Assessing the impact of sublethal concentrations of chemical pollutants on the gut microbiome of pollinators: insights from Osmia bicornis

Pollinators provide a key ecosystem service that underpins sustainable agriculture. For this reason, the recent and widely documented declines in pollinator populations have raised global concern. One of the leading causes of this decline has been identified as the exposure of pollinators to chemical pollutants, which can indirectly affect insects’ health by disrupting their gut microbiome. Most studies on the effects of chemical pollutants on the gut microbiome of pollinators have focused on a few species (Apis mellifera, Bombus spp.), mainly examining insecticides and herbicides at typical field application doses. However, the impact of sublethal concentrations of various chemicals on the gut microbiome of other pollinators, including wild bees, remains largely unexplored, as does the structure of their gut microbiome. Therefore, the aim of this study is to assess the impact of various categories of chemical pollutants on the gut microbiome of one of the most common wild pollinators, Osmia bicornis. In laboratory trials, adult O. bicornis specimens were exposed to sublethal doses of four chemicals - boscalid, copper chloride, glyphosate, and ivermectin - administered both singularly and in combination, to explore potential synergistic effects. These compounds were selected because they have been detected as residues in bee-related matrices (e.g., pollen, nectar), making them relevant for studying their impact on pollinator health. The gut microbiome was studied using quantitative PCR to assess bacterial abundance and high-throughput Illumina sequencing of the 16S rRNA gene to examine the taxonomic composition. The results showed significant reductions in gut bacterial abundance following exposure to copper chloride and boscalid. Moreover, the microbiome composition and diversity were altered by all chemicals under study. Information on the effects of chemical pollutants on pollinator survival will be integrated with the knowledge gained from studying the impact of these substances on the gut microbiome of Osmia bicornis, providing new insights into the chronic effects of sublethal doses of chemicals on pollinator populations.