Bees as effect-detectors of air pollution: an ecotoxicological overview

Particulate Matter (PM) and air chemical pollutants represent increasing environmental and health pressures, particularly in urban areas where climate change boosts the effects of contaminants. In addition to acting as a stressor itself, PM can also transport other hazardous agents, such as heavy metals, pesticides, and organic compounds, increasing their diffusion and adverse effects on the health of living organisms, including humans. Over the years, bees have become one of the most widely used model organism and biological sentinels due to their ability to bioaccumulate contaminants and, thus, to reflect the state of the ecosystem. Despite this, little is known about the ecotoxicological effects of chemical pollutants on bee species. This review synthesizes current knowledge on the use of bees as an eco-bioindicator of air quality and the resulting ecotoxicological effects of pollutants on these organisms. It also proposes an integrated conceptual framework linking biomonitoring, ecotoxicological assessment, PM, and urban environmental pressure. Specifically, we performed an extensive literature review by investigating multiple databases following rigorous criteria as defined by the PECO framework. As a result of the review process, we selected 38 articles adopting bees as indicators of air contamination presence distribution and 15 investigating bees' response to air pollutants. The selected articles analysed different bee species that showed distinct levels of sensitivity to PM although the western honeybee ( Apis mellifera ) was the most widely studied. Most of the studies focused on the bioaccumulation of PM₂.₅, pesticides, and heavy metals. Studies on bee response have observed pesticides as the primary cause of mortality and social behaviour effects. The present review shows an increasing rate of publications in the past few years regarding the use of bees as bioindicators. Despite that, it is still necessary to develop standards and protocols that take into account the different sensitivity among pollinators, the actual concentrations of pollutants present in nature, and their effects (single and combined) on biota. Combining the results with chemical pollution and climate stress would also be necessary, considering the increasing effects of global warming on the environmental air quality.