Diesel Fuel Exhaust Disrupts Scent Signals for Honeybees

Article by Helen Thompson for National Geographic, 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.”

A report showed that 464,231 gallons of fracking fluid containing the toxic chemical 2-BE were injected into West Virginia gas wells and 747,416 gallons of 2-BE bearing fluids were employed in Pennsylvania.  This is the same chemical that showed up in contaminated well water in Pavillion, Wyo. and is likely the cause of the adrenal tumor that Laura Amos of Garfield County, Colo. developed after her well water was contaminated by Encana drilling activity.

You might feel like you are in Toxicology class as you review the Congressional Committee on Energy and Commerce’s recently released report which reveals information regarding chemicals used in hydraulic fracturing.  The information for the report was collected by the Committee from 14 oil and gas service companies which submitted requested data on fracking products used between 2005 and 2009.  This is a very lay-friendly report with only 12 pages of text and tables.  The remaining 18 pages are lists of chemicals.  However, for those who just want the highlights, I’ve tried to pick out them out for this post.

“Between 2005 and 2009, the oil and gas service companies used hydraulic fracturing products containing 29 chemicals that are (1) known or possible human carcinogens, (2) regulated under the Safe Drinking Water Act for their risks to human health, or (3) listed as hazardous air pollutants under the Clean Air Act.”    Of the 29 chemicals,  13 are classified as carcinogens, 8 are Safe Drinking Water Act regulated chemicals, and 24  are hazardous air pollutants.  Many of the chemicals fall into more than one category.  (See chart on page 8 of report.)

Methanol, a toxic air pollutant,  was the most widely used chemical during the time period studied, as measured by the number of compounds containing the chemical.  Other hazardous air pollutants included hydrogen fluoride (systemic poison, potentially fatal), lead (reproductive disorders, high blood pressure, nervous system disease, especially among children), hydrogen chloride and ethylene glycol.

The chemical called 2-BE (shorthand for 2-butoxyethanol) is another common toxic constituent.  It is used as a foaming agent or surfactant.  ”According to EPA scientists, 2-BE is easily absorbed and rapidly distributed in humans following inhalation, ingestion, or dermal exposure.  Studies have shown that exposure to 2-BE can cause hemolysis (destruction of red blood cells) and damage to the spleen, liver, and bone marrow.”   And rare adrenal tumors.  Texas topped the list of states with 12 million gallons of fluid containing 2-BE injected into the ground.   As noted above, WV and PA were below 1 million.

Among the list of carcinogens used are formaldehyde (also a hazardous air pollutant), diesel, naphthalene and chemicals in the BTEX compound group (benzene, toluene, ethylbenzene and xylene).  ”The BTEX compounds appeared in 60 hydraulic fracturing products used in the 5-year period and were used in 11.4 million gallons of hydraulic fracturing fluids.”  Most of those tainted fluids, 9.5 million gallons of the 11.4 million, were used in Texas.   Less than 100,000 gallons were used in Pennsylvania and West Virginia.

“In addition, the hydraulic fracturing companies injected more than 30 million gallons of diesel fuel or fracturing fluids containing diesel fuel in wells in 19 states.”  In a 2004 report, the EPA stated that the use of diesel fuel in fracturing fluids poses the greatest threat to underground sources of drinking water.

“Many chemical components of hydraulic fracturing fluids used by the companies were listed on the MSDSs as “proprietary” or “trade secret.”  The hydraulic fracturing companies used 93.6 million gallons of 279 products containing at least one proprietary component between 2005 and 2009. …In these cases, it appears that the companies are injecting fluids containing unknown chemicals about which they may have limited understanding of the potential risks posed to human health and the environment.”

The report was prepared under the leadership of US  House Representatives Henry Waxman (D-CA), Edward Markey  (D-MA), and Dianna DeGuette (D-CO).