Insect sexual communication is largely influenced by pheromones, chemical attractants that specifically allow males and females of a species to mate. Even the smallest change, such as that observed in the formation of new species, can cause mating to no longer occur, as males and females only find each other by the unmistakable scent of their conspecifics.
Most insect pheromones are odorous molecules containing carbon-carbon double bonds. It is known that ozone can easily destroy such double bonds.
Now, a team of researchers from the Max Planck Institute for Chemical Ecology (in Germany) has shown that rising ozone levels caused by anthropogenic air pollution can degrade the sex pheromones of insects, causing reproduction to never occur.
As the carbon-carbon double bonds of pheromones are broken by ozone, the chemical mating signal becomes dysfunctional.
As the double bonds break down, the specific chemical signal for mating becomes dysfunctional. Specifically, the study authors showed this effect in the vinegar fly Drosophila melanogaster and nine other species of the genus Drosophila.
“Insect pheromones are usually long-chain molecules that carry carbon-carbon double bonds. These double tanks are very sensitive to degradation by oxidants such as ozone or nitric oxide. When it comes into contact with these oxidants, the carbon double bond can break and the long-chain molecule breaks into two shorter pieces. These new molecules are no longer detected by female flies as pheromones”, explains Markus Knaden, research leader, to SINC.
To study the effects of ozone on fruit fly mating behavior, the scientists first developed a system of exposing flies to this gas that could mimic ozone levels in laboratory air. , which are usually measured in summer inner cities.
They quantified the pheromones of the flies using a technique known as gas chromatography coupled to thermodesorption/mass spectrometry (TDU-GC/MS). This method consists of freshly killed flies being individually heated in a small glass tube, so that all their potential pheromones are vaporized and transferred to the GC/MS, a chemical analyzer that identifies and quantifies all molecules in the headspace.
In the experiment, most pheromone amounts decreased after exposure to ozone and some new compounds appeared.
By describing all the Drosophila pheromones, they were able to compare the headspaces of flies that were and were not previously exposed to ozone. Most pheromone amounts decreased after exposure to ozone and some new compounds appeared (probably the degradation products of long pheromone molecules).
“We tested whether ozone exposure had any effect on behavior in small petri dishes, where we checked how quickly males exposed or not exposed to ozone could persuade the female to copulate. The first one (which didn’t have the persuasive mixture of pheromones) took much longer to convince the female”, clarifies the researcher, responsible for the Olfactory Behavior Group of the Department of Evolutionary Neuroethology at the German institution.
In the case of female flies exposed to high levels of ozone, no differences in courtship behavior were observed.
“Basically we see differences when we expose male flies to ozone. In flies, males usually initiate courtship behavior, while the female eventually accepts or rejects the male (based on her pheromones). The former are less judgmental, court the females and don’t care as much when the female’s compounds are degraded by ozone. Therefore, even after being exposed to ozone, a female fly will still be attractive to a male fly,” reveals Knaden.
Only one of the ten species analyzed, Drosophila suzukii, was not affected at all after exposure to high levels of ozone. As is known, this one lacks pheromones, so it courtes based on visual signals.
Exposure to ozone caused males to begin courting other males of their species, not distinguishing them from females.
Another of the most striking findings is that the interruption of sexual communication would also have caused males to exhibit unusual mating behavior towards organized males of their own species.
“We could explain that males started courting after a brief exposure to ozone, because they obviously couldn’t tell ozonated males from females. However, we hadn’t thought of that before. So we were quite intrigued by his behavior,” explains Knaden along with Nanji Jiang, also a co-author of the study, in a press release.
It does not depend on the location, but on the contamination
The discoveries took place in environments with an ozone concentration of 100 ppb (parts per billion). This concentration is sometimes reached in European metropolises. In the case of megacities and their surroundings in India, Mexico and China, these concentrations can be much higher.
“Therefore, it can be assumed that the effects induced by pollutants are even greater in these large cities. It will also be interesting to test whether and how insects in rural areas around megacities are affected and, if so, how far the devastating effect of pollutants extends”, reflects Knaden.
The ozone concentrations in the study were 100 ppb (parts per billion), figures sometimes reached in European metropolises.
The study co-author believes his research has major implications if future studies corroborate that sexual communication in free-living insects has been corrupted, as his laboratory experiments have shown.
“Insects are a very important part of food chains and provide critical ecosystem services (most of our fruits and vegetables depend on insect pollination). So I believe that the insect decline that we are currently seeing (and which, given our findings, could be enhanced by oxidizing contaminants) affects not only some weird entomologists like us who like to observe insects in the field, but also that they may have a great impact on agriculture, on productivity and, therefore, on human health”, he says.
Impacts on other insect species
The authors hope to do more lab and field research in the future on other insect species to find out if their sexual behaviors are affected in the same way. They do not deny a possible correlation.
“Since most insects communicate using pheromones, which have similar basic chemical characteristics (long-chain molecules with carbon double bonds), we expect a severe effect on other insects as well,” Knaden said.
Since most of these animals communicate using pheromones, which have similar basic chemical characteristics, we would expect a severe effect on other insects as well.
Markus Knaden, Research Lead
In particular, they believe the influence may be very evident in the case of moths. “Female moths attract males by emitting sex pheromones hundreds of meters away, so they can be affected, as the pheromones are exposed to air pollution for a long time before reaching the males”, he emphasizes.
Likewise, the authors see a possible link in the case of social insects like bees and ants. They wonder what would happen if the structures of ant colonies or hives suddenly collapsed because pheromone communication stopped working.
The expert recognizes that they still haven’t thought much about animals other than insects, but he considers that, considering that mammals also use pheromones, “potentially they can also suffer from it”.
How to mitigate these effects
From the Max Planck Institute of Chemical Ecology, they consider that to mitigate the impact of ozone pollution at ground level on insect populations, they see no other possibility than to reduce the use of fossil fuels, that is, to immediately reduce the concentration of atmospheric gases pollutants.
Insects have evolved pheromone communication for millions of years and only in the last few decades have they faced the huge increase in oxidizing pollutants.
“Insects evolved granular pheromone communication over millions of years and only faced the huge increase in oxidizing pollutants for several decades. I doubt that the evolution of insects will find a solution in due time”, concludes the researcher.
The study authors acknowledge that there are still some questions to be addressed, such as whether the effects of this laboratory research can be extrapolated to a field study or how drastic these cutting deficiencies caused by contaminants are. However, they urge policymakers not to wait for all the details and urge them to act immediately.
Nan-Ji Jiang, Markus Knaden et al. “Ozone exposure disrupts insect sexual communication.” Nat Communications (March 2023)