First-ever Study Reveals Diesel Exhaust Emissions Adversely Impact Bumblebees and Insects, Say Animal Ecologists

The global decline of insect populations poses a grave threat to ecosystems worldwide. While the impact of pesticides on these insects has been extensively researched, our understanding of the effects of other human-made pollutants has been limited. Researchers in animal ecology at the University of Bayreuth have undertaken a groundbreaking study, investigating the consequences of diesel exhaust particles on bumblebees for the first time.

Their findings, published in Ecology and Evolution and the Journal of Hazardous Materials, reveal that these fine dust particles can inflict significant harm on bumblebee organisms when ingested through their food sources. Diesel exhaust particles, known to cause respiratory ailments in humans, are frequently found in the nectar of flowering plants in the wild, which serve as food sources for bumblebees and other insects. To simulate this scenario, scientists from the University of Bayreuth’s Animal Ecology research group recreated these conditions in a controlled laboratory setting.

The researchers chose the buff-tailed bumblebee (Bombus terrestris), a widely distributed species, as their model organism. They collaborated with the Chair of Technical Thermodynamics and Transport Processes at the University of Bayreuth to generate exhaust particles from a four-cylinder diesel engine commonly found in passenger vehicles. These particles were then introduced into the sugar water provided as daily sustenance to the bumblebees in the laboratory. The amount of particles mirrored what had previously been detected in soil near busy country roads, and analysis revealed that these particles consisted partly of elemental carbon, heavy metals, and other organic compounds such as polycyclic aromatic hydrocarbons (PAHs), which are known to be potentially toxic to humans and linked to cancer development.

The researchers observed notable alterations in the bumblebees’ gut microbiome after seven days of continuous exposure to exhaust particles through their meals. Some bacterial species that typically constitute the primary components of the bumblebees’ intestinal flora increased in abundance, while others decreased. Notably, the bacterium Snodgrassella, crucial for forming a protective biofilm in the intestine, was present in significantly lower numbers. These changes in the gut microbiome are known to weaken the insects’ immunity and their resistance to pathogens, ultimately raising their mortality rates.

In another study, the researchers examined how these particles affect the insects’ immune systems. Over a ten-day period, bumblebees ingested exhaust particles mixed into their sugar water at varying concentrations. At the conclusion of the experiment, their fat content had significantly decreased compared to bumblebees fed a standard diet. This reduction in fat content suggests that the particles initiate detoxification processes in the bumblebees’ bodies, which require increased energy expenditure. These studies collectively suggest that the daily ingestion of exhaust particles through food places significant stress on bumblebee organisms, resulting in increased mortality.

Further evidence of an energy-intensive stress response was found in significant changes in gene expression, where 324 genes exhibited altered expression levels. While the production of RNA molecules increased for 165 genes, it decreased for 159 genes. These changes indicate that the continuous ingestion of exhaust particles over an extended period prompts degradation processes within the bumblebees’ bodies while impeding biosynthesis processes. The researchers plan to conduct additional studies at the University of Bayreuth to explore these relationships in greater detail, examining not only individual insects but also entire colonies and considering other anthropogenic stress factors alongside diesel exhaust.

The authors of these studies stress that significant adverse effects on bumblebees are only observed when the particles are ingested through food. Experiments involving inhalation of the particles by bumblebees did not yield measurable health impacts. Chronic exposure to these pollutants through ingestion, as might occur in plants and soils near polluted areas, is the key factor in damaging bumblebee health.

Under natural conditions, even non-lethal effects of exhaust particles can pose problems for bumblebees, as they are often subjected to multiple stressors simultaneously, such as other environmental pollutants like pesticides and high daytime temperatures in the summer.

The University of Bayreuth emphasizes interdisciplinary collaboration in its efforts to address the decline of insects. This decline, observed worldwide over the past decade, is of great concern because insects play vital roles in various ecosystem functions, including pollination, decomposition of organic matter, and pest control, making them indispensable within food webs. The university is committed to leveraging its expertise to better understand the complex relationships between causes and effects, delving into molecular and cellular biology to address this critical issue.

Source: Bayreuth University

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