Insects may have difficulty locating flowers because air pollutants degrade the chemical compounds responsible for their tantalizing floral scents.
“In recent years, we have become more and more interested in ‘sensory pollution’,” explains Jeff Riffel at the University of Washington in Seattle. This pollution, which results from human activity, can modify the behavior of wildlife by modifying or introducing new stimuli, he explains.
For example, noise pollution has been shown to impact bird song and could be linked to an increase in whale strandings. Light pollution, meanwhile, can disorient many animals, including migratory birds and sea turtles.
But little is known about how human activity affects animals' sense of smell. So, Riffell and his colleagues studied 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, changing their scent.
The team collected the compounds released by the pale evening primrose (Oenothera pallida), a desert flower found in North America. Both pollutants destroyed odorous compounds, but nitrate radicals did so more completely.
To study whether this changed the behavior of the flowers' primary pollinators, the researchers exposed hawkmoth species, including the white-lined hawkmoth (Hyles lineata), to flowers that emitted either the natural floral scent, or to flowers manipulated to release a degraded scent.
Primroses that emit degraded odors are visited 70 percent less frequently than flowers that emit natural odors. This drop in visits could affect the health of the butterfly, says Riffell. It could also have a knock-on effect on the ecosystem as a whole, as researchers calculated that reduced moth visits could lead to a 28% reduction in the amount of fruit produced by plants.
Since the Industrial Revolution, the distance at which butterflies can detect flowers has increased from about 2 kilometers to just a few hundred meters, according to the team's models.
“This is just another reason why we should move to energy sources that don't involve combustion,” says one team member. Joel Thornton, also at the University of Washington. “If we can reduce nitrogen oxide emissions, it will be a victory for air quality as well as the functioning of ecosystems and agriculture.”