Insect symbioses with heritable bacteria have been pivotal in facilitating the use of nutrient-poor plant sap as a primary food source, with bacterial partners supplying essential nutrients. This dissertation explores symbiont replacement dynamics in adelgids, a small family of sap-feeding insects exhibiting complex life cycles and dietary shifts that likely drive changes in symbiont roles and partnerships. In Chapter 2, I sequenced and analyzed the genomes of 10 new adelgid endosymbiont species, representing each major adelgid lineage. I reconstructed metabolic capabilities and symbiont interdependencies, assessed phylogenomic relationships, and estimated symbiosis ages in each lineage. Results indicate that adelgids have undergone many symbiont replacements, with evolutionarily younger symbionts often taking on primary nutritional roles, reversing the typical roles found in other insect symbioses. This pattern supports the hypothesis that feeding on nutrient-rich tissues relaxes selection on symbiont nutritional functions, promoting gene inactivations that necessitate new symbiont acquisitions when adelgids transition to nutrient-poor diets. In Chapter 3, I sequenced the genome of Adelges cooleyi, producing the first high-quality adelgid genome, which enabled a comprehensive phylogenetic analysis of the Aphidomorpha (aphids, adelgids, and phylloxera). Additionally, I explored the evolutionary significance of A. cooleyi's unusually large mitochondrial genome, proposing hypotheses regarding its expansion. In Chapter 4, I investigated how periodic relaxed selection on symbiont nutritional functions may drive symbiont turnover in adelgids. Using RNA-seq, qPCR, and microscopy, I examined gene expression, symbiont abundance, and bacteriocyte counts in A. cooleyi across high- and low-nutrient diets. Results reveal that symbionts contribute more substantially to nutrition under nutrient-poor conditions, supporting the hypothesis that relaxed selection on nutrient-rich diets erodes symbiont functions and facilitates replacement events across adelgid lineages. Together, these findings contribute a deeper understanding of how environmental factors shape symbiont dynamics, providing insights into the evolution of symbiotic relationships in complex life cycles.