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Nighttime pollination is plummeting. Clever sleuthing pinpointed a surprising culprit.

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Nighttime pollination is plummeting. Some clever sleuthing pinpointed a surprising culprit.

A big part of the problem, it turns out, is that nighttime air pollution (more so than daytime pollution) blinds pollinators to the smell of flowers.
February 14, 2024

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Much of the concern about the loss of the world’s pollinators revolves around the declining numbers of bees, butterflies and other insects. But what if a big part of the problem is that the bugs we do have are flying blind?

Insects often rely on odors wafting through the air to home in on their favorite flowers from kilometers away. New research is revealing that nighttime air pollution cancels out those smells in ways that can lead insects astray.

“Pollution from human activity is altering the chemical composition of critical scent cues and altering it to such an extent that the pollinators can no longer recognize it and respond to it,” said Jeff Riffell, a neurobiologist and ecologist at the University of Washington.

Pollination from animals such as insects is critical to supplying many of the fruits and vegetables in our kitchens. It plays a role in reproduction for around 85% of all flowering plants. This vital process is threatened by everything from pesticides to domestic beekeeping.

But what about interfering with smell? Riffell was part of a team of scientists who pieced together a detailed picture of the interplay between a group of moths and a desert flower, and how nitrate (NO3), a common pollutant spewed by everything from car engines to wildfires, can also interfere with pollination.

 

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Fingering the culprit was not easy, requiring a meticulous chain of experiments. Over 200 hours in the Eastern Washington desert, the scientists learned that large, fast-flying moths known as hawkmoths were key visitors to the pale evening primrose, Oenothera pallida, a low-to-the-ground desert plant with spindly, pale green stems topped with delicate pale pink and white flowers.

To understand what drew the moths to the flowers, first the researchers identified the chemical makeup of the scent. In a lab, they exposed two species of hawkmoths wired to instruments that detect electrical activity in the insects’ antennae, to see which molecules were triggers. The moths were particularly tuned to two different flavors of monoterpenes, a class of chemicals found in plant oils.

In the natural world, these chemicals are an ideal way to create a chemical trail. They evaporate quickly in the air. Moths, whose antennae are roughly as sensitive as a dog’s nose, can pick up the scent several kilometers away from a flower.

But there is an Achilles heel. When the researchers exposed the monoterpenes to NO3, it reacted with the oils, causing them to degrade by between 67% and 84%. While NO3 is less of a problem during the day because it breaks down in sunlight, it accumulates at night, when many pollinators, including the hawkmoths, are active.

To see how this translated into changes in moth behavior, the researchers tested moths’ attraction to flowers with natural odors versus ones altered to simulate exposure to urban levels of NO3, first in a wind tunnel and then in the eastern Washington desert. The effect was dramatic. In the desert, nighttime visits to artificial flowers with the pollutant-altered chemistry fell by 70%. That would translate into roughly 28% less fruit forming from those flowers, the researchers reported last week in Science.

“The NO3 is really reducing a flower’s ‘reach’—how far its scent can travel and attract a pollinator before it gets broken down and is undetectable,” said Riffell.

The results point to potential pollination roadblocks well beyond Washington state. Using a global model of air pollution, the scientists found that large parts of the globe, chiefly urbanized areas, have NO3 levels that would significantly impair pollination by creatures relying on these same monoterpenes. Hotspots included much of Europe, the Middle East, India, eastern China, and North America. In many of these places, the distance from which an insect can pick up the smell of a flower has fallen more than 75% compared to preindustrial times, the scientists found.

The research doesn’t point to an easy solution. But it does alert people to another way the Anthropocene is making life harder for pollinators, said Joel Thornton, a University of Washington atmospheric scientist.

“Our approach could serve as a roadmap for others to investigate how pollutants impact plant-pollinator interactions, and to really get at the underlying mechanisms,” he said.

Chan et al. “Olfaction in the Anthropocene: NO3 negatively affects floral scent and nocturnal pollination.” Science. Feb. 8, 2024.

Photo illustration by Floris Van Breugel

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