Technical Abstract: Plant-feeding insects have been recently found to use microbes to manipulate host plants. Gall midges are one of the largest groups of insects that manipulate host plants extensively. In this study, we systematically analyzed for the first time bacteria associated with the Hessian fly (HF, Mayetiola destructor), an important pest of wheat and a model system for studying gall midge – plant interactions. Diverse bacteria were found in different developmental HF stages. Fluorescent in situ hybridization detected a bacteriocyte-like structure in developing eggs. Bacterial rDNA was also detected in eggs by PCR using primers that targeted to different bacterial groups. These results suggested that HF hosted different types of bacteria that were transmitted to the next generation maternally. Deprivation of bacteria from the insect through antibiotics resulted in high mortality of HF larvae, indicating that symbiotic bacteria were essential for the insect to survive on wheat seedlings. A preliminary survey identified various types of bacteria associated with different HF stages, including bacterial genera Enterobacter, Pantoea, Stenotrophomonas, Pseudomonas, Bacillus, Ochrobactrum, Acinetobacter, Alcaligenes, Nitrosomonas, Arcanobacterium, Microbacterium, Paenibacillus, and Klebsiella. Similar bacteria were also found in HF-infested wheat, suggesting that HF larvae transmit bacteria into plant tissue, and that these transmitted bacteria may play a role in the wheat-HF interaction.