Pollinators in a haze can’t find their flowers

Industrial pollution interferes with the scent of wildflowers and disrupts night time pollination.

Moths – crucial yet underappreciated plant pollinators – find flowers in the dark using their keen sense of smell, even detecting scents at kilometre range. 

In new research, with global implications for the future of agriculture, University of Washington, US researchers show nitrate radicals (NO3) degrade wildflower scent cues making it harder – and sometimes impossible – for moths to find and pollinate flowers at night. Their findings are published in Science.

Nitrate radicals arise when nitrogen oxides – the product of combustion in cars, power plants and other sources – react with ozone (O3) in the atmosphere. Natural sources of NO3 include wildfires and lightning.

“The NO3 is really reducing a flower’s ‘reach’ — how far its scent can travel and attract a pollinator before it gets broken down and is undetectable,” says Jeff Riffell, professor of biology at the University of Washington.

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Pale evening primrose flower during field experiments in eastern Washington / Credit: Jeremy Chan / University of Washington

Through a series of lab and field experiments, the researchers investigated the effects of NO3 on chemical scent cues produced by the pale evening primrose (Oenothera pallida). The wildflower has a distinctive aroma known to attract diverse pollinators, particularly nocturnal moths.

Using flower scent samples gathered in the field, the researchers analysed the mix of chemical compounds which make up the moth-friendly fragrance.

Then, using mass spectrometry they observed reactions between individual scent compounds and NO3, finding some – like monoterpene, a compound known to attract moths – were nearly eliminated by the pollutant.

In the lab, the help of a wind tunnel, the researchers tested the accuracy of two moth species – tobacco hawkmoth’s (Manduca sexta) and white-lined sphinx (Hyles lineata) – in locating and flying towards the scent of pale evening primrose; with and without the presence of nitrate radicals. 

When typical night-time levels of NO3 were introduced, the moths’ ability to find the scent source dropped dramatically. The tobacco hawkmoth’s accuracy fell 50%, while the white-lined sphinx – the flower’s chief pollinator – was unable to find the source of the aroma.

In the field, the researchers also recorded moth visitation – with and without NO3 – finding moth visits dropped by 70% once the pollutant was introduced.

The findings highlight the dangers of human sources of pollution, the researchers say.

“Pollution from human activity is altering the chemical composition of critical scent cues, and altering it to such an extent that the pollinators can no longer recognise it and respond to it,” says Riffell.

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A white-lined sphinx pollinating a pale evening primrose flower / Credit: Ron Wolf / University of Washington

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