Cyfluthrin, atrazine, and prothioconazole alter gut bacterial diversity in <i>Bombus terrestris</i> (Hymenoptera: Apidae)

Across agricultural systems, crop production relies heavily on insect-mediated pollination. Both honey bees and bumble bees contribute substantially to crop productivity, with honey bee colonies providing large foraging workforces. On the other hand, bumble bees' specialized pollination behaviors offset the limitations of honey bees, making them especially well-suited for greenhouse production. However, pesticides may pose significant risks to bumble bee health. Here, we used 16S rRNA gene sequencing to examine the effects of treatment with cyfluthrin, atrazine, and prothioconazole on the gut bacterial community of the European bumble bee (Bombus terrestris Linnaeus, 1758). Only atrazine treatment elicited a concentration-dependent response in intake and mortality. In bumble bees that survived treatment with cyfluthrin and atrazine, beta diversity of the gut bacterial community was significantly altered. In contrast, prothioconazole significantly altered both alpha and beta diversity, suggesting a stronger impact on gut microbial structure. Cyfluthrin significantly increased the relative abundance of Proteobacteria while decreasing Firmicutes, atrazine significantly reduced Proteobacteria while increasing Firmicutes and Bacteroidota, and prothioconazole significantly reduced only Bacteroidota. At the genus level, cyfluthrin significantly decreased Lactobacillus and increased Pseudomonas and Brevundimonas, atrazine significantly increased Apibacter and Lactobacillus, and prothioconazole significantly decreased Apibacter and Bifidobacterium. Snodgrassella, Gilliamella, Apibacter, and Lactobacillus remained the dominant genera across all treatments. The results of this study clarify pesticide-specific effects on the bumble bee gut microbiome, providing exploratory evidence for potential modes of action and informing future risk-related studies.