The Ashen Wave (Idaea deversaria) is very similar in size and appearance to the Plain Wave (I. straminata). The Ashen Wave usually has small black dots in the wing fringes, whereas Plain Wave lacks these. Other subtle differences are described in Hausmann (2004), The Geometrid Moths of Europe, Volume 2. Perhaps best separated by genitalia dissection.
There are no records in the system yet in Bulgaria.