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Streetlights lure mayflies by the millions to their deaths, as seen on this bridge in Navarra, Spain. Researchers are testing ways to keep the insects closer to the water.

Eduardo Blanco/Minden Pictures

Can scientists help insects survive their fatal attraction to light at night?

Each summer, on bridges across the world, mayfly massacres occur. First, warm weather prompts the transformation of the insects’ aquatic larvae. Within hours, the short-lived, flying adults pop out of streams, rivers, and lakes, eager to mate and lay eggs by the millions.

But bridges illuminated with artificial light can lure the newly emerged adults away from the water to a futile death before breeding. Others, fooled by the sheen of reflective pavement, drop their eggs on the bridge road instead of the water. Because mayflies control the growth of algae and are food for fish, the fate of these humble insects may reverberate through ecosystems, says Ádám Egri, a biological physicist at the Centre for Ecological Research in Budapest, Hungary, who is working to save endangered mayflies there.

Mayflies aren’t alone in their fatal attraction to what researchers refer to as ALAN: artificial light at night. Studies from around the globe are finding worrisome impacts on insect mating and abundance, says Stéphanie Vaz, an entomologist at the Federal University of Rio de Janeiro’s main campus. In the past year, researchers have published the first experimental and regional studies of the problem, and in March, Insect Conservation and Diversity devoted a special issue to the topic.

Some researchers think brighter nights may be a factor in recently documented insect declines, says Stephen Ferguson, a physiological ecologist at the College of Wooster. With insect numbers dropping by 80% in some places and 40% of insect species headed for extinction by some estimates, “Some researchers have started to make more noise about the ‘insect apocalypse,’” Ferguson says. “ALAN is almost certainly one of the drivers.”

Even as they begin to raise the alarm, scientists are pointing to simple solutions. Egri, for example, has found that mounting bright lights low on the sides of bridges keeps the mayflies close to the water. But researchers are “still at the very beginning of the story of global, ecologically friendly artificial lighting,” he says.

Many insects and other animals are drawn to light because they depend on the Moon or Sun for navigation, Ferguson says. And light at night is increasing by an average of 2% to 6% and up to 40% per year in remote places, according to ALAN researcher Franz Hölker at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, who calculated this estimate using satellite, energy use, and other data. Cities are using more light-emitting diodes, whose blue light appears brighter than the yellow glow of sodium vapor streetlights.

Even dark areas are no longer very dark. “Protected areas are not able to buffer these light intensities as we thought,” Vaz says. On Moon-less nights, artificial sky glow now exceeds the combined light of stars and other natural sources on 22% of the globe’s total land, with biodiversity hot spots disproportionately affected, Brett Seymoure, a behavioral ecologist at Washington University in St. Louis, and his colleagues report in the preprint elibrary SSRN.

Given the many other factors also hurting insects, such as habitat degradation and climate change, linking light to species’ declines is challenging. “It is a very understudied field,” Hölker says. But scattered studies suggest the impact may be powerful. He and others have calculated that Germany’s 9 million streetlights attract about 1 billion insects a night, many of which die or are killed by bats and other predators. Researchers have estimated that at least one-third of the insects swarming around artificial lights die of exhaustion or are eaten by predators.

In Grand Teton National Park, a new system of dimmer, reddish lights attracts fewer insects—and lets visitors see the stars.

Barber Lab/Hunter Cole

One recent study underscores the magnitude of the effect. On the night of 27 July 2019, the glow of Las Vegas lights lured massive numbers of migrating grasshoppers into the air above the city, according to a 31 March paper in Biology Letters. The clouds of grasshoppers were visible on weather radar; by estimating numbers of insects seen on radar before, during, and after the swarm, Elske Tielens, an ecologist at the University of Oklahoma, Oklahoma City, and her colleagues calculated that at its peak, the swarm weighed 30.2 tons and contained 48 million grasshoppers.

There were “more grasshoppers in the air on that single July night than human visitors to Las Vegas in a whole year,” Tielens says. “This is probably happening on smaller scales in many places, and with many more insects,” Ferguson adds.

In the Netherlands, a consortium of universities, nonprofit organizations, industry, and government is exploring light’s effects on local ecosystems through the Light on Nature project. It set up long-term experiments in seven sets of plots in dark areas. The researchers lit up some plots with lights of different colors and monitored bat and insect communities. Between 2012 and 2016, moth numbers remained steady in dark plots but decreased 14% in lighted areas, Roy van Grunsven, an entomologist at Dutch Butterfly Conservation, and colleagues reported in June 2020 in Current Biology.

“This study represents the only published experimental evidence to date” about ALAN’s long-term effects, says Douglas Boyes, an entomologist at the UK Centre for Ecology and Hydrology in Wallingford. “The bottom line is that moths are being bombarded with unnatural night conditions that their sensory systems are not adapted for,” Seymoure adds.

Most of the research on artificial light so far has taken place in temperate climates. But Vaz’s modeling studies point to light pollution as a possible cause for a decline in firefly diversity in Brazil’s Atlantic Forest. And Jessica Deichmann, an applied ecologist at the Smithsonian Conservation Biology Institute, documented what happens when electric lights were first turned on in a remote tropical forest in Peru. “I’ve witnessed firsthand the truly massive storm clouds of insects drawn to lights when they are first installed, and this sight is hard to forget,” she says. Most of the insects, particularly flying ants and flies, die of exhaustion or are eaten.

She worries the nightly tolls will curtail pollination and other ecosystem services provided by these species. So, like more and more ALAN researchers, she is seeking solutions. Her team set up experimental plots in the forest lit by lights of different colors and discovered amber lights attracted 60% fewer insects than white light.

But what’s good for some flying insects may be bad for others, as Tufts University graduate student Avalon Owens described in January at a virtual meeting of the Society for Integrative and Comparative Biology. Owens evaluated how fireflies and other flying insects reacted to red, blue, and amber light in Kellettville, Pennsylvania, a rural area with little light pollution and so many Photinus carolinus fireflies that the town hosts an annual firefly festival. Observing fireflies in the wild, “I found red light is ‘best,’ and amber is ‘worst’ for interfering with courtship,” she says.

In the lab, she found that in amber light, “females go almost completely dark,” leaving males no way to find them, she and her colleagues reported in the special issue.

Egri and his colleagues, too, tested the impact of color, hanging beacons of different hues low on a bridge, then photographing and counting mayflies. Blue lights, being even brighter than the yellowish road lights, kept more insects close to the water. For two springs now, blue beacons installed on the Tahitótfalu bridge in northern Hungary have shone for 3 hours past sunset, while lights on the roadway are dimmed. This seems to work, Egri says. “No mayflies left the river.”

Elsewhere, dimmer, redder lights are being tested, including at a visitor center in Grand Teton National Park. But Egri says his own effort and others “are still too little.” Deichmann agrees that more ambitious measures are needed. For the sake of insects and ecosystems, “It is absolutely essential to ensure substantial areas of our planet remain dark forever.”

*Correction, 5 May, 9:45 a.m.: An earlier version of this story misspelled Douglas Boyes’s name.