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This small geometrid is pale tan in ground colour with heavy brown markings, consisting of antemedian (AM) and postmedian (PM) lines, dark marginal shading, discal spots and often black marginal blotches. Probole are best recognized by the extended outward toothlike projection of the forewing PM line. Virtually identical to (and previously considered to be conspecific with) P. amicaria. The distinguishing trait in wing markings is the angle of the PM line above the "tooth": it is perpendicular to the wing margin where it meets the edge in alienaria, while in amicaria the line is directed more toward the wing apex, and is essentially perpendicualr to the anal (not the costal) margin. Some have treated P. aienaria and amicaria as forms of the same species (eg. McGuffin 1987). Ferguson (in Hodges 1981) treats them as separate species, and according to Handfield (1999), alienaria is a generalist feeder on deciduous shrubs while amicaria feeds only on species of dogwood (Cornus). There also appear to be differences in the male antennal structure among some populations, at least in eastern North America (Handfield 1999). This interesting situation certainly warrants further research in Alberta.

The caterpillar is a twig mimic, and extremely variable in colouration, ranging from green to reddish or tan and brown. The pupa overwinters (Wagner et al. 2001). Adults come to light, but are also diurnal. Not uncommon in forest habitats with a diverse shrubby understory.