Went for a walk through the fields and found literally hundreds of these dark metallic blue leaf beetles (hamushi) on the plants growing beside the stream. It’s obviously a common species.

streamchrysomelidaechrysomelidaechrysomelidae
chrysomelidaechrysomelidae

Coleoptera: Chrysomelidae (leaf beetles): Fleutiauxia armata (kuwa-hamushi)
[identification to be confirmed]