Drought-tolerant cactus Opuntia stricta sustains livestock in Brazil’s semi-arid Northeast but suffers yield losses from the armored scale insect Diaspis echinocacti. Symbiotic bacteria are thought to underpin scale fitness; however, their response to pest pressure remains unexplored. We characterized the bacterial communities of D. echinocacti collected from cladodes displaying low, intermediate, and high infestation (n = 3 replicates per level) using 16S-rRNA amplicon sequencing, processed with nf-core/ampliseq. Shannon diversity declined from low to high density, and Bray–Curtis ordination suggested compositional shifts, although group differences were not significant (Kruskal–Wallis and PERMANOVA, p > 0.05). The obligate endosymbiont “Candidatus Uzinura” dominated all samples (>85% relative abundance) irrespective of density, indicating a resilient core microbiome. PICRUSt2 predicted a contraction of metabolic breadth at higher infestations, with convergence on energy- and amino acid biosynthesis pathways. Taken together, increasing pest density was associated with modest loss of diversity and functional streamlining, rather than wholesale turnover. These baseline data can guide future work on microbiome-based strategies to complement existing scale-insect control in dryland cactus systems.