Air pollution messes with moths’ ability to smell flowers

Byproducts of car exhaust disrupt pollination by degrading the floral scents that insects use to track down their favorite plants, according to new research.
Photo illustration of hawkmoth navigating to flower during vehicle exhaust emission.
Photo illustration of hawkmoth navigating to flower during vehicle exhaust emission. Image courtesy of Floris Van Breugel.

One of life’s many pleasures is stopping to smell the roses, but flowers don’t just make their sweet scents for human enjoyment. The fragrances are biological signals that broadcast a plant’s location to potential pollinators. Delightful (or occasionally dreadful) smells drifting on the breeze enable plants to attract insects and other animals to stop by and spread some pollen. Unfortunately, air pollution is getting in the way, according to a study published February 8 in the journal Science

The researchers behind the new paper assessed the impacts of ozone (O3) and a nitrate radical (NO3) on some moths’ ability to detect and pollinate evening primrose flowers at night. They found that these pollutants, common byproducts of car exhaust and burning fossil fuel, react with and deactivate key attraction chemicals in the flower’s scent. In the presence of nitrate radicals, significantly fewer moths visit primrose flowers. The plants rely on moths and other nocturnal pollinators to produce fruit, and the scientists’ results suggest that–amid air pollution–evening primrose flowers are less able to propagate the next generation. It’s a troubling set of findings that carries implications far beyond just one insect’s diet or one flower’s seed production.

“Pollinators play a huge role in community ecology; they’re critical for the fitness of plants. If you affect that, then you’re going to have ecosystem-wide impacts,” says Jeff Riffell, co-senior study author and a biology professor at the University of Washington. “Pollinators are also critical for our food system and food security,” he adds– mess with the wrong insects and humans could end up paying the toll too. 

Pollution isn’t always as simple as a deadly chemical spilled into a lake. Less direct, sensory pollution can harm animals in all sorts of surprising ways. There’s the city lights that draw migrating birds into collisions with building windows and the noisy boats that can deafen squids. The way humans alter animals’ olfactory environments can also be damaging. This most recent study builds on previous research that’s also found air pollution can mess with pollinators’ ability to smell

Image of Manduca sexta moth visiting paper flower emitting Oenothera flower scent. CREDIT: Image courtesy of Charles Hedgcock.

Yet the new research adds to the scientific record in a few notable ways. It was one of the first to explore this particular moth and flower system in such high detail. And the study keys in on the exact chemical compounds at play, demonstrating a precise explanation for the problem, says Jeremy Chan, lead study author and a postdoctoral researcher at the University of Naples Federico II. By understanding the exact mechanisms and reactions at play, Chan and his co-researchers were able to expand their analysis from a single ecological partnership to the impacts that nitrate radical and ozone pollution could be having on plants and pollinators worldwide. “We could extrapolate more confidently about where this is going to be an important problem, how long it’s been a problem, and what we might actually do about it,” says Joel Thornton, co-senior study author and an atmospheric science professor at the University of Washington. 

Ultimately, the researchers found that in many parts of the globe, ozone and nitrate radical pollution may be stymying pollinators’ ability to detect their host plants. In most of the northern hemisphere, these pollutants could be reducing the distances at which insects are able to locate flowers by 75% or more–from more than five kilometers in the pre-industrial age to fewer than 400 meters in the present.  

Reaching that global conclusion required a lot of preceding steps and discoveries. Chan, Riffell, Thornton, and their colleagues began their investigation all the way back in 2017 by first observing pale evening primrose plants (Oenothera pallida) and their insect visitors. The scientists recorded a variety of pollinators stopping by the flowers during both day and night, but they noted that two types of hawkmoths were especially frequent nighttime visitors: White-lined sphinx moths (Hyles lineata) and moths in the genus Manduca, which includes tobacco hornworms. In their first field experiment, the researchers excluded pollinators at different times of day and demonstrated that nighttime pollination played a bigger role than daytime pollination in whether or not primrose flowers produced viable fruit.  

Then, they analyzed the primrose’s heady scent. Through gas chromatography and mass spectrometry, they identified each of the flower’s smell compounds. The researchers used tiny, disembodied insect antennae from moths and bees–connected to electrodes–to determine which of those many chemicals the insects were most likely to respond to. They found that a chemical class called monoterpenes were uniquely attractive to the pollinators. 

From there, the scientists took a turn as perfumers composing a synthetic scent to match those attractants. When they exposed their fake flower scent to nitrate radicals and ozone, they found the key monoterpenes degraded and disappeared over time.

In wind tunnel experiments, Chan and company tested how well hawkmoths located different scent sources, including their fake floral scent, a real flower, and their primrose perfume mixed with nitrate radicals and ozone in similar concentrations to what might be expected in an urban setting. In a natural, unpolluted environment hawkmoths might fly 80 kilometers in a night and be able to home in on flowers from kilometers away. But the study results showed that, in the presence of nitrate radicals, white-lined sphinx moths could not locate a primrose flower at all–from just two meters away–and tobacco hornworms were about 50% less successful at finding their food source. They replicated these same results in field trials where they found no difference in insect visitation between their synthetic scent and real flowers, but observed a 70% reduction in hawkmoth visitation with pollution-exposed scent lures. 

Combining their data on the primrose pollination rates and the impacts of air pollution, the researchers posit that the levels of nitrogen trioxide present in many populated areas could make primrose plants 28% less successful at producing seeds, simply based on the loss of hawkmoth pollination alone. The real impacts on plants could be much larger, as daytime pollinators are also known to suffer reduced smell capacity around certain pollutant sources, like diesel exhaust. 

All of these findings together allowed the researchers to run their global model of NO3 and O3 pollution and present a theory for how insects worldwide might be losing their ability to detect flowers from faraway. 

There are, however, some important limitations to note. For one, because they only studied one plant and two types of moth–it’s possible other insects and flowers have different chemical systems that aren’t as affected by the same pollutants, says Chan. Additionally, he adds that nitrate radicals are more common at night because they rapidly degrade in sunlight, so the study is most relevant to nocturnal pollinators. However, other compounds (like hydroxyl radical) could be playing similar roles in the daytime atmosphere, says Thornton. More research is needed to establish the impacts of air pollution on different plant and pollinator systems and on daylight pollination, Thornton and Riffell both say.  

Nonetheless, the study offers thorough and unsettling insight into yet another way humans are gunking up ecological systems. “The beauty of this research is that it is truly multidisciplinary, combining both laboratory and field experiments,” says Mark Elgar, a professor of evolutionary biology at the University of Melbourne. Elgar has previously studied the impacts of particulate pollution on pollinator smell, but was not involved in the February study. The new research, he says, demonstrates that insects face multiple stressors from air pollution. “We’d be nuts not to continue to investigate this.” Elgar adds. 

Hiding amid the ominous news, there is one small silver lining: Since the 1980’s, environmental protections around in many countries, such as enforced standards on car emissions, have significantly reduced ozone and nitrate radical pollution, Chan says–which means it’s possible we could make further reductions. “It’s just more motivation,” Thornton says, “for moving our transportation and energy needs away from fossil fuel combustion and to other sources of energy.” Greener transportation and fewer emissions would mean we’d all breathe (and smell) a little easier–moths included.