Polluted Scents

Ozone and other constituents of smog destroy at least some of the floral perfumes that pollinators rely on to find their meals, scientists report.

Bees might suffer some from the effects of these smog constituents, which pollute urban and rural areas alike. But the foragers most likely to be confused by air pollution’s degradation of floral scents, entomologists suspect, are pollinators that rely less on sight than bees do to find nectar.

Flower scents’ vulnerability to ozone and other reactive chemicals is not new. Until now, though, no data existed on how quickly pollution might extinguish these natural perfumes, explains Jose D. Fuentes of the University of Virginia in Charlottesville.

To probe this process, his team recorded meteorological conditions — such as air temperatures and wind speeds — from a snapdragon farm and fed the data into a computer program. The researchers then calculated chemical reactions between three of the most common floral scent molecules used by pollinators and three airborne products of fossil-fuel combustion: ozone, nitrate and hydroxyl radicals. Under pristine-air conditions, scent molecules could drift unchanged over distances of a kilometer or more, the calculations showed. The strength and length of that plume diminished dramatically, however, in the presence of smog constituents.

Within just 200 meters, for instance, half of the average intensity of a scent plume was already lost, the researchers report in a recent issue of Atmospheric Environment. For one scent molecule studied, beta-ocimene, 75 percent of the perfume would vanish within 300 meters. In some cases, Fuentes notes, the pollutant reactions chemically alter a perfume rather than rendering the air scentfree.

Such dramatic scent changes or losses over short distances “was a real surprise,” he says.

The report by Fuentes’ group “is certainly intriguing,” says Laurie Adams of the Pollinator Partnership, based in San Francisco. Its analyses help identify the potential for “many signals that nature depends on to go askew.”

Although it’s easy to assume bees would be the big losers from vanishing floral scents, Geraldine Wright of NewcastleUniversity in the United Kingdom has her doubts.

“A bee’s response to floral odors is very plastic,” she points out. If a scent changes, “bees can learn another scent very quickly and adapt.” Besides, she notes, “some flowers don’t produce any scent and bees still forage on them,” relying on visual signals.

Stephen Buchmann, international coordinator of the North American Pollinator Protection Campaign, agrees that compared to bees, giant hawk moths, bats and other pollinators of night-blooming plants stand to lose a lot more from pollution damage to floral perfumes.

Some of these animals rely heavily on scents as they travel long distances each night in search of food. It’s possible that for them, he says, ozone and other combustion pollutants’ impacts might be likened to “an industrial disease for pollinators.”

Wright questions whether pollinators actually constitute the biggest at-risk group from scent destruction. Pheromones — potent scented chemicals that insects and other species release to attract mates — are, like the sweet smell of flowers, hydrocarbons that could be altered over short distances by air pollutants. Normally, she notes, a male moth can orient toward a female using “very low concentrations of pheromones that were emitted miles away.”

Fuentes concedes that compared to pollinator attractants, pheromones are perhaps an even more important class of biomolecules at risk from ozone and other reactive pollutants — and one that his group intends to explore soon.

He also points out that theoretical modeling can only suggest problems. Field testing will be needed to confirm that the computer results actually occur under real-world conditions. And his team is poised to begin such field testing this summer.