Air Pollution Affects How Moths Smell Flowers

For pollination to work, flowers lure insects with their sweet aromas. The scents are chemical signals that attract the pollinators, who have a preference for certain odors in a symbiotic relationship that has evolved over millions of years.

But as air pollution has increased, it has become difficult for some pollinators to break through the ozone haze to smell their floral targets. In a new study, researchers found that tobacco hawkmoths specifically aren't attracted to flower aromas when ozone levels are high. However, the insects are able to learn that odors affected by ozone can still lead to nectar.

"We know that most insects heavily rely on olfaction to find their food and mating partners. As many of the known flower odors are chemically fragile and can easily become degraded by oxidants, we wondered how oxidants like ozone that increase due to pollution affect the relation between flowers and their pollinators," study leader Markus Knaden, who heads a research group in the Department of Evolutionary Neuroethology at the Max Planck Institute in Germany, tells Treehugger.

For the study, Knaden and his team chose the tobacco hawkmoth (Manduca sexta) because it is attracted to flowers not just by their smell, but it also uses a visual system to find its target.

The researchers analyzed the composition of the hawkmoth's favorite flower odors — with and without increased ozone. Then they watched how the moths responded in a wind tunnel as they investigated both the original floral odor and the ozone-altered odor.

"We were shocked that ozone does not only slightly decrease the attraction of flower odors to the tobacco hawkmoths but completely ruins it," Knaden says.

The study was published in the Journal of Chemical Ecology.

The Ability to Learn

The researchers were curious whether ozone would prevent insects from finding their food or whether they eventually could figure out that even polluted flowers could lead them to nectar. They tested whether the insects would accept an unattractive scent as a food cue if they smelled it while being offered sugar solution as a reward.

In the real world, the researchers knew, a floral odor is changed as it moves downwind from the flower and mixes with the ozone in the air. To see if moths could recognize ozone-changed floral scents even without receiving the sugar solution, the researchers created an experiment where the moths followed the ozone-altered aroma, but were rewarded with the original scent and the flower containing the sugar nectar.

"While we anticipated that Manduca sexta could learn new floral scents and hoped that they would be able to learn the polluted floral scent of their host flower, we were amazed to see that Manduca sexta could learn the polluted floral blend in a number of different ways, including learning a polluted scent that was decoupled from a sugar reward. This type of learning, which we were surprised to find in Manduca sexta, could be very important in insects’ ability to use learning to cope with their rapidly changing environments,” says first author Brynn Cook from the University of Virginia in a statement.

Although the tobacco hawkmoth was able to learn, not all insects might be able to adapt in this way.

"The consequences of pollution can be far reaching," Knaden says. "At the same time at least our study animal the tobacco hawkmoth was able to cope with this situation by targeting the flowers through vision and then immediately learning the ozone-changed scent of the flower. However there might be many insect species that do not have such an accurate visual system or are just not 'clever' enough to learn the changed odors. We are therefore afraid that pollution might affect many insects on their search for food (and by that might reduce the insects‘ pollination service)."

The researchers hope to continue their work with other pollinators.

"The study shows how complex it can be to figure out the effects of pollution," Knaden says. "It will now be interesting to test insects with less powerful vision and/or lower learning capabilities."