Many heritable symbionts invade and persist in host populations by conferring benefits, including host protection. In the legume crop pest Aphis craccivora (Koch), infection with the heritable symbiont Hamiltonella defensa confers protection against parasitoid wasps, an important natural enemy often employed in biological control. Despite this benefit, not all A. craccivora carry H. defensa. A major aim of this thesis was to estimate the vertical transmission efficiency of H. defensa in A. craccivora and identify costs that may limit its spread. We found that vertical transmission rates were high but not 100%, and clear costs to infection in population cage studies. We also attempted to better characterize H. defensa infection in A. craccivora, by examining the genetic diversity of bacterial strains and associated viruses within this system. Overall, we found little symbiont diversity, which is consistent with the aphid being an invasive species with a mostly asexual persistence.