Ennomos erosaria, the September thorn, is a moth of the family Geometridae. The species can be found in the Palearctic realm in western Europe and from central Scandinavia. Its range extends to the northern Mediterranean and east to the Caucasus and Russia.It is widespread in mixed and deciduous forests in Europe. The south eastern occurrence reaches Turkey and the Caucasus. The main habitat is dry deciduous forests and parks. In the Southern Alps, the species rises to an altitude of about 1600 metres. Similar species: other Ennomos species.