Air pollution interferes with the ability of bees and other insects to follow the scent of flowers to their source, undermining the essential process of pollination, a study by three University of Virginia researchers suggests.
Their findings may help unlock part of the mystery surrounding the current pollination crisis that is affecting a wide variety of crops. Scientists are seeking to determine why honeybees and bumblebees are dying off in the United States and in other countries, and the new study indicates that emissions from power plants and automobiles may play a part in the insects' demise.
Scientists already knew that scent-bearing hydrocarbon molecules released by flowers can be destroyed when they come into contact with ozone and other pollutants. Environmental sciences professor Jose D. Fuentes at the University of Virginia -- working with graduate students Quinn S. McFrederick and James C. Kathilankal -- used a mathematical model to determine how flowers' scents travel with the wind and how quickly they come into contact with pollutants that can destroy them. They described their results in the March issue of the journal Atmospheric Environment.
In the prevailing conditions before the 1800s, the researchers calculated that a flower's scent could travel between 3,280 feet and 4,000 feet, Fuentes said in an interview, but today, that scent might travel 650 feet to 1,000 feet in highly polluted areas such as the District of Columbia, Los Angeles or Houston.
"That's where we basically have all the problems," Fuentes said, adding that ozone levels are particularly high during summer. "The impacts of pollution on pollinator activity are pronounced during the summer months."
This phenomenon triggers a cycle, the authors noted, in which the pollinators have trouble finding sufficient food, and as a result their populations decline. That, in turn, translates into decreased pollination and keeps flowering plants, including many fruits and vegetables, from proliferating.
Fuentes said scientists now have a more sophisticated understanding of the signals for which insects are searching, and that air pollution rapidly eliminates as much as 90 percent of flowers' aroma.
"We now know what the pollinators are looking for when they're actually looking for the flowers," he said.
Most bees have poor eyesight, which makes scent particularly important, the researchers wrote.
Since 2006, honeybee colonies in the United States have been suffering from a widespread phenomenon known as colony collapse disorder (CCD), in which adult worker bees abandon an otherwise-healthy hive.
John P. Burand, an associate professor at the University of Massachusetts at Amherst who is studying bee colony collapses, said the effects of air pollution described in the new study are probably not directly related to that phenomenon. But, he added in an e-mail: "There is no doubt that air pollution and air quality is having an effect on bees and other pollinators. It appears there is more than one factor that is contributing to the CCD phenomenon we are seeing with bees, and certainly air pollution in some fashion may be playing a role."
Burand, working with two other University of Massachusetts researchers and an insect ecologist at the University of Maine at Orono, just received a $150,000, three-year grant from the Agriculture Department to analyze microbes carried by bees that pollinate apples, squash and pumpkins. They are working with colleagues to compare the bacteria, viruses and fungi in healthy bee colonies with those in dysfunctional hives.
Richard Poirot, an air-quality planner at Vermont's Department of Environmental Conservation who helps advise the federal government on its national ozone standards, said it makes sense that the chemical reaction of floral hydrocarbons and pollutants such as ozone would reduce the power of a flower's scent and affect the insects that depend on those aromas.
"It does make sense that it certainly would be another stress factor" on pollinators, Poirot said, though he added that pollinators are declining for an array of reasons not related to pollution. "The question is, how significant is it?"
Timothy H. Tear, a senior scientist at the advocacy group the Nature Conservancy who studies the impact of air pollution on ecosystems, said the recent study confirms the extent of ozone's effects on habitats up and down the East Coast.
"We know that ozone levels continue to be high and go well beyond EPA standards for public health," Tear said. "What's been pretty consistent is the more we look at air pollution's impacts on natural resources, the more we find those impacts to be."
Tear and his colleagues have recently completed a survey of how atmospheric pollution is affecting biodiversity in the Eastern United States and concluded that high levels of ozone can decrease forest growth by as much as 30 percent.
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