The young leaves of bay trees (Laurus noblis) become distorted and curled up, often turning yellow.
Cause
Adult bay suckers curl the leaf margins before laying their eggs. The larvae feeds underneath the curled up leaf and often can be see if you unfurl the curl. The fully grown larvae excrete a white fluffy substance which is more obvious.
Harm
Most infestations are not too severe and only look unsightly. Bad infestations may weaken the plant.
Treatment
Organic - cut out infected leaves as they appear.
Chemical - Spray with a suitable contact insecticide and cut out infected leaves. Be careful not to use newly sprayed leaves in cooking.