How diesel particulate matter threatens insects

Bumblebees and co. can pick up fine dust particles when collecting nectar from flowers near the road. © Heike Feldhaar

Problematic flower nectar: ​​Experiments on bumblebees show that exhaust particles from diesel vehicles ingested through food can cause significant harm to insects. Stress, such as that which can occur when visiting flowers near the road, significantly weakens the animals. This effect could also contribute to the death of insects, say the scientists.

Some of the losses are frightening: Studies show that insect populations have declined drastically in recent decades in many regions of the world. This is particularly worrying because they fulfill key ecological functions: they pollinate plants, shape material cycles and form the basis of nutrition for many other animals. But what causes insects so much trouble? As studies show, it is probably a mixture of different factors: In addition to climatic changes, the loss of habitat and food sources, environmental pollution with certain substances harms insects.

So far, the focus has been primarily on the effects of pesticides from agriculture. A research team from the University of Bayreuth is now looking into the contribution of other anthropogenic pollutants. In two studies they have now examined the possible effect of diesel exhaust particles ingested through food on insects. They chose the well-known earth bumblebee (Bombus terrestris) as their model organism.

Fine dust-contaminated nectar in sight

As the researchers explain, the notorious fine dust particles can become a burden not only through the air we breathe. In the area of ​​roads they accumulate in the nectar of plant flowers, which bumblebees and others feed on. To produce test substances, the team first generated the typical exhaust particles in a four-cylinder diesel engine. They then mixed this with sugar water, which was used as a substitute for nectar in feeding experiments on bumblebees. The concentrations were based on exposures such as those that can occur on busy country roads.

As is known from previous studies, an unfavorable change in the vital intestinal flora can represent a form of stress on insects. The current study showed that after supplying the bumblebees with the contaminated food for seven days, the researchers noticed a significantly changed composition of the intestinal microbiome. In particular, colonization with a type of bacteria known to form a biofilm that protects the intestines was limited. Studies have already shown that such changes in insects can, among other things, weaken their resistance to pathogens.

Altered intestinal flora and stress symptoms

In the second sub-study, the Bayreuth scientists then examined the question of the extent to which there are physical consequences of continuous consumption of contaminated food. They found that the test animals had a reduced fat content in the body of the test animals compared to bumblebees fed a normal diet. “This is an indication that the particles have triggered detoxification processes that are associated with increased energy consumption,” explains first author Frederic Hiplein. “We have also observed that bumblebee mortality is increasing. These studies also prove that daily intake of exhaust particles through food puts the insects’ organisms under stress,” says Hiplein.

The scientists report that further evidence of this effect was provided by the investigation of gene activities in the test animals: The analysis of the transcriptome showed that the expression of 324 genes had changed as a result of the contaminated feed. “There is much to suggest that the altered gene expression is a stress reaction that attacks and weakens the insects’ energy resources,” says co-author Heike Feldhaar.

As the team emphasizes, the results also fundamentally showed that there were no significant consequences when feeding the contaminated food only once or with longer interruptions. “The decisive factor for damage to the bumblebees is that the intake of exhaust particles is chronic, i.e. repeated over a longer period of time. However, if plants and soil are contaminated, such chronic exposure to the pollutants is possible,” says co-author Matthias Schott.

The team now wants to continue the research. The head of the “Influence of fine dust on insects” project, Heike Feldhaar, says: “We are planning further studies in the near future to clarify the connections in more detail. “We don’t just want to look at individual insects, but rather entire colonies, and include other anthropogenic stress factors in our research in addition to diesel exhaust fumes,” says the scientist.

Source: University of Bayreuth

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