Bees and butterflies have trouble smelling flowers in polluted air.
The researchers found that diesel exhaust and ozone made it harder for pollinators to find flowers and reduced plant production rates.
Air pollutants make it difficult for butterflies and other pollinators to find flowers, which in turn affects food stocks for humans.
Insects such as bees and butterflies rely on a keen sense of smell to pollinate the food we eat and the flowers in our gardens.
"Our sense of smell is terrible compared to most other organisms," says Robbie Girling, associate professor of agroecology at the University of Reading in England. "[Pollinators] do indeed use scents to navigate, move, and communicate with each other."
But Girling, along with other researchers at the University of Reading, the UK Center for Ecology and Hydrology and the University of Birmingham, discovered that air pollutants can drive pollinators out of odor. A study they publish today in Environmental Pollution shows that ozone and diesel exhaust significantly reduce the presence of pollinators, the number of times pollinators visit plants, and how many seeds the plants produce.
Previous research suggests that common pollutants such as ozone and diesel exhaust alter the scents of flowers and make it harder for pollinating insects to find plants. However, not much light has been shed on what the effects of this are on pollination rates.
Some insects can first smell it when chemical compounds from a flower land on their antennae. James Ryalls, a Leverhulme Trust Early Career Fellow at the University of Reading and one of the study's authors, says they then take this scent cloud back to the plant like a treasure map.
After feeding, Girling says, insects like honeybees learn which compounds make the tastiest flowers and come back to them like Pavlov's dog. But ozone and diesel exhaust, a byproduct of factory and vehicle emissions, can cloud these perfumes.
"[Contaminants] can disrupt the signal they use, so they may no longer find the flower," says Ryalls.
Thus, Girling and Ryalls began to understand the impact of ozone and diesel exhaust on insects and pollination in the natural environment. Previously, all work on this topic was done in a laboratory. At the University of Reading farm, researchers uncovered octagonal rings of about 26 feet. In each of the rings, they pumped either ozone, diesel exhaust, a combination of both, or none at all. The rings also contained black mustard plants; Previous research had shown that pollutants impair the species' floral scent.
The rings were open to ambient air to allow local insects to access them. Next, the team observed how often pollinators such as bees, flies, butterflies and moths entered the rings and visited one of the mustard plant's flowers. The results were harsh. In rings with a combination of ozone and diesel exhaust, the presence of pollinators was reduced by 70 percent compared to rings without pollutants; visits to flowers also decreased by 90 percent.
The researchers also found a 31 percent reduction in pollination rate by measuring the number of seeds in the pods produced by the plants. They also noted that air pollutants have little or no direct effect on the plants themselves. (The scientists hand pollinated a few of them and found that seed production did not change significantly due to their exposure to pollutants.) So this means that the reduction in pollinator visits directly results in a reduction in seed production.
But Girling and Ryalls were surprised at how dramatically pollutants were affecting pollinators, especially since they couldn't pump as much ozone and diesel exhaust into the rings as they would like due to equipment limitations. "We were thinking, 'Oh, we're not going to see anything here,'" Girlings says. "So when James came back with the first results, I had him go and recheck them."
The researchers were able to maintain levels of ozone and nitrogen dioxide (found in diesel exhaust) at 35 parts per billion and 21 parts per billion, respectively. These levels were about half the standards set for safe ozone and nitrogen dioxide levels set by the Environmental Protection Agency.
These findings have an impact not only on biodiversity, but also on the food in your fridge, Girling says. “Seventy percent of all the different crops we eat require insect pollination,” she explains. She adds that the lack of insect pollination could cause food prices to rise due to low supply or for growers to resort to more labor-heavy measures such as hand pollination.
While Girling and Ryalls say that air pollution doesn't kill all insects or indicate end time, they emphasize that it's one of the many stressors faced by pollinators.
“Insects are currently under a lot of pressure from human influence,” Girling says. "And when you start pushing things from different directions, at some point they just can't stand it. And they collapse."
