Bucculatrix cristatella is a very small dull grey moth that is rarely seen as an adult. The feeding signs of the larvae are quite subtle too. We have very few records of the moth but it is one species that is almost certainly under-recorded.
Feeding signs on Yarrow
Signs can be found now on new growth Yarrow fronds. Look for browned tips to the leaves and then for the pale white-ish ribbed cocoon and the smaller white silk blobs that are the remains of temporary moulting habitations. There are other causes of the brown leaf ends so you really need a cocoon to be sure of the record.
B.cristatella - Cocoon on Yarrow.
To give you some idea of scale, the cocoon is about 3mm long.