The Impact of Air Pollution on Pollinators' Ability to Find Flowers

Some night-blooming flowers may lose their distinct scents due to air pollution, potentially affecting pollination.

In laboratory and field tests, it was found that certain pollutants in the night air destroy important scent molecules when they come into contact with the aroma of a pale evening primrose. This could make it harder for moths and other nocturnal pollinators to detect the fragrance and find the flower, according to research published in the February 9 Science.

The results underline the fact that air pollution has implications not just for human health but also for ecosystems and food security. Joel Thornton, an atmospheric scientist at the University of Washington in Seattle, said, “Pollination is crucial for agriculture”.

Ozone, nitrogen oxides and other pollutants have been introduced to the air through increasing industrialization. Sunlight usually breaks down ozone during the day, but at night, the pollutant builds up and interacts with nitrogen dioxide, resulting in nitrate radicals. Some evidence suggests these reactive particles impact plant fragrances, although the specifics were unknown.

The researchers, including Thornton, extracted scent molecules from a pale evening primrose and introduced the scent into a wind tunnel which contained hawk moths. The team observed that the moths were capable of “flying upwind and tracking the odor,” according to biologist Jeff Riffell from the University of Washington.

However, the moths’ behavior dramatically altered when pollutants such as ozone and nitrogen dioxide were included. They flew erratically and frequently failed to find scent signals.

The team identified the specific cues that hawk moths use to locate flowers by exposing their antennae to each molecule in the scent. Molecular analysis showed two types of aromatic compounds, monoterpenes, were shredded when they met nitrate radicals, dramatically reducing the scent of the primrose.

Riffell expressed surprise that a seemingly minor change in concentration of just two compounds — out of more than 20 — could ruin the flower’s appeal. Jose Fuentes, an atmospheric scientist at Penn State who was not part of the study, compared the effect to asking someone to fetch a coffee while blindfolded.

In order to test the findings in a real-world setting, ecologist Jeremy Chan, now based at the University of Naples Federico II in Italy, laid out a field of real and fake primroses around 280 kilometers east of Seattle. These fake flowers emitted either an artificial primrose scent or the scent along with the chemicals which interact to form nitrate radicals. Their visits to the flowers were monitored with cameras.

Nightly visits to both real and fake flowers that gave off the pure primrose scent typically occurred between two and three times. By contrast, visits to flowers with polluted scents were less than one a night on average.

Fuentes claimed that scientists must further investigate how the behaviors observed may disrupt food sourcing. He warned that if insects are finding it increasingly difficult to pollinate in areas with worsening air pollution, it will pose a threat to “pollination, crop production [and] the health of native plant species”.