Dasineura odoratae is a midge causing gall on Viola odorata, the Sweet Violet.  The margins of the leaf are rolled upwards, and are hairy.  White larvae, later orange-yellow, inside.

[Note: Redfern & Shirley 2023 state that the host associations for these species on Viola are tentative]

Similar galls are formed on other species of Viola

Photograph of the gall and host plant.  You must ensure the host species has been identified correctly. See our Wildflower species galleries for guidance on identifying Violets. https://www.naturespot.org/family/violaceae

See British Plant Galls, Redfern & Shirley, 2023