Adult flatheaded woodborer beetles are flattened, compact beetles, a shiny grey or black colour. At low temperatures the thorax is black, whereas it is greyish when it is warm, with six black patches. Capnodis tenebrionis is about 15-25 mm long. The eggs are white, oval-shaped, and 1-1.5 mm in length. The first instar larvae are 3-4 mm long; older larvae grow up to 70 mm. Larvae are white with a flattened, clearly segmented body that is strongly tapered behind the pronotum. The pronotum is very broad and rounded with a brownish dorsal plate. The head has strong black mandibles.
The adults appear at any time from spring to autumn. Those emerging from pupae in spring only live for a few months, whereas those emerging in summer overwinter. They feed on the cortex of twigs and young branches. Oviposition only occurs above 25ºC, and only in dry soil. The females usually lay their eggs close to the base of the tree stem. The newly emerged larvae are able to locate the roots at distances of 60 cm from the hatching site. They penetrate the roots and feed in the cortex. Larval development lasts for 6-18 months in the field. The larvae eat their way through the roots and stem forming long galleries, which are initially small but become larger as the larvae grow, and leave behind compressed frass. The favoured pupation site is at the base of the stem.