The larva of the gall midge Semudobia betulae galls the catkins of Birch.  The gall is small, <2 mm long, and in each individual fruit.  The wings of the fruit are distorted and reduced in size.  The gall has a distinct circular window pit; this is a pre-formed exit hole for the insect, which pupates inside the gall.  Eventually, the catkin becomes brown where the fruits are galled; the remainder of the catkin stays green.

2 other Semudobia species gall birch catkins, eventually causing browning of the catkin; see Redfern and Shirley (2011) for details of the galls 

Gall Adult

Tease the catkin apart to see the galls - it helps to do this under a microscope

The individual fruits inside the catkin must be examined and photographed in close up.

Rare but probably under-recorded.

Unknown