Are Flowers Smell So Sweet?

Ozone shields our planet from the sun’s harmful ultraviolet rays, but it may be also be keeping bees away from their favorite flowers, according to a new study. Ozone doesn’t just live high in Earth’s atmosphere; near the ground, it contributes to smog, and ground-level ozone has gradually increased in most places because of industrial pollution from vehicles and fossil-fuel burning. Ozone is known to react with and break down molecules called volatile organic compounds. That’s a concern because flowers emit certain kinds of these compounds that, like a natural perfume, entice bees and other pollinators to visit the flowers. In the laboratory, the scientists exposed groups of them to ozone levels ranging from 0 parts per billion (ppb) of ozone to 120 ppb, which is found in some urban areas. A machine called a mass spectrometer measured levels of scent molecules as far as 4.5 meters away from the flowers. At 4.5 m away, levels of most of the scent’s component molecules ranged from 17% to 31% lower at 120 ppb than at 0 ppb ozone... Little to no decrease occurred at 0 m away, showing that ozone’s effect increases with distance. Additional lab tests showed that unaltered scent is attractive to a common type of bumblebee (Bombus terrestris); bees preferred spending time in scent-laced air over scent-free air, the researchers say. Because some molecules’ levels decreased more than others in the lab tests, the scent’s strength isn’t all that’s at stake: The scent itself may change a bit, too. The findings suggest pollinators could struggle more over time to locate flowers, as ozone pollution in most parts of the world will keep rising as a result of industrialization and climate change.