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A honeybee investigates an oilseed rape flower.

A honeybee investigates an oilseed rape flower. A new study provides evidence of how fuel exhaust changes the chemical composition of floral odors, scrambling the scent signals for pollinators.

Photograph by Nigel Cattlin, Alamy

Helen Thompson

for National Geographic

Published October 3, 2013

To a bee, no two flowers smell quite the same. When honeybees forage for flowers, they search for, learn, and memorize distinctive floral scents and return to the hive to tell other bees what they’ve found through their famous waggle dance.

It is an important ritual that is being disrupted by one of the most pervasive forms of air pollution—diesel exhaust—according to a new study published Thursday in Scientific Reports. The research pinpoints the mechanism by which the fuel-combustion pollutants degrade certain chemicals in floral odors. The absence of those chemicals affects honeybees’ ability to recognize the scent. (See related quiz: “What You Don’t Know About Cars and Fuel.”)

Engine exhaust is hardly the only threat facing the honeybee. It is well recognized that exposure to multiple pesticides can impair bees’ olfactory skills, while ground-level ozone, or smog, and ultraviolet (UV) radiation can also degrade floral odor compounds that bees pick up on. Authorities around the globe are grappling with how to address the little-understood cyclical diseases that are causing colonies to dwindle. (See related, “The Plight of the Honeybee.”)

The new study offers insight into the specific hazard for pollinators from the fumes from cars, trucks, trains, ships, and heavy machinery. Significantly, the study indicates that honeybees haven’t been helped by the "cleaner" diesel now used in Europe and the United States due to regulations that over the past decade removed sulfur from the fuel. The researchers used ultra-low-sulfur diesel fuel in their experiment. (See related: “Pictures: Cars That Fired Our Love-Hate Relationship With Fuel.”)

Odor Cues

Thousands of chemical compounds contribute to flower odors, so honeybees (Apis mellifera) need a discerning sense of smell. “A honeybee might see a red flower, and say oh is this a flower that I want to visit, and [it] uses odor cues to figure out if it’s worth visiting,” said Quinn McFrederick, an ecologist at Fresno State University in California. Odor cues can tell bees which flowers have the most nutritious nectar and pollen for harvesting.

Scientists have long thought that air pollution masked these key floral scents, but the new study provides evidence of how the exhaust actually changed the chemical composition of the odors. Using an odor palette from a common target for honeybees, oilseed rape flowers (Brassica napus), a research team at the University of Southampton in the United Kingdom exposed the compounds to diesel fumes from a generator fueled by ultra-low-sulfur diesel. Almost immediately, the diesel fumes started breaking down two of the flower odor compounds: farnesene and terpinene. After training honeybees recognize the flower scent, the researchers removed both degraded compounds from the mix.

“To our surprise, really, we saw that even changes in one of the very minor constituents of the mixture caused a major change in the responsiveness of the bee to the smell,” said Tracey Newman, a neurobiologist at the University of Southampton and a co-author of the study.

The researchers said one component of diesel exhaust takes the blame for this degradation: NOx gases, compounds that contain both nitrogen and oxygen, reacting with volatile floral odors. Although the scientists used diesel fuel, which powers the majority of cars in Europe and nearly all heavy vehicles around the world, NOx gases also are emitted by gasoline, or petrol, and even alternative fuels like biodiesel and ethanol. (See related, “Biofuel at a Crossroads.”) “The bottom line is I don’t think one can start pointing one’s finger at biodiesel, diesel, or petrol,” said Guy Poppy, an ecologist and co-author on the study. It’s a larger issue with internal combustion engines, he said.

Both the United States and the European Union use nitrogen dioxide (NO2) levels as a proxy for all NOx gases and have set limits for the amount of NO2 in the air, but not for nitric oxide (NO) levels. Poppy said, “These are the sorts of emissions that are sometimes left out from the discussions about climate change because these emissions are not ones as heavily associated with greenhouse gases.” (See related “Pictures: A Rare Look Inside Carmakers’ Drive for 55 MPG.”)

Flower Chemistry

Oilseed rape flowers aren’t the sweetest smelling blooms. “They’re actually a bit stinky,” Newman said. But, their odors are very well understood, and these two degraded compounds appear to be a key element of odor communication for bees. Other bee species and other pollinators rely even more heavily on scent over longer distances. So, the findings could have major implications for other pollinators as well, said McFrederick, who was not affiliated with the study.

Interestingly, the degraded compounds in this experiment were present only at low levels, and removing terpinene by itself led to a significant decline in bee recognition in the experiment. “That suggests that that in some way kicks off a particular pathway in the odor perception abilities of the animal,” said Newman.

The researchers’ next step is to look at the impact of diesel on the honeybee nervous system.

How will these findings play out in the real world? “The study clearly illustrates that airborne pollution can perniciously impact the ability of bees to locate food,” said Jose Fuentes, a meteorologist at Penn State University who was not associated with the study. Fuentes spelled out two cautions: The experimental levels of pollutants were high even for urban rush hour; and the impact of the NOx gases might actually be an indirect one. That’s because NOx gases notoriously react with air and sunlight to make ground-level ozone, or smog, which may be the actual culprit in disrupting the floral odor compounds.

