Ecological control studies of the Asian longhorned beetle (Anoplophora glabripennis) distinguish susceptible, resistant, and dead-end trap trees as functionally different plant categories. We used shotgun metagenomics to examine bacterial and archaeal profiles detected in adult gut samples after 72 h exposure to three dietary plants or prolonged water-only starvation. The study included 24 metagenomes, with three biological replicates per DietGroup × SexGroup combination. No time-zero gut samples were available, so the observed patterns remain superimposed on the beetles’ field history. The retained catalogue contained 152,895 bacterial genes and 9 archaeal genes. The original observed-richness difference was strongly correlated with host-depleted read depth and was not supported after common-depth rarefaction. Genus-level Bray–Curtis analysis detected a DietGroup × SexGroup interaction that persisted after depth adjustment and exclusion of low-yield samples. This interaction was exploratory because of the small within-cell sample size. Raw Bray–Curtis analysis of KEGG Orthology profiles showed a DietGroup association, but this association was not robust to direct-depth adjustment or Aitchison analysis. CAZy profiles were descriptive and showed no significant DietGroup effect. These results indicate short-term, depth-sensitive associations between dietary treatment and gut-sample bacterial and archaeal profiles. They do not establish resident status, microbial activity, or a physiological mechanism.