Street lights lure millions of mayflies to their deaths, as seen on this bridge in Navarre, Spain. Researchers are testing ways to keep the insects closer to the water.
Eduardo Blanco / Minden Pictures
By Elizabeth Pennisi May. 4, 2021, 5:40 p.m.
Mayfly massacres occur on bridges around the world every summer. First, warm weather leads to the transformation of insect water larvae. Within hours, the short-lived, flying adults emerge from streams, rivers, and lakes, eager to mate and lay eggs.
Bridges illuminated with artificial light, however, can lure the newly created adults away from the water into a futile death before breeding. Others, fooled by the sheen of the reflective pavement, drop their eggs on the bridge road instead of the water. Since mayflies control the growth of algae and are food for fish, the fate of these humble insects can reverberate in ecosystems, says Ádám Egri, a biological physicist at the Center for Ecological Research in Budapest, Hungary who works to save endangered mayflies there .
Mayflies are not alone in their deadly attraction for what researchers call ALAN: artificial light at night. Studies from around the world show worrying effects on mating and abundance of insects, says Stéphanie Vaz, an entomologist on the main campus of the Federal University of Rio de Janeiro. Last year researchers published the first experimental and regional studies on the issue, and in March Insect Conservation and Diversity devoted a specific topic to the topic.
Some researchers believe brighter nights could be a factor in recently documented insect declines, says Stephen Ferguson, a physiological ecologist at the College of Wooster. With insect numbers falling by 80% in some places and 40% of insect species estimated to be critically endangered, “some researchers have started making more noise about the ‘insect apocalypse’,” says Ferguson. “ALAN is almost certainly one of the drivers.”
As soon as they sound the alarm, scientists point to simple solutions. For example, Egri has found that mounting bright lights on the sides of the bridges keeps the mayflies close to the water. However, the researchers are “still at the beginning of the story of global, environmentally friendly artificial lighting,” he says.
Many insects and other animals are drawn to light because they depend on the moon or the sun for navigation, Ferguson says. According to ALAN researcher Franz Hölker from the Free University of Berlin, who calculated this estimate using satellite, energy consumption and other data, night light increases by up to 40% per year. Cities use more light-emitting diodes, the blue light of which is brighter than the yellow glow of street lamps with sodium vapor.
Even dark areas are no longer very dark. “Protected areas cannot buffer these light intensities as we thought,” says Vaz. On moonless nights, the artificial glow of the sky now exceeds the combined light of stars and other natural sources on 22% of the total land in the world, with biodiversity hot spots being disproportionately affected. Brett Seymoure, a behavioral ecologist at Washington University in St. Louis, and colleagues report at the SSRN preprint library.
Given the many other factors that harm insects as well, such as: B. Habitat degradation and climate change, associating light with species decline is challenging. “It is a very understaffed area,” says Hölker. But scattered studies suggest that the effects can be strong. He and others have calculated that Germany’s 9 million street lights attract around 1 billion insects each night, many of which die or are killed by bats and other predators. Researchers have estimated that at least a third of insects swarming around artificial light die of exhaustion or are eaten by predators.
In Grand Teton National Park, a new system of darker, reddish lights attracts fewer insects – and lets visitors see the stars.
Barber Lab / Hunter Cole
A recent study underscores the extent of the effect. On the night of July 27, 2019, the glow of Las Vegas lights drew large numbers of locusts into the air over the city, according to a publication in Biology Letters on March 31. The grasshoppers’ clouds were visible on the weather radar; Elske Tielens, an ecologist at the University of Oklahoma, Oklahoma City, and her colleagues estimated the number of insects seen on radar before, during, and after the swarm and calculated that the swarm was 30.2 tons at its peak weighed and contained 48 million locusts.
On that single night in July, “there were more locusts in the air than human visitors to Las Vegas in an entire year,” says Tielens. “This is likely happening on a smaller scale in many places and with a lot more insects,” adds Ferguson.
In the Netherlands, a consortium of universities, non-profit organizations, industry and government is studying the effects of light on local ecosystems as part of the Light on Nature project. Long-term experiments were carried out in seven plots in dark areas. The researchers illuminated some plots with lights of different colors and monitored bat and insect communities. Between 2012 and 2016, moth numbers remained constant in dark plots, but declined by 14% in lighted areas, reported Roy van Grunsven, entomologist at Dutch Butterfly Conservation, and colleagues in Current Biology in June 2020.
“This study is the only published experimental evidence” of the long-term effects of ALAN, says Daniel Boyes, an entomologist at the UK Center for Ecology and Hydrology in Wallingford. “The bottom line is that moths are being bombarded with unnatural night-time conditions that their sensory systems are not suited to,” adds Seymoure.
Most research on artificial light so far has been carried out in temperate climates. However, Vaz’s model studies indicate that light pollution is a possible cause of a decline in firefly diversity in the Brazilian Atlantic forest. And Jessica Deichmann, applied ecologist at the Smithsonian Conservation Biology Institute, documented what happens when electric lights are switched on for the first time in a remote tropical forest in Peru. “I’ve seen firsthand the really massive storm clouds of insects that were attracted to lights when they were first installed, and it’s hard to forget the sight,” she says. Most insects, especially flying ants and flies, die of exhaustion or are eaten.
She fears that the nightly tolls will limit pollination and other ecosystem services of these species. Like more and more ALAN researchers, she is looking for solutions. Her team set up experimental areas in the forest that were lit by lights of different colors and discovered that amber lights attracted 60% fewer insects than white light.
But what is good for some flying insects can be bad for others, as Tufts University graduate student Avalon Owens described at a virtual meeting of the Society for Integrative and Comparative Biology in January. Owens assessed how fireflies and other flying insects reacted to red, blue, and amber light in Kellettville, Pennsylvania, a rural area with low light pollution and so many Photinus carolinus fireflies that the city hosts an annual firefly festival. Watching fireflies in the wild: “I found red light was the best and amber was the worst for disrupting advertising,” she says.
In the laboratory, she discovered that when the light was amber, “women become almost completely dark” and that men have no way of finding them, she and her colleagues reported in the special issue.
Egri and his colleagues also tested the effect of colors, hung beacons in different hues low on a bridge, and then took photos and then counted mayflies. Blue lights, even brighter than the yellowish street lamps, kept more insects close to the water. The blue beacons installed on the Tahitótfalu Bridge in northern Hungary have been lit three hours after sunset while the lights on the road are dimmed for two sources. That seems to be working, says Egri. “No mayflies left the river.”
Darker, reddish lights are being tested elsewhere, including at a visitor center in Grand Teton National Park. But Egri says that his own efforts and others “are still too little”. Deichmann agrees that more ambitious measures are needed. In the interests of insects and ecosystems, it is imperative that essential areas of our planet remain dark forever.
