One of the reasons given for the substantial decline of pollinators in many parts of the world – along with habitat loss, disease and insecticides – is environmental pollution. Now, researchers have discovered that diesel exhaust could be contributing to this because of the effect it has on flower odors.

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Of the 100 crop species that provide 90% of the world’s food, over 70 are pollinated by bees.

In the Journal of Chemical Ecology, a study from the Universities of Southampton and Reading, both in the UK, reports how of the 11 most common single compounds in flower scents, five can be so chemically degraded by toxic nitrous oxide (NOx) gas from diesel exhaust that honey bees no longer recognize them.

We already know that NOx is harmful to humans, and there is also evidence that it can confuse bees’ sense of smell, which they rely on to locate food; they eat nectar and pollen from flowers.

Lead author Dr. Robbie Girling, a lecturer in agro-ecology and sustainable agriculture at Reading, says:

“People rely on bees and pollinating insects for a large proportion of our food, yet humans have paid the bees back with habitat destruction, insecticides, climate change and air pollution.”

According to a United Nations report published in 2011, of the 100 crop species that provide 90% of the world’s food, over 70 are pollinated by bees.

In 2000, the value of crops pollinated by bees was estimated at $14.6 billion in the US alone.

For the study, the researchers exposed some of the most common volatile blends of compounds found in flower odors to diesel exhaust. They found it altered the blend of common flower volatiles significantly. For example, they note “myrcene was considerably reduced, β-ocimene became undetectable, and β-caryophyllene was transformed into its cis-isomer isocaryophyllene.”

Then, using proboscis extension reflex (PER) assays, the team showed that the alteration the diesel fumes caused in the floral compounds was enough to reduce the ability of honey bees to recognize the odors.

PER assay is a well-established method developed 50 years ago to study how honey bees perceive and learn about floral odors, which signal the nectar and pollen resources a colony needs for survival. It uses the fact that when a bee’s antenna detects a compound it associates with food, the insect extends its proboscis (sticks its tongue out).

And finally, the researchers showed that it was the chemically reactive nitrogen oxides in the diesel exhaust gas that degraded the floral volatiles.

The team does not think that air pollution from diesel vehicles is the main reason for the decline in bee populations, but the findings suggest it may have a worse effect than initially thought.

Coauthor Guy Poppy, a professor in biological sciences at Southampton, says it is clear bees are coming under pressure from a range of sources, and this latest study highlights vehicle emissions as one of them. He adds:

Whilst it is unlikely that these emissions by themselves could be affecting bee populations, combined with the other stresses, it could be the tipping point.”

Some of the study authors are now investigating whether diesel fumes directly affect the bees themselves.

Earlier this year, Medical News Today learned how another team of researchers discovered a widespread risk of infectious diseases to wild bees. In the Journal of Animal Ecology, a team from Royal Holloway-University of London, revealed that a network of viruses that were previously linked to managed honey bees may now be a threat to bumblebees in the wild.