The treble-bar or St. John's wort inchworm (Aplocera plagiata) is a moth of the family Geometridae.

the species was first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae.

This species varies considerably in size (wingspan 37–43 mm) and colouration but is generally grey with three characteristic dark fascia across each forewing, giving it its common name.

The egg is whitish, without gloss, micropylar rosette 11- to 12-leaved, the sides with regular polygonal reticulation, each cell again more irregularly subdivided.

The larva is also very variable, being green to reddish brown with alternating darker and lighter stripes.