The larvae of the fly Cerodontha iraeos mine the leaves of Yellow Iris (Iris pseudacorus). The mine takes the form of an irregular corridor, essentially parallel to the leaf blade. The mine is white when seen from above. Frass in one, rarely more, big black lump. Larva solitary.
Unless identified by a recognised expert, a photo showing mine held up to the light is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed.
Wherever Yellow Iris is present.
Mines might be encountered whenever Yellow Iris is in leaf.
Pupation occurs within the mine.
Widespread and fairly frequent in Britain – though not well recorded.
Status in Leicestershire and Rutland not known.
Leicestershire & Rutland Map
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Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015