2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Biologists have found that after reacting with ozone, the smell of tobacco no longer attracts hawkers, but after a single training, insects remember the association of an unfamiliar aroma with nectar. Learning abilities, however, are limited: the hawk makers could not remember the sequence of smells, but they recognized similar scents. The authors of the article, published in the Journal of Chemical Ecology, believe that pollinator training can reduce the negative effects of air pollution from oxidants.
The evolution of plants and their pollinators is closely related: both need the pollinator to be able to find and recognize a flower by smell, despite the inevitable differences in the structure of volatile organic substances, even between populations of flowers of the same species. Therefore, pollinators, firstly, are guided by whole classes of substances, and not by specific compounds, which may differ in different flowers; secondly, they know how to learn: they remember new smells after meeting a plant, generalize experience about different substances with a similar structure, and change their preferred scents.
After the industrial revolution, the content of ozone and other oxidants in the troposphere increased sharply. They can react with double bonds between carbon atoms in VOC molecules and thus change their structure and smell. As a result, pollinators do not recognize familiar scents and, in order to successfully find a flower, they need to rebuild and learn a new smell.
Scientists from Germany, Holland and the United States, led by Markus Knaden of the Max Planck Institute for Chemical Ecology, decided to find out if tobacco hawk moths (Manduca sexta) can recognize and remember the smell of organic tobacco volatile substances (Nicotiana alata) that have reacted with ozone. The tobacco odors were mixed with ozone or air, analyzed by gas chromatography and mass spectrometry, and introduced into the hawker chamber through two tubes.
In the second part of the experiment, the researchers taught the butterflies to fly to a particular smell: first, they looked at which of the two scents (studied or control) the hawkers preferred before training, then they attached an artificial flower with a sweet solution to the tube from which the smell came. The hawkers could see a flower, drink "nectar" from it, and remember the association of smell and sweet flower.
Learning Experiment Design
Ozone changed the amount and ratio of several primary volatile organic compounds in the composition of odors and created several secondary ones - as a result, the original and mixed with ozone aromas were noticeably different. The hawk moths preferred artificial flowers that smelled like tobacco without ozone, although the ozone itself did not repel or attract insects. However, after training, the butterflies remembered that the smell of tobacco changed by ozone was associated with the sweet flower and flew to the tube with it much more often (p <0, 0001).
Scientists suggested that hawk makers could not learn a new smell from scratch, but generalize knowledge about a different, but similar, unchanged smell. Indeed, after learning with flowers that smelled of tobacco, insects reacted more actively to the aroma mixed with ozone (p <0, 0001).
Finally, the scientists conducted the training in more natural conditions. The fact is that in nature, a modified smell cannot lead an insect directly to a flower: the plant itself emits an unchanged aroma, and it remains clean around the flower. While the substances react with ozone, the aroma has time to disperse over some distance. To simulate this situation, the researchers trained butterflies to memorize a sequence of two smells: first, a neutral smell (linalool) flew out of the tube, but when the hawk moth flew 20 centimeters to the artificial flower, the aroma was changed to an attractive one (2-phenylethanol).
The hawkers did not remember the sequence of smells and, after training, did not find the smell of linalool attractive. So, probably, butterflies do not remember a new smell by itself, but recognize it after meeting a similar one and find a flower along a gradient of similar aromas.
Learning ability can help insects and pollinated plants in conditions of increasing concentrations of tropospheric oxidants. However, different insect species have different learning abilities, and in general, air pollution still poses a serious threat to pollinators, and may also be one of the reasons for the global decline in insect numbers.
Not only pollinating insects can find food by smell. Earlier, for example, Marcus Knaden and his colleague found out that ants can associate different smells with food and remember such associations for life.