by Sexual communication with insects relies heavily on pheromones, chemical attractants that specifically allow males and females of a species to mate. Sex pheromones are discriminative between males and females of a species. Even the smallest differences, such as those seen in the formation of new species, ensure that mating no longer takes place, because males and females find each other only through the unmistakable scent of their conspecifics.
Most insect pheromones are odor molecules containing carbon-carbon double bonds. Such double bonds are known to be easily destroyed by ozone. “We already knew that environmental pollutants such as ozone and nitric oxide degrade flower scents, making flowers less attractive to their pollinators. Since compounds with carbon double bonds are particularly sensitive to ozone depletion and almost all insect sex pheromones carry such double bonds, I wondered if air pollution also affects how well female and male insects find and recognize each other during mating,” says Markus Knaden, head of the Odor-Guided Behavior Group at Department of Evolutionary Neuroethology and is lead author of the study.
To study the effects of ozone on the mating behavior of the model fly Drosophila melanogaster, scientists developed for the first time an ozone exposure system for flies that could mimic ozone levels in the air, as they are often measured today in cities during the summer. To do this, the researchers had to create a continuous flow of air with precisely defined ozone levels, which is complicated by the fact that ozone is not a stable chemical compound and breaks down easily. At the same time, flies often carry very small amounts of pheromones even under normal conditions. “So we needed a technique that would allow us to measure even small amounts of pheromones in individual flies that had either been exposed to ozone or not before the measurements. To do this, we used what’s known as a thermal desorption unit coupled with a gas chromatograph/mass spectrometer, which allowed us to measure tiny amounts of odors emitted by individual flies,” said first author Nanji Jiang, describing the techniques challenges.
Ozone destroys the carbon-carbon double bonds in pheromones
In the experiments, male flies were exposed to slightly elevated concentrations of ozone. The scientists then measured whether the flies were still emitting their pheromone. When flies were exposed to 100 ppb (parts per billion, corresponding to a concentration of 10-9) of ozone for two hours, measured pheromone levels were significantly reduced compared to a control group exposed only to ambient air. Except for the male model flies Drosophila melanogasterthe researchers also examined male flies of eight related species of the genus Drosophila. In a single species, Drosophila busckiithe release of specific male pheromones was unaffected after exposure to ozone, but these compounds also do not contain carbon-carbon double bonds and therefore do not react as readily with ozone.
Ozone drastically corrupts the mating behavior of flies
The researchers then tested the attractiveness of the male flies to their mates. The observations they made were disturbing, which may be mainly due to the role of the respective pheromones. These are emitted from the male media Drosophila species and increase their attractiveness to females. At the same time, males use smell to distinguish females from other males: While their pheromone attracts females, it repels other males. During mating, males transfer their pheromone to females. Newly mated females that smell the pheromone are no longer attractive to other males for the next two hours. Consequently, high ozone levels not only made females less attractive than males. In contrast, ozonized males were suddenly interesting to their male counterparts. “We knew that high ozone levels could affect insect mating systems because breaking down carbon double bonds, and therefore pheromones, by oxidation is not rocket science in chemistry. However, we were shocked that even slightly elevated ozone concentrations had such strong effects on the fly. In fact, we initially wanted to focus on interactions between males and females. We could explain that the males started courting each other after a short exposure to ozone because apparently they couldn’t tell the ozonated males from the females. However, we hadn’t thought of this before. Therefore, we were quite confused about the behavior of ozone-exposed males, which lined up in long courtship chains,” Nanji Jiang and Markus Knaden summarize the effects of ozone on mating behavior.
The research team also observed the effects of high levels of ozone in the air on the mating behavior of others Drosophila kind. Even males of the species Drosophila busckii were less successful at mating after exposure to ozone, although ozone does not alter the pheromone that has been described as emitted by D. busckii males. However, other so far unidentified ozone-sensitive compounds may also play an additional role in their mating behavior. In eight of the other nine species studied, the research team observed unusual courtship behavior from males to other males exposed to ozone. Interestingly, one species, D. suzukiiwhich is known to lack pheromones, but courts based on visual cues, was not affected at all by increased ozone levels.
Air pollution is contributing to the global decline of insect populations
Most insect pheromones contain carbon-carbon double bonds. Ozone is therefore thought to interfere with sexual communication in many insect species. “Insects and their pheromones have evolved over millions of years. In contrast, the concentration of air pollutants has increased dramatically since industrialization. It is unlikely that insect communication systems, which have evolved over the course of evolution, are in position to adapt to the new conditions within a short period of time if suddenly there are no more pheromones. The only solution to this dilemma is to immediately reduce pollutants in the atmosphere,” says Bill Hansson, head of the Evolutionary Neuroethology Department and co-founder of the Center Max Planck Next Generation Insect Chemical Ecology (nGICE), which is dedicated to the study of the effects of climate change and air pollution on insects and their chemical communication.In particular, research focuses on the effects of anthropogenic climate change on insect ecosystem services, to outbreaks of invasive insect species and spread of disease vectors in Europe.
Scientists in Jena want to study the effects of ozone on a wider range of insects, including moths that normally follow pheromone plumes over long distances. Sex pheromones are also critical cues for insects to distinguish between conspecific and closely related species. “We would like to know whether high ozone levels lead to increased rates of hybridization when closely related fly species share their habitat. Finally, chemical communication in insects is not limited to mating behavior. All social insects such as bees, ants and wasps, use chemical cues to identify members of their colony. We’re also studying whether social structure in ant colonies is affected when ants return from foraging trips where they were exposed to elevated levels of pollutants. You don’t even want to imagine what happen if social structures in ant colonies or hives suddenly collapse because pheromone communication no longer works,” worries Markus Knaden, who also studies the behavior of desert ants.
High ozone levels are not only harmful to human health. The current way of life in industrialized countries has a very high cost to the environment and the climate. many indirect effects are not even known. The present study provides an additional explanation for why insect populations are declining dramatically worldwide, apart from the application of insecticides and habitat destruction. If chemical communication is disrupted by pollutants in the air, they cannot reproduce at a sufficient rate. This can also affect many pollinators such as bees and butterflies. The fact that 80% of our crops must be pollinated by insects makes it clear what scale this problem can take in the future if we do not manage to drastically reduce air pollution.
