Insect pheromones are odor molecules used for chemical communication within a species. Sex pheromones play a crucial role in the mating of many insects. Species-specific odors attract males and females of the same species. At the same time, they maintain the natural boundaries between species. The research team has previously shown that elevated ozone levels severely disrupt chemical communication within fly species: Ozone breaks the carbon-carbon double bonds found in most insect pheromones. As a result, male flies can no longer distinguish between females and other males and therefore court both sexes.

In the mating experiments, the flies were exposed for two hours to ozone concentrations that are often measured on particularly hot days in our cities. The scientists gave ready-to-mate females the opportunity to choose between a male of the same species and a male of a different species. After a few hours, they separated the females from the males and allowed them to lay eggs. To determine whether the female had mated with a male of her own species or another species, the researchers analyzed the sexual organs of the male offspring, as species and hybrids can be distinguished on the basis of their morphology. The results of these tests showed that hybridization occurred more frequently under the influence of ozone, while few hybrids were found when the flies were previously exposed only to ambient air.

Outside the laboratory, other oxidizing pollutants such as nitrogen oxides, which cannot be tested in laboratory experiments because of their toxicity, can amplify the effect of ozone. Limit values already exist for these pollutants because of their harmful effects on humans. "The limits for air pollutants should be re-evaluated, considering that even small amounts of these substances have a significant impact on the chemical communication of insects," says Markus Knaden. "As we are currently facing a dramatic decline in insect populations regarding their total biomass and their biodiversity, we should try to better understand and counteract all possible factors that potentially favor this decline."