Small oval galls on male oak catkins, early in year.  The galls are covered in white straggly hairs and often have a dark tip.  The base is surrounded by anthers.

The agamic gall is the familiar artichoke gall.

There are other small oval galls found on catkins - e.g. Andricus quadrilineatus and Neuroterus politus

Close examination of a specimen is needed; photos should show hairs.

Oak catkins

Spring

see Bladmineerders website