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Bees fail to find flowers due to exhaust fumes

In air polluted by the exhaust fumes of petrol engines, bees pollinate considerably fewer flowers because they can no longer smell them.

By searching flower heads for nectar and pollen, bees don't just ensure the sustenance of their colony, they also enable pollination of the flowers and therefore enable the reproduction of the plants. It's the classic example of a symbiosis. Both sides profit, to the point of depending on each other. That's why flowers emit signals to catch the attention of the bees.

When looking for flowers rich in nectar, the scent of the blossoms are of primary importance to bees. Although they initially find flowers from a distance through their shapes and colours, the closer they get to them, the more they rely on their sense of smell. Through their antennae, bees detect the scents emitted by the flowers, allowing them to assess their nectar content and decide which ones are worth visiting.

Field experiments have already shown that the air pollution caused by internal combustion engines drastically reduces the rate of pollination from insects, by reducing the number of their visits. A field study in the US determined in 2022 that the number of flower visits from insects dropped by about 90% when the air was contaminated with diesel exhaust fumes, despite the measured pollution levels still being considerably lower than what the U.S. Environmental Protection Agency (EPA) still considers safe.

A motorcycle blows exhaust gas into flowers
A dirtbike blows a cloud of blue two-stroke exhaust fumes into a bed of tulips.

Recording produced for the purpose of illustrating the article, © Abgase.org

For one thing, the exhaust fumes reduce the size of the flowers' scent plumes, the smell of the flowers is covered and concealed. At the same time, it was assumed that the ground-level ozone caused by the exhaust gas reacts with the odour particles and changes the scent itself. This fall, a study conducted by several British universities reproduced these effects in the laboratory. Especially at the boundaries of the odour plumes, where likelihood of reacting with the ozone is highest, the exhaust fumes of gasoline engines chemically alter the odour particles, therefore preventing bees from finding and visiting the flowers rich with nectar and pollen.

Although the experiments were conducted with diesel exhaust, the ground-level ozone which is activating this reaction is not just caused by diesel engines. Ozone is not emitted directly from the exhaust pipe, it's not a component of the exhaust gas itself. It is formed when sunlight acts upon nitrogen oxides and volatile organic compounds (VOCs) in the fumes. Diese engines are infamous for their high nitrogen oxide (NOx) emissions.

But two-stroke engines (see photo and video) also measure up very poorly. For systemic reasons, a good 20–30% of the fuel mixture is expelled form the exhaust pipe unburned. For that reason, the exhaust fumes of two-stroke engines contain great amounts of unburned hydrocarbons – which are volatile organic compounds – and therefore also heavily contribute to the formation of ground-level ozone.

For an ironic cherry on top: in discussing the results of the British study, one of the scientists involved points out the danger this effect poses not only to food security, but also the growing of plants used in the production of biofuels. However, the nitrogen oxides responsible for the formation of ground-level ozone are emitted all the same when burning alternative fuels such as biodiesel!

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