Urban environments expose honeybees and flowers to more diesel exhaust, but there are many important factors affecting the success of hives. The kind of neighborhood gardens found in urban and suburban areas also might provide bees with longer lasting food sources than in rural areas. Field studies could shed more light on the impact of air pollution.  “What we need to know is [for] a flower sitting in a field next to a car in a motorway, whether there actually is going to be a plume of smell coming from that flower and whether it’s going to be significantly affected because of the exhaust fumes,” said Poppy. The worst-case scenario would be to find a drastic reduction in honeybee foraging and pollination.

While it’s unclear how much impact diesel pollutants might have on pollination, the new study indicates that exhaust should be added to a growing list of known threats.

“Honeybees living in a modern world face many stresses,” including diseases, insecticides, and atmospheric pollutants, Poppy said. “Probably bees can cope with most of these stresses in isolation or when just two or three of them come together. But, when they all come together simultaneously, one might start to see significant effects and that might explain some of the things we’re seeing … with pollinators being lost around the world.”

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

ciaran payne
ciaran payne

I would suggest that the answer, which is already in development, is Electric Motor drive for all forms of road vehicles, although what to do about aviation and shipping is another matter entirely and it would be interesting to know if they also are a contributory factor to the Bee issues. To progress a solution should be the effort all countries to get behind this research and provide real long term funding soon before it becomes a more  serios issue. 

liz gyno
liz gyno

Can someone plz tell me the solution for this problem? (according the article, what is the solution?)

liz gyno
liz gyno

Can someone plz tell me what is the solution to this problem? (referring to the article)

Weber Chen
Weber Chen

the content of the floral scents in environment is very low. Therefore, the airborne pollution, even at the near zero level might affect the ability of honeybee to forage the scent signals for pollinator.

Allen Schaeffer
Allen Schaeffer

All internal combustion engines emit NOx; it's too bad the story headlines don't better reflect the broader issue at hand here, as noted by the researcher himself.

In the US, the climate change debate is primarily on emissions of carbon dioxide (CO2) as well as some short-lived climate agents.  NOx, along with hydrocarbons is a primary ingredient of ozone or smog which is certainly a high priority in the US, so NOx is by no means left out of consideration.

Emissions from new diesel engines today are near zero levels for both particulate matter and NOx emissions - from diesel powered passenger cars to heavy duty trucks and buses to off road machines and equipment, marine vessels and generators.

The researchers provided no information about the age or state of condition about the generator used in the study to generate the NOx emissions.  Advanced engine controls and emissions aftertreatment systems in todays new engines ffectively reduce the emissions of these compounds to near zero levels.

More at

Dan Winter
Dan Winter

Phil Callahan proved the smell / olfaction mechanism (including insects) was more infra-red wave mechanics than particulates-  he did this by measuring which infrared frequency transmissivity was damped by the key fuel ketones...  more on callahan:​CALLAHAN/​callahan.html

Their 'field' must be centripetal

Peace Seeker
Peace Seeker

Filmed over a period of a year we follow a few charismatic South African beekeepers as they come to terms with and react to this potentially disastrous infection. The wild South African bees have through the years self managed all previous foreign diseases but little is known on how the bees will react to this new bacterial invasion. The viewer is taken on several personal journeys into learning what actually makes the African bee 'wild' and how has it, until now, managed to be generally unaffected by the rest of the worlds problems.

The film brings forward the dilemmas people are faced with when working with modern day livestock and the constant threat of diseases spreading globally.

A very personal film that allows viewers into a reality of which very little is known publicly.

www (dot) cultureunplugged (dot) com/documentary/watch-online/play/12375

Steve Factor
Steve Factor

Another reason to avoid burning toxic liquids for personal transportation. Use electric vehicles with solar power.

Mark Goldes
Mark Goldes

Breakthrough technologies now being born can end automotive exhaust.

One example is an engine that needs no fuel. A small, desktop, prototype is en-route.  If it performs as anticipated, it opens the potential to replace stationary diesel engines of all sizes with 24/7 power availability. It will also recharge some models of electric cars.

See for more information.

This will open the door to other neglected new science that can provide recharge to all electric cars from on-board systems.

Such cars, trucks and buses, may eventually pay for themselves when parked, by selling electricity. No wires required. See MOVING BEYOND OIL on the same site.

Bees and humans will all benefit from ending the need to burn fossil and radioactive fuel. That achievement is now on the horizon. The necessary work can, and must be, greatly accelerated.

liz gyno
liz gyno

@Weber Chen What is the solution for this problem? (according to the article, what is the solution?)

liz gyno
liz gyno

@Allen Schaeffer What is the solution for this problem? (according to the article, what is the solution?)

Michael MacBride
Michael MacBride

@Steve Factor the toxic waste (both airborne and not) from the creation of the batteries required for your "solar" electric car is as high or higher even then if you just burned the fossil fuel in a new engine. 

liz gyno
liz gyno

@Mark Goldes What is the solution for this problem? (according to the article, what is the solution?)

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