Curious damage?

My fourth Prominent came last night to the Robinson trap at Thrupp, Oxon - this gloriously-coloured Iron Prominent. The other newcomer for me this year was a Least Black Arches, little scrap of a thing. Also in the eggboxes were four Clouded Drab, three Pebble Prominent, two Powdered Quaker, one each of Early Grey, Streamer, Common Quaker, Pale Prominent, Hebrew Character and Flame Shoulder, plus three Brindled Beauties, one of them with these curiously symmetrical bare-looking patches on the forewing. Any ideas about the cause of this?  Martin Wainwright