Confirmed: Night Lights Drive Pollinators Away From Plants

It’s hard enough to grab an insect during the day, so try doing it in the dark, while wearing night-vision goggles. These eyepieces offer little in the way of depth perception, and “catching insects without that is not easy,” says Eva Knop from the University of Bern. Nevertheless, she and her colleagues persisted, and became better with practice.

Over several summer nights, they would head out into 14 Swiss meadows, half of which were illuminated by artificial lights, and half of which were kept in darkness. For several hours, the team would wander slowly through the fields, peering into local flowers, and catching every insect that they found. And their census, conducted over two consecutive summers, provides the first clear evidence that night-lighting deters or distracts pollinating insects. That, in turn, harms the plants they would typically visit.

Insects help to keep the world green, by spreading the pollen of 88 percent of flowering plants. Those species account for 30 percent of crop production, with a total value of $361 billion—so a world full of buzzing insect wings is also one of full human stomachs. But pollinators are in trouble. Despite the recent good news that honeybee populations have bounced back slightly in the last year, the general trend is still a downward one in Europe and North America. A third of bee and butterfly species are in decline, beset by parasitic mites, destructive diseases, toxic pesticides, and changing climate. And recently, scientists have started considering another culprit—light pollution.

Already, much of the planet’s surface is lit by artificial lights, and that proportion is growing at a rate of 6 percent a year. This metastasizing illumination spells trouble for wildlife. It can disrupt daily rhythms of mating and feeding, send migrating birds off course, fatally draw turtle hatchlings away from the ocean, and force insects to hover around light sources until they die from exhaustion.

These disruptions matter to plants because a lot of pollination is done after dark—not by the usual bees and butterflies, but by moths, bats, beetles, and more. This night shift is often unappreciated. When it comes to pollination, “we realized there’s almost nothing known about what goes on at night,” says Knop.

To rectify that, she temporarily installed street lamps in seven rural meadows. By comparing these sites to meadows that were kept in the dark, Knop’s team showed that the lamps reduced the frequency of insect visits by 62 percent. “Some species will fly directly to the light and are deterred from visiting flowers,” says Knop. “We also saw that many species would just sit on the ground and not move in the vicinity of the lamps.”

In their absence, at least one plant suffered. The team found that cabbage thistles—a species that’s common to all the meadows, and that’s usually visited heavily by nocturnal pollinators—produced 13 percent less fruit in the lit field than in the dark ones.

That’s an important result. Cabbage thistles are also pollinated by daytime insects, but it’s clear that the day shift couldn’t compensate for the inactivity of the night shift. And ultimately, they’ll suffer, too. Cabbage thistles can pollinate themselves, producing offspring that are clones of the parents. But in the long-term, this strategy would reduce their genetic diversity and leave them vulnerable to pests and diseases. If their numbers go down, they’d provide less food for the daytime pollinators. “There are these potential cascading effects when the nocturnal pollinators disappear,” Knop says.

“This is a very important study, which clearly demonstrates that artificial light at night is a threat to pollination,” says Franz Hölker from the University of Hamburg. He and others had no proof of this, but they suspected as much. For example, in one study, his team found that street lamps attract moths from up to 23 meters away, potentially distracting them from plants. In Europe, these lamps tend to be anywhere from 25 to 45 meters apart. Aligned in a row, they offer us a sense of security. But to moths, they might as well be a wall.

Night lights aren’t going away any time soon, but they could be changed. “LEDs are replacing older light sources, and they have higher levels of blue light that are particularly attractive to insects,” says Knop. Altering their color would help to reduce the harm they cause.