Wingspan 9 - 10mm.
A rare species (proposed Red Data Book) with few recent records. It occurs on marshes and river banks south of a line from the Severn estuary to The Wash.
It is single-brooded, the adults on the wing in June and July. The larvae mine the leaves of Greater Pond-sedge (Carex riparia), and less often, Lesser Pond-sedge (C. acutiformis), the mines most easily being found in March.
One of the smallest of the Monochroa species with four pale spots on the costal cilia. To determine beyond doubt dissection is recommended for poorly marked specimens.
The specific scientific name refers to "arundinetum", for reed bed, the habitat, not the food plant.