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These beetles mostly target Pseudotsuga spp. (Douglas-firs) but may also be found on Abies spp. (firs) (Bright 1976). They are most commonly found in brightly lit sections of the trees and also where the bark layer is relatively thin.

Adult beetles are 2.6 to 3.5 mm long with black stout bodies (Bright 1976). In both males and females, the abdominal sternite is not concaved, but rather ascends vertically to meet the elytra (which extend over the abdomen) (Bright 1976). A median tubercle may be found on the 2nd sternite and the base of the tubercle is joined with the posterior margin of sternite 2 (Bright 1976, LaBonte et. al. 2003).

Females will create galleries in the wood and may lay up to 60 eggs in each gallery. Upon hatching in mid-June to July, the larvae will feed on the wood of host trees, creating feeding galleries that extend from the parental galleries (Bright 1976). Pupation occurs just below the bark. This species of bark beetle will overwinter and only one generation is produced in a single year (Bright 1976).

Adult S. unispinosus will attack tree limbs that are in brightly-lit areas and where bark layer is thin. As a result, these beetles generally target smaller branches or twigs. Larvae will feed on woody tissue in the host trees.