The use of genetically modified symbionts is a new approach to control the spread of insect-transmitted pathogens by reducing vector competence. A symbiont-control strategy is being developed to reduce the spread of Xylella fastidiosa by Homalodisca coagulata. In this study, the fitness of the transformed symbiont in comparison to its wild type, the pathogen and other symbiotic bacteria was assessed, as it relates to its establishment and persistence inside the foregut of H. coagulata. Multiplexed TaqMan-based quantitative real-time PCR was used to detect and quantify bacterial cells remaining at the end of several post-exposure experimental treatments.