Air pollution is changing the scent of flowers and confusing insects

Insects may be struggling to locate flowers because air pollutants are degrading the chemical compounds responsible for their enticing floral scents.

“In recent years, there has been a growing interest in ‘sensory pollution’,” says Jeff Riffell at the University of Washington in Seattle. This pollution, which arises from human activity, can change wildlife behaviour by changing or introducing new stimuli, he says.

Noise pollution, for example, has been shown to impact birdsong and may be linked to an increase in whale strandings. Light pollution, meanwhile, can disorientate a range of animals, including migratory birds and sea turtles.

But little is known about how human activity has been affecting animals’ sense of smell. So, Riffell and his colleagues investigated the effects of anthropogenic pollutants on plant pollinators.

They focused on ozone and nitrate radicals, pollutants created by the interaction of vehicle emissions with gases in the atmosphere. Both are known to react with compounds given off by flowers, altering their smell.

The team collected the compounds released by the pale evening-primrose (Oenothera pallida), a desert flower found in North America. Both pollutants broke down the scent compounds, but nitrate radicals did so more completely.

To study whether this changed the behaviour of the flowers’ primary pollinators, the researchers exposed hawk moth species, including the white-lined sphinx (Hyles lineata), to flowers that emitted either the natural flower scent or flowers manipulated to release a degraded scent.

The primroses that released the degraded scents were visited 70 per cent less frequently than the flowers releasing natural sent. This drop in visitation could affect hawk moth health, says Riffell. It could also have a knock-on effect on the wider ecosystem, because the researchers calculated that the decline in moth visitations could result in a 28 per cent reduction in the amount of fruit the plants produce.

Since the industrial revolution, the distance at which hawk moths can sense flowers has shrunk from about 2 kilometres to just a few hundred metres, according to the team’s models.

“This is just another reason that we should switch to energy sources that do not involve combustion,” says team member Joel Thornton, also at the University of Washington. “If we can reduce nitrogen oxides emissions, it’ll be a win for air quality as well as ecosystem functioning and agriculture.”