Foul-smelling vapors cause problems for pollinators

A team led by University of Washington researchers has discovered a major cause of declining nocturnal pollinator activity – and humans are largely to blame.

Researchers discovered that nitrate radicals (NO₃) in the air degrade odorous chemicals released by a common wildflower, thereby significantly reducing the olfactory cues that nocturnal pollinators rely on to locate the flower. In the atmosphere, NO₃ is produced by chemical reactions between other nitrogen oxides, which are themselves released by the burning of gas and coal from cars, power plants and other sources.

The results, published in the journal Science, are the first to show how nighttime pollution creates a chain of chemical reactions that degrade olfactory signals, leaving flowers undetectable by smell. The researchers also determined that pollution likely had global impacts on pollination.

The team, co-led by Jeff Riffell, a UW biology professor, and Joel Thornton, a UW atmospheric sciences professor, studied the pale evening primrose (Oenothera pallida). This wildflower grows in arid environments in the western United States. They chose this species because its white flowers give off a scent that attracts a diverse group of pollinators, including moths, which are one of its most important pollinators.

At field sites in eastern Washington, researchers collected odor samples from pale evening primrose flowers. Back in the lab, they used chemical analysis techniques to identify the dozens of individual chemicals that make up the scent of wildflowers.

“When you smell a rose, you smell a diverse bouquet made up of different types of chemicals,” Riffell said. “The same is true of almost all flowers. Each has its own scent composed of a specific chemical recipe.”

Once they identified the individual chemicals that make up the wildflower’s scent, the team used a more advanced technique called mass spectrometry to observe how each chemical in the scent reacted to NO.₃. They discovered that reacting with NO₃ almost eliminated some scent chemicals. In particular, the pollutant decimated levels of monoterpene odor compounds, which the moths found most attractive in separate experiments.

Moths, which smell through their antennae, have an odor detection ability that is roughly equivalent to that of dogs and several thousand times more sensitive than the human sense of smell. Research suggests that several species of butterflies can detect odors from miles away, according to Riffell.

Using a wind tunnel and a computer-controlled odor stimulation system, the team studied the extent to which two species of butterflies – the white-lined hawkmoth (Hyles lineata) and the tobacco hawkmoth (Manduca) sexta) – could locate and fly towards odors. When the researchers introduced the normal odor of the pale evening primrose, both species readily flew toward the source of the odor.

But when researchers introduced perfume and NO₃ at levels typical of a nocturnal urban environment, Manduca’s accuracy dropped by 50% and Hyles, one of the main nocturnal pollinators of this flower, was unable to locate the source at all.

Experiments in a natural environment have confirmed these results. In field experiments, the team showed that moths visited a fake flower emitting an unchanged scent as often as they visited a real one. But if they treated the perfume with NO first₃moth visitation levels have dropped by up to 70%.

“The no₃ “really reduces the ‘range’ of a flower, which is how far its scent can travel and attract a pollinator before it decays and is undetectable,” Riffell said.

The team also compared the impact of daytime and nighttime pollution conditions on odorous chemicals in wildflowers. Nighttime pollution has a much more destructive effect on the chemical composition of perfume than daytime pollution. Researchers believe this is largely due to the degradation of solar radiation.₃.

The team used a computer model simulating both global weather and atmospheric chemistry to locate areas most likely to have significant problems with communication between plants and pollinators. Areas identified include western North America, much of Europe, the Middle East, central and southern Asia, and southern Africa.

“Apart from human activity, certain regions accumulate more NO₃ due to natural sources, geography and atmospheric circulation,” said Thornton, who added that natural sources of NO₃ include wildfires and lightning. “But human activity produces more NO₃ everywhere. We wanted to understand how these two sources – natural and human – combine and where levels might be so high that they could interfere with pollinators’ ability to find flowers.

The researchers hope their study is just the first of many that will uncover the full extent of the pollinator failure.

“Our approach could serve as a roadmap for others to study the impact of pollutants on plant-pollinator interactions and to truly understand the underlying mechanisms,” Thornton said. “You need this kind of holistic approach, especially if you want to understand how much plant-pollinator interactions are broken down and what the consequences will be.”

The study highlights the dangers of human-caused pollution and its implications for all pollinators and the future of agriculture.

“Pollution from human activity changes the chemical composition of critical olfactory signals, and changes it to a point where pollinators can no longer recognize and respond to it,” Riffell said.

About three-quarters of the more than 240,000 species of flowering plants rely on pollinators, Riffell said. And more than 70 species of pollinators are endangered or threatened.