Solitary Bees Acquire and Deposit Bacteria via Flowers: Testing the Environmental Transmission Hypothesis Using <i>Osmia lignaria</i>, <i>Phacelia tanacetifolia</i>, and <i>Apilactobacillus micheneri</i>

Microbial environmental transmission among individuals plays an important role in shaping the microbiomes of many species. Despite the importance of the microbiome for host fitness, empirical investigations on environmental transmission are scarce, particularly in systems where interactions across multiple trophic levels influence symbiotic dynamics. Here, we explore microbial transmission within insect microbiomes, focusing on solitary bees. Specifically, we investigate the environmental transmission hypothesis, which posits that solitary bees acquire and deposit their associated microbiota from and to their surroundings, especially flowers. Using experimental setups, we examine the transmission dynamics of <i>Apilactobacillus micheneri</i>, a fructophilic and acidophilic bacterium, between the solitary bee <i>Osmia lignaria</i> (Megachilidae) and the plant <i>Phacelia tanacetifolia</i> (Boraginaceae). Our results demonstrate that bees not only acquire bacteria from flowers but also deposit these microbes onto uninoculated flowers for other bees to acquire them, supporting a bidirectional microbial exchange. We therefore find empirical support for the environmental transmission hypothesis, and we discuss the multitrophic dependencies that facilitate microbial transmission between bees and flowers.