Wingspan 13-18 mm.
This momphid is superficially similar to the more widespread M. subbistrigella, differing from that species in its larger size and by the presence of distinct white costal streaks.
The moth was first identified in Britain in 1950, and whilst most records are from southern England it seems to be spreading north.
The larvae feed on rosebay willowherb (Chamerion angustifolium). The first generation (May-June) can be identified by a gall (swelling) within the stem. The second generation in July-August feeds either in a seed pod with a small hole, or in a stem gall high up on the plant.
The species flies in two generations, July to August and September to May, and can be seen flying around its foodplant in the sunshine.