Marsh Mallow Moth Hydraecia osseola
Wingspan 40-50 mm.
Restricted in Britain to a few scattered localities in Kent and East Sussex, this moth inhabits marshes, fens, river banks and ditches.
It flies in September, and is attracted to light, but doesn't tend to wander far from its foodplant.
The larvae feed on the stems and roots of its namesake plant, marsh-mallow (Althaea officinalis).