Basic colour: Metallic blue.
Pattern colour: None
Number of spots: None
Pronotoum: As basic colour
Leg colour: As basic colour
Confusion species: Oulema erichsoni
, Confusion species: Oulema obscura
The taxonomic status of O. septentrionis in Ireland is currently under review as there is evidence from dissections of the aedeagal flagellum that specimens from Ireland and Normandy are a melanic form of O. melanopus. The final outcome of this work is awaiting publication.
Habitat: Various, especially near water.
Host plant: Oats (Avena) and Bulrushes (Typha)
Food: Host plants.
Other notes: Similar to O. obscura but separated by the more elongate elytra (1.30-1,45 times as long as wide, compared with 1.25 in O. obscura). Separation from O. erichsoni requires dissection; the lamella of the aedeagus is relatively elongate and somewhat pointed (though generally not sharply so) in O. erichsoni, but broad and blunt in O. septentrionis.