Leaves become puckered and blistered. White fluff can be seen on the plant.
The larval/grub stage an insect feeds underneath the curled up leaf. You can sometimes see them if you unfurl the curl. The fully grown larvae excrete a white fluffy substance which can also often be seen.
Most infestations are not too severe and only look unsightly. Bad infestations may weaken the plant.
Organic – Cut out infected leaves as they appear. Box are very hungry plants and it would be well worth feeding them with a mulch of well rotted farmyard manure to keep the well and growing vigorously. So in the spring and again in the autumn put a good thick layer of manure around the base of plants.
Chemical – Spray with a suitable contact insecticide and cut out infected leaves.