Larvae of unijuga can normally be separated from the several dozen other Salicaceae-feeding species in North America (others separable include relicta, parta and the four closely related cara, carissima, amatrix and concumbens). Characteristics of unijuga include rumpled and rugose body, dark gray to grayish brown body color, overlayed with heavier thick speckles and banding (cf. stippled banding), dull orangish pinnacula dorsolaterally; A5 patch slightly distinguished, with mild hump; numerous but short lateral filaments; prominent wide black head capsule band (wider than in other species) from ocelli up sides and connecting over lobes, contrasting with paler vertices with orangish hue; frons with prominent dark vertical line (cf. absent or partial or more lightly colored in others); venter with mauve shading (approaches ilia/umbrosa in intensity), black spots on all segments, wide on A3-A6. A frequent underwing in the north (along with relicta, semirelicta, briseis) and the only species recorded to date from interior Alaska. Commonly feeds on Populus, including P. deltoides and P. tremuloides. (NOTE: the images of semirelicta in the 2011 Wagner et al. Owlet Caterpillars volume are in fact unijuga).