Air Pollution Makes Flowers Smell Less Appealing to Pollinators, Study Suggests

Nocturnal hawk moths are less likely to visit primroses in air polluted by nitrate radicals, which break down important wild fragrances, researchers find

A photo-illustration of a hawk moth landing on a primrose flower, with an exhaust pipe polluting the interaction.
In a photo illustration, a hawk moth lands on a flower with an exhaust pipe polluting the interaction. Floris Van Breugel

Scientists have revealed a lesser known effect of burning fossil fuels: the disappearance of the world’s natural, vibrant scents.

In a new study published Thursday in the journal Science, researchers suggest the relentless stream of emissions from cars, factories and other industrial sources is interacting with and breaking down wild plant fragrances, an essential tool for attracting pollinators.

The team of nine researchers investigated the olfactory symbiosis between primrose, a wildflower that blooms only at night, and hawk moths, nocturnal insect pollinators that are drawn to the flowers’ aroma.

Their methods showed that some of air pollution’s most common chemical agents—ozone and nitrate radicals—significantly deteriorate the wildflower’s scent, deterring the moths from landing on polluted plants.

“We worry a lot about exposure of humans to air pollution, but there’s a whole life system out there that’s also exposed to the same pollutants,” study co-author Joel Thornton, an atmospheric chemist at the University of Washington, tells the New York Times’ Emily Anthes. “We’re really just uncovering how deep the impacts of air pollution go.”

The researchers traveled to a field in eastern Washington where primrose grew and used plastic bags to capture air samples containing the flowers’ fragrance. Back in the lab, they deconstructed this scent into 22 unique chemical components that might attract hawk moths, with their ultra-sensitive antennae.

“They’re as good as a dog in terms of their chemical sensitivity,” Jeff Riffell, a sensory neurobiologist at the University of Washington and a study co-author, tells the New York Times.

A primrose with a transparent bulb over the petals, with tubes and cords connecting to the apparatus
A primrose during field experiments in eastern Washington. Jeremy Chan

By examining electrical activity in moths’ antennae, the team determined which components of the primrose fragrance were most attractive to the insects. Chemicals called monoterpenes, which are found in many fruits and vegetables and help give bark and coniferous leaves their signature smells, were a moth favorite, the team found.

With this knowledge, they created their own eau de primrose, then sullied it with chemicals to simulate a flower exposed to common air pollutants. When just ozone was added, the scent’s monoterpene concentrations fell by about 30 percent. But when they mixed in nitrate radicals, which primarily act at night when hawk moths are out, the sweet-smelling monoterpenes dropped by 84 percent.

The fragrance’s weakening translated directly into two different hawk moth species visiting the artificial primrose less and less. In an isolated wind tunnel, the team set up a fake flower with the homemade polluted-primrose scent. Compared to an unaltered primrose in the wind tunnel, tobacco hawk moths reduced their visitation by 50 percent and white-lined sphinx moths by 100 percent—the latter species stopped visiting the polluted flower entirely.

“Pollinators [are] literally starving,” Ilaria Negri, a zoologist at the Catholic University of the Sacred Heart who was not involved in the study, tells Science’s Elizabeth Pennisi. “That pollinators are unable to find already scarce food due to pollution is challenging for many species.”

a man lies on his stomach in a grassy field. He closely investigates a white primrose wildflower; equipment stands in the background
Researcher Jeremy Chan, a co-author of the study, observes wildflowers at the research site in eastern Washington. Jeremy Chan

When the researchers mixed their fake flower into a field with other healthy primroses, hawk moths visited the polluted imposter 70 percent less over one night, as compared to the others. In turn, this reduction would lead primroses to produce 28 percent fewer seeds, the scientists write.

Finally, the team designed computer models to map how scents interacted with the preindustrial atmosphere. They found that, since then, the level of air pollution in most cities around the world has reduced the distance from which pollinators like hawk moths can detect scents by up to 75 percent.

“The beauty of this research is that it is truly multidisciplinary, combining both laboratory and field experiments,” Mark Elgar, a professor of evolutionary biology at the University of Melbourne in Australia who was not involved in the study, tells Popular Science’s Lauren Leffer. “We’d be nuts not to continue to investigate this.”

The findings demonstrate the impact of “sensory pollution,” or human-made factors that can mess with animals’ perception of the world. For example, artificial lights can disorient insects or migrating birds, and the din of cars, planes and ships can deafen aquatic creatures and interfere with animals’ reproduction.

“In recent years, there has been a growing interest in ‘sensory pollution,’” Riffell tells New Scientist’s Chen Ly.

The new findings suggest that pollutants affect animals’ sense of smell, too. And if other pollinators react to pollution in a similar way to hawk moths, then agricultural output, biodiversity and the microworlds of insects—which are crucial for the health of humans and other wildlife—may all be impacted when scents disappear.

“It’s just more motivation,” Thornton tells Popular Science, “for moving our transportation and energy needs away from fossil fuel combustion and to other sources of energy.”

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