Even
“clean” diesel and biofuel generate gases that pose threat to pollinators.
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.
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 famouswaggle 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.”
