Insects find locating their favourite flowers more difficult when pollution and other odours get in the way, new research has shown.
Tobacco hornworm moths feed on the nectar of plants which can grow hundreds of metres apart.
Researchers found the moths tracked the flower's scent better in clean air, when other smells were not present.
They discovered the other odours changed how the moth's brain processed the plant's scent.
The findings are reported in the journal Science.
Adult tobacco hornworm moths, Manduca sexta, have wingspans of 4in (10cm) and can travel up to 80 miles (129km) in an evening, looking for food and mates.
Their ability to detect scents is similar to that of dogs, and both are several thousand times more skilful than humans.
They pollinate the trumpet-like blossoms of Sacred Datura, Datura wrightii, but the calories from one feed are only enough to fly for 15 minutes, making accurate navigation essential.
Little is known about how insects tell the difference between flower odour and the variety of natural and man-made scents in the air.
The research team from the University of Washington and University of Arizona, conducted fieldwork in the south-western United States, where the moths' search for Sacred Datura can be hindered by the scents of neighbouring plants.
The plants often grow in dense groups of creosote bush, Larrea tridentata, which give off some of the same aromatic chemicals as Sacred Datura.
These same volatiles from vehicles may affect pollinators like honeybees or bumblebees'
End Quote Jeffrey Riffell University of Washington
The scientists used a chemical detection device (called a proton transfer-reaction mass spectrometer) to study the odours given off by the flowers in the wild.
They then flew moths in a wind tunnel where they controlled the smells the moths were encountering. The insects' neuron pathways were also recorded to see which were activated.
The team found the moths located Sacred Datura most easily when its scent was at lower intensities (frequencies between 1-2 hertz).
They tested moths' responses to the flower's scent when different odours, including the creosote bush and man-made pollutants, were present. They found the other smells masked Sacred Datura's scent making it more difficult for the moths to recognise and track.
The authors found this was because the other smells affect odour-processing neurons in the moth's brain, altering the insect's perception of the plant's scent.
Jeffrey Riffell, lead author, said: 'Local vegetation can mask the scent of flowers because the background scents activate the same moth olfactory channels as floral scents.'
He said they were surprised to find that even scents that were only vaguely similar to those that are attractive to moths, such as those from car and lorry exhausts, also misled them.
'Nature can be complex but an urban environment is a whole other layer on top of that,' Mr Riffell added.
'These moths are not important pollinators in urban environments, but these same volatiles from vehicles may affect pollinators like honeybees or bumblebees, which are more prevalent in many urban areas.'
The researchers plan to study other pollinators and hope to identify more chemicals that can alter insects' ability to recognise flowers.
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