Plants show pollinators the way to their flowers through “enchanting” scents. But air pollutants can obviously impair this effect: at high ozone levels, flowers smell less tempting for butterflies, according to a study on the model insect tobacco hawks. Although the research shows that pollinators can learn that “polluted” scents also promise nectar, the possible widespread impairment of communication between plants and insects is worrying, say the researchers.
Air pollutants such as ozone are known to have a negative impact on human health. But not only we are exposed to them – it stands to reason that insects can also suffer from the substances in the air. So far, however, little is known about how air changes affect communication between flowers and their visitors. Pollination is a crucial ecosystem service that butterflies and moths take on in addition to bees, bumblebees and the like. In addition to visual stimuli, many plants also use scents to guide insects to their flowers. These are chemical signals for which the respective pollinator insect has an innate predilection that has developed in the course of the long mutual evolution of the partners.
Smell tests with tobacco hawks
The international research team has now investigated the question of whether man-made high ozone concentrations in the air affect how attractive moths perceive the scent of flowers. “The tobacco hawk was the perfect model insect for our study. Flowers, which tobacco enthusiasts find attractive, usually have the same chemical compounds in their scent, ”says study director Markus Knaden from the Max Planck Institute for Chemical Ecology in Jena.
The researchers first investigated how the highly reactive ozone changes the scent of flowers: Using gas chromatography, they determined the composition of the volatile substances with and without increased ozone content. For the “contaminated” fragrances, the scientists used concentrations of the pollutant that can occur today on hot days in the natural habitat of tobacco enthusiasts. They tested the reactions of the insects in behavioral experiments in the wind tunnel, where the animals were exposed to the original flower scent and the mixture of scents changed by ozone.
As the researchers report, the air pollutant had a drastic effect: “We were surprised that the moths completely lose their innate preference for the scent of tobacco flowers in the presence of ozone,” says Knaden, summing up the result of this part of the study. The result implied two possible explanations: Either the tobacco enthusiasts no longer find the scent really tempting, or they can no longer locate their source of food. In order to clarify what is behind the observations, the researchers examined the extent to which the moths can learn that even contaminated flower scents promise a sweet reward.
Able to learn – but is that enough?
To do this, they tested whether tobacco enthusiasts can learn the initially unattractive ozone blossom scent as a food guide if they smell it while they are administered sugar solution at the same time. To this end, the scientists developed an experiment in which the moths had to follow the ozone-altered scent to bloom. It was confirmed that the butterflies can relearn: they can learn through experience to combine the scent of ozone flowers with food, the experiments showed. Accordingly, a lengthy evolutionary process is not necessarily necessary for adaptation. “The behavior we have seen in tobacco hawks shows that they may be able to learn new stimuli quickly to cope with their rapidly changing environment,” says lead author Brynn Cook of the University of Virginia at Richmond.
But does that mean that air pollution may not be such a major threat to pollination and pollinators? The researchers speak out against this conclusion: “Learning can be a key to insects recognizing their host plants despite air pollution or changed climatic conditions. However, it remains unclear to what extent pollinators in nature have any chance to learn that ozone-altered flower scents can be food clues. In addition, other pollinator species may be less capable of learning than the tobacco hawk. Species that specialize in certain flowers may not be as flexible with learning, ”says Cook. “Our investigation represents a starting point for further ones. Now, above all, we need field studies to answer the critical questions of which flowers and insects are most affected by which pollutants and why, ”the scientist concludes.
Source: Max Planck Institute for Chemical Ecology, original publication: Journal of Chemical Ecology, doi: 10.1007 / s10886-020-01211-4