Flowers Are Losing Their Smell

April 16, 2008 — -- Air pollution is killing the smell of flowers, possibly eliminating the "scent trail" that helps guide those terribly important pollinators, like bees, to the plants that depend upon them for survival, scientists believe.

The discovery could be one of several factors in the "colony collapse disorder" that is wiping out honey bees around the world.

While it is still too soon to determine the full impact of air pollution on the symbiotic relationship between insects and the flowers they pollinate, researchers at the University of Virginia are confident they have shown that pollutants are killing the scent trail, and that could turn out to be extremely significant.

Before the industrial revolution, the trail extended at least half a mile from the flower, but today at that distance "it's almost completely destroyed," said Quinn McFrederick, a doctoral candidate in biology at the university and lead author of a study that in the current issue of the journal Atmospheric Environment.

Scientists have known for some time that airborne chemicals like ozone, hydroxyl and nitrate radicals -- major components of smog -- alter the chemicals produced by flowers that give them a specific smell. But it had not been known how that affected the trail that helps lead insects to the flowers.

Scents that could travel for more than half a mile in the 1800s now probably travel less than about 600 feet, according to Jose D. Fuentes, professor of environmental sciences at the university and a co-author of the study.

"This makes it increasingly difficult for pollinators to locate the flowers," Fuentes said.

In a telephone interview, McFrederick said that the scent trail deteriorates even very close to the flowers, and that could discourage insects, especially bees and moths, from even sampling the flower to see if it contains the nectar they need for survival. And if they pass up the flower, it will not receive the pollination it needs. So both the pollinator and the pollinated suffer.

At this point the research consists of a mathematical model into which the researchers inserted the known impact of various pollutants on the molecules carrying the scent. They then extrapolated out to various distances to see how much of an impact that would have. But the findings haven't been tested in "the real world," McFrederick said. He and his colleagues hope to do that soon.

The findings are intriguing, but no one knows just yet how significant they really are.

"We don't know an awful lot about how insects actually use these scent trails," he said. It's unknown how much of a scent is required for the insect to detect it, and no one knows yet if new chemicals produced by the reaction between scent molecules and air pollution can also be detected by insects. But what is known is that scent is important in the overall pollination process.

Bees and many other insects depend primarily on vision to find flowers. But the researchers believe that scent, detected at a considerable distance from the flowers, may tell the insects the general direction of the flowers. So insects travel in that direction until they actually see the flowers, and then depend on scent somewhat to decide which flowers to visit. Some other insects, like nocturnal moths, must depend very heavily upon scent, McFrederick said.

And if that's the case, "plants that don't depend on animal pollinators would do better than plants that do depend on animal pollinators," he added. "Plants that can be pollinated by the wind, or plants that can pollinate themselves, might be expected to do better and their populations to be proportionally larger in areas where there is lots of pollution."

Two years ago an international team reported that a 25-year study had found just that in the Netherlands and parts of Great Britain. When the bee population declined, so did the plants that the bees pollinate.

"In Britain, pollinator species that were relatively rare in the past have tended to become rarer still, while the commoner species have become even more plentiful," Stuart Roberts of the University of Reading said at the time. "Even in insects, the rich get richer and the poor get poorer."

That trend has not been documented yet in the United States, but there is no debate about the decline in pollinators. In the last 50 years the bee population that farmers depend upon for pollination has declined by 50 percent, according to one study. The decline in bees has been blamed chiefly on diseases spread by mites and viruses, as well as pollution and pesticides.

Now, scientists may be able to add another element to the equation. The sweet aroma coming from flowers isn't as strong as it once was, and that's probably happening all over the globe.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.