Idaea inquinata (Lepidoptera, Geometridae) is a potential pest of stored food, mainly dry herbs. In this study, the role of the diet in shaping the composition of I. inquinata gut bacterial community and its impact on the insect performance (i.e., proportion of adult emergence and duration of postembryonic development) were investigated. Larvae were reared on three diets (Matricaria chamomilla, Angelica archangelica and artificial diet) with different nutritional compositions. A DNA metabarcoding approach targeting V1-V2 and V4 regions of the bacterial 16S rRNA was adopted to characterize the bacterial community associated with adults and larvae fed on different diets, both in terms of composition and diversity. The core microbiota of this species was found to include some bacterial genera commonly associated with Lepidoptera (i.e., Pseudomonas, Acinetobacter, Sphingomonas, Sphingobacterium). The number of the observed amplicon sequence variants was not affected by the developmental stage and neither by the diet, while the Shannon index H’ and the Pielou’s evenness were found to be significantly affected by the diet type. The lowest taxa diversity (H’ = 1 ± 0.3) and the most unbalanced bacterial community (J’ = 0.27 ± 0.1) were found for individuals reared on M. chamomilla. Individuals fed with this fiber-rich diet had the longest developmental time. No difference in the microbiota structure was also observed between adults and larvae, while a significant difference was observed among individuals reared on different diets. The obtained results suggest that most likely the core microbiota of adults is not acquired from the environment but gets past the metamorphosis.