Insect reproductive behavior is influenced by ozone exposure.
It's commonly understood that ozone harms the respiratory system of humans, but it also poses substantial threats to insects. Numerous studies and investigations suggest that pollutants contribute to their significant decrease. The consequences are severe, even extending to the items found in supermarkets.
Climate change, habitat loss, and pesticides are frequently pointed out as the main causes of the substantial decline in insect populations. However, scientists are continuously uncovering more evidence of the harmful influences of air pollutants. For instance, ground-level ozone and nitrogen oxides affect the attraction of insects to floral scents and animal mating behaviors, as demonstrated by various studies.
A thorough analysis concluded that the performance of pollinating insects in gathering food and their reproduction rates decreased significantly when the concentrations of air pollutants were high. A ** researchers from the University of Reading in the UK** analyzed 120 studies, involving 40 insect species from 19 different countries, for this meta-analysis recently published in the journal "Nature Communications". Surprisingly, air pollutants did not significantly impact plant-eating aphids and other pests.
Misleading Scent
"We are dealing with a detrimental situation in which air pollution harms beneficial insects but leaves pests unscathed," says James Ryalls, the first author of the study. This could potentially "result in increased crop damage, reduced yields, and fewer food items in supermarkets." Ozone had a particularly notable impact on pollinators.
Researchers from Jena have delved deeper into the effects of ozone on insects, focusing particularly on its impact on their sexual attraction.
"Male fruit flies mark themselves with their sexual pheromones and use them to attract mates, similar to how humans use perfume," explains Markus Knaden from the Max Planck Institute for Chemical Ecology in Jena. However, ozone can destroy this scent, "making male flies less appealing to females and causing male flies to suddenly pursue other male flies." The reason for this behavior is that male flies are unable to distinguish between ozone-affected males and female flies that do not use perfume in general. "As a result, male flies have to dedicate more time and energy to encounter a female and mate with her," concludes Knaden.
"We were aware that elevated ozone levels could impact the mating systems of insects," says Knaden. "However, we were amazed at how even slightly increased ozone concentrations had such strong effects on fly behavior."
Most insect sexual pheromones contain carbon-carbon double bonds that can be easily split by oxidizing pollutants such as ozone or nitrogen oxides, explains Knaden.
Bewildered Females
In another experiment, Knaden's team introduced female fruit flies of the species Drosophila melanogaster to other fruit fly species. The males were exposed to an ozone level of 100 ppb (parts per billion), a level that can be found in large cities, for two hours. After their scent was destroyed, the females responsible for choosing a mate often failed to identify which male was the best match and mated with other species' males. This resulted in fly hybrids that could not reproduce.
"The males are less effective, and the females choose inappropriate mates," concludes Knaden. "The damages accumulate."
Knaden predicts that the impact is also visible in the natural environment: "20 minutes of high ozone concentration is enough to eliminate the perfume layer of male fruit flies. It takes them two days to restore their original scent layer. If ozone levels increase daily in the summer, their perfume wanes continually," says Knaden. Field trials are planned.
Ozone is produced through intense sunlight from car exhausts or solvents in paints and varnishes. Nitrogen oxides are released directly from car exhausts and formed, for example, during coal and oil combustion.
Unrecognizable Flower Scents
Air pollutants can also disrupt the attraction of flowers to insects in various ways. A US team demonstrated that ozone (O3) and the nitrate radical (NO3 radical) break down certain compounds that produce attractive flower scents. Researchers observed 300 nightshade flowers for a total of 200 hours and found that the flowers were hardly noticeable or not at all to certain nocturnal moths due to the loss of the scent substance. The number of flower visits by the night moths decreased by around 70%, and the fruit production of the nightshade by around 30%.
British researchers exposed black mustard, a member of the cruciferous vegetable family, to a concentration of diesel exhaust and ozone that falls under current air quality standards in a field study. The number of insects such as bees, flies, moths, and butterflies decreased by 62 to 70% on the test field, and their flower visits by 83 to 90%. The researchers suspect that pollutants not only attack flower scents but also the sense of smell of insects.
Further Research Required
Pollinating insects are threatened from multiple angles, including climate change, habitat loss, and increased pesticide use, state Laura Duque and Ingolf Steffan-Dewenter of the University of Würzburg in a meta-study from 2024. However, the effects of air pollution have only been examined in a limited number of plant-pollinator systems to date, they criticize in the journal "Frontiers in Ecology and the Environment". Additional research is required in this area.
In addition to the effects on mate selection and flower recognition, pollutants can impair the learning abilities and memory of insects, their orientation, and overall fitness. Frequently, air pollution increases the time insects spend searching for food, reducing their efficiency.
The extent to which air pollutants contribute to the decrease in insect populations isn't fully clear yet due to a limited amount of research, according to Duque. Despite the reduction of numerous pollutants in Germany, there are studies suggesting harmful effects of air pollutants at present concentrations. It's ideal to continue lowering our emissions and collaborate with other global nations in this effort.
Recent studies have reported a decline in insect populations in Germany. In 2017, the Entomological Society of Krefeld published data in "PLOS One" indicating a drop of over 75% in the total biomass of flying insects from 1989 to 2016. A 2019 study led by researchers from the Technical University of Munich showed a 67% decrease in the total biomass of insects and arthropods in grasslands and about 40% decline in forests from 2008 to 2017.
Knaden suggests reconsidering air pollution limits as small quantities of these substances can have substantial impacts on insect chemical communication. Given the current significant decline in insect populations in terms of both biomass and biodiversity, there's a need to better comprehend and combat all potential factors contributing to this decline.
The findings from the University of Reading's study suggest that air pollutants, such as ozone and nitrogen oxides, negatively affect the attraction of insects to floral scents and animal mating behaviors. This could potentially lead to increased crop damage, reduced yields, and fewer food items in supermarkets (Sex: insect mating behaviors).
Furthermore, Markus Knaden from the Max Planck Institute for Chemical Ecology in Jena explains that ozone can destroy the sexual pheromones of male fruit flies, making them less appealing to females and causing them to pursue other male flies. This behavior affects the mating system of insects and requires more time and energy from the males (Sex: animal mating behaviors).
