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Head white, antennae banded with brown on each segment, labial palpi porrect, dorsally white, ventrally brown. Thorax white, tegulae and metathorax brown. Legs brown. Forewings strongly hooked, a characteristic shared by many species in the genus Ypsolopha. Dorsal surface of forewings brown, white on the basal one third of the wing, extending lengthwise, but not reaching the external wing margin. There is a well-defined, whitish "spine" pointing upwards and away from the wing base. Hindwings uniformly dark grey. Ventral wing surfaces are grey. Ypsolopha dentella can be confused with two species in Alberta. The first one is Y. canariella, which has yellow scales instead of white, the basal white band on forewings reaches the external wing margin and it lacks a "spine". The second one is Plutella xylostella, which has a wavy basal band and narrower hingwings with longer fringes. In fact, many early workers such as Meyrick and Ford confused these two species with one another (Alford 1971). However, they can be separated from one another based on the size (P. xylostella is much smaller than Y. dentella), lack of hooked wings on P. xylostella and absence of the characteristic projections that form the "diamond back" of P. xylostella when it rests with its wings closed (Alford 1971).

Young larvae are green and inconspicuous. Older larvae change colour and become pinkish-red on the dorsal surface (I. Kimber, web reference). Feeding occurs in a loose web. Pupation takes place in elongated silken cocoons among debris on the ground.