<p><i>Haematobia irritans</i> (L.), the Horn fly, is an obligate bloodsucking ectoparasite of cattle. This is an insect that lives on cattle dung during its early stages (egg, larva, and pupa), whereas in its adult stage, it develops an exclusively hematophagous diet. This lifestyle shift involves a significant adjustment in metabolic capacities and could exert considerable pressure on shaping the community of microorganisms associated with each developmental stage. In this work, we hypothesize that anaerobic microorganisms dominate the larval microbial communities, while aerobic microorganisms dominate the adult stage. To test this hypothesis, we performed an amplicon sequencing of the 16S rRNA gene analysis to describe the bacterial community across the Horn fly's life stages. We found that early stages, such as the egg and larva, were dominated by anaerobic Clostridiales and then transitioned to one overwhelmingly dominated by Gammaproteobacteria in adulthood. A notable decline in alpha biodiversity was observed during the pupal-to-adult transition, accompanied by considerable species turnover, indicating the existence of ecological filtering processes. We also estimated genera as part of the microbial core, as well as differentially abundant bacteria that may be relevant for development in both the larval and adult stages. These findings provide a clear picture of the microbial ecological succession processes associated with the bacterial symbionts of this important insect pest.</p>