Host-associated microbiomes are essential mediators of insect ecology and adaptation, yet their structure and the environmental factors that shape them remain poorly understood. We employ DNA metabarcoding to characterise the microbiome of the chestnut weevil, Curculio sikkimensis, infesting seeds of several Fagaceae hosts across multiple populations in Japan. We demonstrate that both environmental context and host plant species influence weevil-associated bacterial communities, and discuss dominant bacterial taxa. Additionally, we extend the analysis to other insect species also found in the same hosts, and discuss patterns.