Diesel exhaust gases found to harm insects: Animal ecologists study the effects on bumblebees for the first time

The decline of insects threatens many ecosystems worldwide. While the effects of pesticides are well researched, there has been a lack of knowledge about the effects of other anthropogenic pollutants. Animal ecologists at the University of Bayreuth have now studied the effects of diesel exhaust particles on bumblebees for the first time.

In two new studies published in Ecology and Evolution and the Journal of Hazardous Materials, they show that these fine dust particles can significantly damage the organism of bumblebees if they are permanently ingested through food.

Exhaust particles from diesel-powered vehicles can cause respiratory or lung diseases in humans. In the wild, they often end up in the nectar of plant flowers, which bumblebees and other insects feed on. Scientists of the Animal Ecology research group at the University of Bayreuth have recreated this constellation in the laboratory.

As a model organism, they chose bumblebees of the widespread species Bombus terrestris (buff-tailed bumblebee). In cooperation with the Chair of Technical Thermodynamics and Transport Processes at the University of Bayreuth, they generated exhaust particles produced by the combustion processes in a four-cylinder diesel engine of the type often found in passenger cars. These particles were added to the sugar water fed daily to the bumblebees in the laboratory.

The amount corresponded to the amount of diesel exhaust particles that had already been detected in soils near busy country roads. Analysis of the particles in the Bayreuth laboratories has now shown that they consist partly of elemental carbon, but also contain heavy metals and other organic substances such as polycyclic aromatic hydrocarbons (PAHs). PAHs are suspected of being toxic to humans and promoting the development of cancer.

Changes in the gut microbiome: Indications for a weakening of the immune system

After the bumblebees had consumed exhaust particles at every meal for seven days, the scientists observed a significant change in the composition of the intestinal microbiome: Of the bacterial species that normally form the main components of the bumblebees' intestinal flora, some were much more abundant, while others were less abundant.

In particular, the bacterium Snodgrassella, which is important for the formation of a biofilm protecting the intestine, was present only in very low numbers. Such changes in the gut microbiome are known in research to weaken immunity and resistance to pathogens in insects, thereby increasing their mortality.

Decreasing fat content of the body and increased mortality

In another study, the focus was on how the particles affect the insects' immune system. For ten days, the bumblebees ingested exhaust particles mixed into the sugar water in varying concentrations. After that, their fat content had decreased significantly compared to bumblebees fed with normal food.

"The reduced fat content is an indication that the particles trigger detoxification processes in the bumblebees' bodies, which are associated with increased energy consumption. These studies also suggest the conclusion: the daily intake of exhaust particles through food puts the bumblebees' organism under stress. We observed that their mortality increases significantly," says first author Frederic Hüftlein M.Sc., Ph.D. student at the Animal Ecology research group.

Changes in gene expression: further evidence for an energy-intensive stress response

Significant changes in gene expression, the production of vital proteins controlled by genes, were also revealed. Analysis of the transcriptome—this is the totality of RNA molecules produced at a given time—revealed that the expression of 324 genes had changed. The production of RNA molecules was intensified for 165 genes, but decreased for 159 genes.

The observed changes can be interpreted as indications that the degradation processes in the bumblebees' organism are promoted by the degradation particles ingested with food over a longer period of time, while processes of biosynthesis are slowed down.

"There is much to suggest that the altered gene expression is a stress response that attacks and weakens the insects' energy resources. At the University of Bayreuth, we are planning further studies in the near future to elucidate these relationships even more precisely. We want to look not only at individual insects but also at entire colonies and include other anthropogenic stress factors in addition to diesel exhaust," says Prof. Dr. Heike Feldhaar, head of the subproject "Influence of particulate matter on insects" in the Bavarian project network BayÖkotox.

Harmful effects only in the case of chronic uptake of the particles via food

The authors of the new studies emphasize that significant adverse effects of diesel exhaust on bumblebees can only be determined if the particles were ingested via food. Experiments in which the particles were inhaled by the bumblebees showed no evidence of adverse health effects.

"When the bumblebees were fed the particles once or only several times over the course of 48 hours, there were no measurable significant reactions. There was also little change in the fat content of the bumblebees' bodies. The decisive factor for damage to the bumblebees is that the uptake of the exhaust particles is chronic, i.e. repeated within a longer period of time. If plants and soils are polluted, chronic exposure to the pollutants is conceivable," reports Dr. Matthias Schott from the Animal Ecology research group.

The authors also point out that under natural conditions even a non-lethal effect of the exhaust particles can be problematic for bumblebees. This is because bumblebees in the environment are usually exposed to several stress factors at the same time, for example other environmental pollutants such as pesticides or also high daytime temperatures in summer.

Interdisciplinary collaboration in Bayreuth

"For about ten years, a rapid decline of insects has been observed in numerous regions of the world. This development is worrying because insects perform or contribute to many important ecosystem functions such as pollination, decomposition of organic material and pest control. They also form an indispensable link in food webs."

"It is now clear that environmental pollution is one of the main reasons for this decline. At the University of Bayreuth, we want to use our expertise to help clarify the relationships between causes and effects, including at the molecular and cell biological level," says Prof. Dr. Christian Laforsch, Chair of Animal Ecology, referring to the close interdisciplinary collaboration in the BayÖkotox project.