The importance of bees in our food system often goes unappreciated. Just by going about their daily business, these insects are responsible for pollinating three-quarters of the 100 crop species that provide roughly 90 percent of the global food supply. The most recent estimate for the economic value of this bee activity is that it’s worth over $200 billion.
But in recent years, an alarming number of bee colonies across North America and Europe have begun to collapse. As part of the phenomenon, formally known as Colony Collapse Disorder, worker bees fail to return to the hive after their pollen-collecting trips nearby. We still don’t fully understand what’s driving this trend, but the list of culprits likely includes pesticides, viral infections, intensive agriculture and perhaps even the practice of feeding bees high fructose corn syrup in place of the honey we take from them.
New research, though, suggests there may be an overlooked problem: the exhaust fumes produced by diesel-powered engines. As described in a study published today in Scientific Reports, a group of researchers from the UK’s University of Southampton found that the pollution produced by diesel combustion reduces bees’ ability to recognize the scent of various flowers—a key sense they use in navigating and finding food sources.
“Honeybees have a sensitive sense of smell and an exceptional ability to learn and memorize new odors,” Tracey Newman, a neuroscientist who worked on the study, said in a press statement. “Our results suggest that that diesel exhaust pollution alters the components of a synthetic floral odor blend, which affects the honeybee’s recognition of the odor. This could have serious detrimental effects on the number of honeybee colonies and pollination activity.”
To come to the finding, the group used extract from rapeseed flowers to create a scent that mimics the natural smell of several different flowers that the bees normally pollinate. In a sealed glass vessel, they mixed the scented air with diesel exhaust at a variety of concentrations, ranging from those that meet the EPA’s standards for ambient air quality to worst-case scenarios—concentrations of diesel pollutants (specifically the highly reactive NOx gases, nitric oxide and nitrogen dioxide) that greatly exceed these standards but are commonly detected in urban areas.
At all concentrations, just one minute after they added the pollutants, gas chromatography testing revealed that two of the main flower-scented chemicals in the original blend were rendered undetectable, degraded by the nitrogen dioxide. Previously, they’d trained 30 honeybees to remember the flowers’ scent—by rewarding them with a sip of sucrose when they extended their proboscis in response to smelling it—but when the scent had been altered by the exposure to diesel fumes, just 30 percent of the bees were still able to recognize it and extend their proboscis. They confirmed that the NOx gases in particular were to blame by repeating the experiments with isolated versions of them, instead of the whole range of diesel pollutants, and arriving at the same results.
It’s a small study on one bee population using one flowers’ scent, but it’s a concern. That’s because, although the study specifically looked at NOx gases that resulted from the burning of diesel, the gases are also produced by your car’s gasoline-burning engine. When NOx measurements are averaged out, few areas exceed the EPA’s standards, but in many urban locales during periods of high traffic, NOx levels can be much higher—high enough, this testing suggests, to disrupt bees’ ability to smell flowers.
It follows that diesel fumes could play a role in Colony Collapse Disorder: If bees are less effective at navigating and finding nectar, they might be more likely to get lost in large numbers. Colony collapse is typically characterized by the continual disappearance of worker bees during their travels—so it’s possible that the effects of engine exhaust plays a role.
“Diesel exhaust is not the root of the problem,” said Newman said in a press briefing. “But if you think of a situation where a bee is dealing with viral infections, mites, all the other stresses it has to deal with—another thing that makes it harder for the bee to work in its environment is likely to have detrimental consequences.”