Contaminated flowers smell less sweet to pollinators, study finds

The damage that air pollution can cause is extensive and well known: chemicals produced by human activities can trap heat in the atmosphere, change the chemistry of the oceans, and harm human health in countless ways.

Now, a new study suggests that air pollution could also make flowers less attractive to pollinating insects. Compounds called nitrate radicals, which can be abundant in urban night air, severely degrade the odor emitted by pale primrose, reducing visits by pollinating hawkmoths. the researchers reported in Science on Thursday.

This sensory pollution could have far-reaching effects, interfering with plant reproduction and decreasing the production of fruits that feed many species, including humans. It could also threaten pollinators, which depend on flower nectar for sustenance and are already experiencing global declines.

“We are very concerned about human exposure to air pollution, but there is an entire system of life that is also exposed to the same pollutants,” said Joel Thornton, an atmospheric chemist at the University of Washington and author of the new study. “We’re really finding out how profound the impacts of air pollution are.”

The project was led by Dr. Thornton; his colleague Jeff Riffell, a sensory neurobiologist and ecologist at the University of Washington; and his joint PhD student, Jeremy Chan, who is now a researcher at the University of Naples.

The study focuses on the pale primrose, a plant with delicate flowers that open at night. Their key pollinators include hawk moths, which have exquisitely sensitive odor-detecting antennae. “They’re as good as a dog in terms of chemical sensitivity,” Dr. Riffell said.

The aroma of a flower is a complex olfactory bouquet that contains many chemical compounds. To identify the ingredients of primrose’s distinctive scent, scientists placed plastic bags over the flowers, capturing samples of the fragrant air. When the team analyzed these samples in the laboratory, they identified 22 different chemical components.

The scientists then recorded the electrical activity of the moths’ antennae when they were exposed to these aromatic compounds. They found that the moths were especially sensitive to a group of compounds called monoterpenes, which also help give conifers their fresh, evergreen smell.

The researchers used these attractive scents to create their own simulated primrose scent. Then, they added ozone and nitrate radicals, which can form when pollutants produced by burning fossil fuels enter the atmosphere. Ozone, which is formed in the presence of sunlight, is abundant during the day, while nitrate radicals, which are broken down by sunlight, are more dominant at night.

The scientists first added ozone to primrose scent and observed some chemical degradation, with concentrations of two key monoterpenes dropping by about 30 percent. They then added nitrate radicals to the mix, which proved much more harmful, reducing these key moth attractants by up to 84 percent compared to their original levels. They had “almost completely disappeared,” Dr. Thornton said.

To test the effects on two species of hawkmoths, the scientists placed a fake flower, emitting the simulated primrose scent, at one end of a wind tunnel. Moths released at the other end often found their way to the flower.

But when the fake flower gave off a fragrance degraded by nitrate radicals, the moths hesitated. The tobacco hawkmoth’s flower visitation rate dropped by 50 percent, while white-lined sphinx moths no longer visited the flower at all. The researchers found that adding ozone alone had no effect on the moths’ behavior.

The scientists replicated these findings in nature by placing artificial flowers on primrose plants. Flowers that emit a pollution-degraded fragrance received 70 percent fewer hawkmoth visits over the course of a night than those that give off an intact odor, the researchers found. They calculated that this drop would reduce primrose pollination enough to significantly decrease fruit production. “The chemical environment is playing a really profound role in shaping these ecological communities,” Dr. Riffell said.

Researchers believe the problem extends far beyond the hawkmoth and primrose. Many pollinators are sensitive to monoterpenes, which are common in floral odors. Using computational models, the researchers calculated that in many cities around the world, pollution has reduced odor detection distances by more than 75 percent since the pre-industrial era.