This is the Victorian collector’s classic all-time favourite, the ‘Blue Underwing’. In the British Isles it is now only an immigrant, though was formerly resident in certain parts of Kent and Norfolk during the middle part of the 20th century.
Nowadays, only a handful per year are recorded, mainly in the south and south-east of England, September being the most likely month.
The large greyish caterpillar feeds mainly on aspen (Populus tremulosa)
Forewings grey. Pale central blotch, zigzagged outer central and undulating inner central cross-lines. Hindwing black with broad curved violet-blue central cross-band, scalloped outer margin and broad white fringe. Well camouflaged at rest on trees trunks, hind wing (underwing) has a blue stripe on a black background.
If found report to MWHG/County Lepidoterist as soon as possible and provide photographic proof if possible.