Effects of traffic-derived airborne particulate matter on social Hymenoptera

Biodiversity loss is threatening the functionality of ecosystems worldwide. The decline in insect biomass and diversity is of special concern as they provide many vital ecosystem services, including pollination, nutrient cycling, and pest control. The reasons for this decline are multifactorial, with the main drivers being habitat destruction, land use intensification, climate change, invasive species, and pollution. Among pollutants, research focused on the effects of pesticides and fertilisers due to their extensive application in the environment. Another group of anthropogenic pollutants is airborne particulate matter, such as diesel exhaust particles. It could be dangerous as it is ubiquitous and may contain harmful substances. While negative effects on human health have been reported, the impact on insects is still largely unknown. As airborne particulate matter is very small, it may enter an insect’s body via the tracheae or by the ingestion of contaminated food. Social Hymenoptera, such as ants, social wasps, and social bees, are an ecologically important and widespread group of insects. They have a reproductive division of labour, breed cooperatively, and generations overlap. Encounters with an array of pollutants may happen in their typically large foraging areas from where they transport food into their colonies. Therein, pollutants might accumulate in the food storage and affect the different life stages. In my thesis, I investigated the effects of airborne particulate matter, primarily diesel exhaust particles, on the buff-tailed bumblebee Bombus terrestris and the black garden ant Lasius niger. First, I assessed the lethal and sublethal effect of diesel exhaust particles on B. terrestris in the laboratory after oral exposure. I could show that chronic exposure to high doses leads to increased mortality, while single exposure and lower concentrations did not affect the bumblebee’s survival (Article 1). Chronic oral exposure to diesel exhaust particles caused shifts in the composition of the workers’ gut microbiome and gene expression. I found a significantly lower abundance of the common bacterium Snodgrasella, which is associated with protection against gut parasites. Exposed workers showed changes in the gene expression associated with metabolism and stress, also indicating potential health issues (Article 2). In a field experiment, I tracked the homing and foraging behaviour of bumblebees after exposure via air to evaluate the effects of diesel exhaust particles under natural conditions. While a one-time exposure did not affect the homing flight duration and subsequent foraging, it significantly delayed the take-off to start the homing flight. The delay was mainly caused by a struggle to take off vertically out of the exposure box, which may indicate underlying physiological constraints (Article 3). Colony founding is one of the most important and, at the same time, most vulnerable stages in the life cycle of social insects. I regularly exposed bumblebee colonies at the early founding stage to diesel exhaust particles. However, the development of the treated colonies did not differ from the control colonies, indicating no harmful effects (Article 4). To compare single and multiple stressor effects on the ant L. niger, we exposed wild-caught queens at the colony-founding stage to soil containing different combinations and concentrations. Diesel exhaust particles, microplastic particles and fibres, or brake abrasion did not affect any of the investigated colony founding parameters. In contrast, manure application caused prolonged egg development and a smaller number of pupae and workers. This highlights the potential harm of manure application to soil-dwelling insects (Article 5). When trying to generalize the results from my thesis, I need to be aware of some limitations that I had to accept. In my thesis, choosing field-realistic doses was one of the biggest challenges for me as there is a lack of reliable data on environmental concentrations. Additionally, I mostly studied single stressor effects, even though insects encounter various other stressors, such as parasites or limited food availability, in the wild. The slight changes caused in my studies might indicate problems to the organisms if encountering multiple stressors. Insects may be able to compensate for the impacts of one stressor but will eventually be overstrained by multiple stressors. It also must be considered that B. terrestris and L. niger are very abundant species, especially in urban areas. Thus, they could be more tolerant towards anthropogenic pollution or have already adapted to higher levels of air pollution than other species. Nevertheless, the novel approaches and results from my thesis lay an important foundation for future research on the effects of airborne particulate matter on insects. My thesis adds to the understanding of the role these pollutants play in the global insect decline. I am looking forward to future studies that build on this work to investigate these pollutants in multiple stressor setups and look at the effects on other, less common species.