit at night and others remained dark. Instead of looking at moth adults that can fly kilometers during their lives, the researchers counted caterpillars, which traverse just meters. At night, the team knocked dozens of species from roadside hedgerows or swept up larvae from grasses, catching nearly 2,500 caterpillars. 

Hedgerows under bright LED lights contained 52 percent fewer caterpillars than dark sections, while areas under duller sodium lamps housed 41 percent fewer. On grassy sections, LED lights cut the population by 33 percent, while sodium lamps had little effect. LED lamps emit a broader spectrum of light than other lamps, which may explain their heightened influence. Caterpillars were fatter in lit sections, which probably indicates abnormal development, Boyes says, but how exactly LED light harms caterpillars remains unclear.

The United Kingdom’s moth population has shrunk by a third in 50 years, but since less than 3 percent of the country lies under strong illumination from streetlights, habitat loss and climate change are more likely to blame than the lights, Boyes says. Still, the work highlights a relatively easy way to give some insects a break, he says. Just turn down the lights, or place filters on LEDs that narrow the spectra of light they shine down.