Like a moth to a flame, all kinds of insects are drawn to glowing light at night—but this obsession often drives bugs to exhaustion and death. Indeed, many studies have indicated that human-caused light pollution is contributing to the devastation of millions of insects. But something as simple as changing a light’s hue may reduce the attraction for many types of insects, thus preventing insect harm and death, according to a new study published last month in the journal Insect Conservation and Diversity.
Study author Jessica Deichmann, a conservation biologist at the Smithsonian’s National Zoo and Conservation Biology Institute (SCBI) and her colleagues found putting an amber-colored filter over an LED bulb substantially reduced the amount and variety of insects drawn to the glow. The team also found disease-carrying insects were disproportionately attracted to white LED light, which could mean avoiding white light around a workspace or a home, whether in the Amazon or the mid-Atlantic, could be beneficial to human health.
It has been previously shown that the color of “artificial light at night,” or ALAN, can reduce insect casualties. While past research has found insects favor white light over amber light, Deichmann’s paper is the first to demonstrate the impact of different light hues in a tropical rainforest, specifically in settings that have never been illuminated. Deichmann says she’s seen the impact when forests are lit up for the first time.
“You basically bring the moon down to earth from the perspective of an insect,” she says. “It brings these massive clouds of bugs to circle, circle, circle and die.”
The study is valuable because it’s “one of the very few from tropical forest environments,” says Alan Stewart, an ecologist at the University of Sussex in England.
“As the adoption of ALAN continues to expand into pristine environments that have had no prior experience of artificial illumination, especially in the tropics, there is an urgent need to understand the consequences for whole communities of insects,” writes Stewart, who is also the editor of the special issue where the paper appeared, in a commentary article accompanying the paper.
Deichmann and her colleagues tested their hypotheses in a virgin lowland rainforest area of northern Peru that was slated to be developed by a gas and oil company. The scientists’ goal was to recommend lighting that would cause the least ecological harm. The company has still not developed the land as of early 2021, in part because of the Covid-19 pandemic.
The researchers set light traps in 12 different locations. The traps were illuminated with an LED white bulb, yellow-filtered bulb, or an amber-filtered bulb. A trap without light was used as the control. The scientists counted the number and type of insects attracted to the traps during two different time periods at night.
Some 15,000 insects were captured, belonging to 763 unique species. A significantly greater variety of species—and far more individual insects—were captured in the white LED traps. The amber-filtered traps had 34 percent fewer species and 60 percent fewer insects in total than the white light traps. The team also suggests fewer disease-carrying insects gather at amber lights. “Our evaluation of known insect disease vectors showed a substantial reduction of potential vectors at amber lamps,” the study authors write in the paper.
More work needs to be done to find out which insect vectors are attracted to which light wavelengths, says Stewart. “But if this is a general pattern, then it has obvious implications for the kind of lighting that should be used around housing and human settlements,” he adds.
Two groups of bioluminescent insects, however, were actually more attracted to the amber-colored light. “Switching to longer wavelength lighting to limit the adverse impacts on the majority of insects in that setting could have the unfortunate side effect of attracting these bioluminescent groups,” Stewart writes in his commentary article.
Deichmann says her team’s findings about amber light and bioluminescent insects line up with what has previously been shown in temperate forests, which she found surprising. ALAN is known to make these insects lethargic or inactive, preventing reproduction. The light can be confusing to bioluminescent insects that rely on light signals to mate. Light pollution has had a similar effect on fireflies in many areas of the United States, Canada and other nations, for example. (The Xerxes Society for Invertebrate Conservation outlines the research on ALAN and other threats to fireflies in its 2019 report.)
While ALAN influences insect movement, feeding and mating, it’s hard to tease out the impact on entire insect populations that face so many other stressors, including habitat loss, pesticide use and climate change, Stewart says.
“Insects are so diverse that generalizations are hard to make,” he says, adding that Deichmann’s results “do seem to accord with a lot of previous work in other habitats.”
Proving the harmful impact of white light and the mostly less-harmful effects of amber light in a rainforest is important, says Brett Seymoure, a behavioral ecologist at Washington University in St. Louis, who was not involved in the study.
“If you find something with a big ecological effect in the rainforest, it is probably applicable to all biomes that have trees,” says Seymoure, who has studied ALAN in various settings.
As many commercial operators and homeowners are shifting to LEDs, which tend to fall somewhere in the blue-white spectrum, Seymoure says the new results may have important implications beyond tropical rainforests..
Why care about the fate of bugs? Insects are crucial to human life, as they provide decomposition and nutrient cycling services, and pest control and pollination of crops, in addition to being food sources for birds and animals. Researchers have valued these ecosystem services at $57 billion a year in the U.S. alone, according to Deichmann’s paper.
Homeowners who want to minimize attraction of disease-causing insects and maximize insect survival should install amber-colored bulbs in outdoor lights, says Deichmann.
“It’s really easy,” she says, noting that amber bulbs can be found at many hardware stores.
“If you have a porch light make sure it focuses the light where you need it. It shouldn’t be shining up into the sky,” where it can attract and confuse insects and wildlife.
Deichmann recommends cut-off lighting that doesn’t shine upward into the night sky or motion detector switches for outdoor lights, and Seymoure suggests closing curtains and keeping unnecessary indoor lights off.
“If you are more the type of person who wants to take action, then talk to your city council about it, talk to your mayor, talk to your businesses,” says Deichmann. Individuals can petition city officials to put up yellow or amber street lights, which still provide enough light for human safety, she says.
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