Gall height 2 mm
This is a mite which produces tiny red pustule galls on the upper surface of Common Alder (Alnus glutinosa). They are are green to begin with, becoming red or purple later on. They may cover the whole upper surface of the leaf, and in some cases this may prohibit growth of the leaf.
The mites leave the gall via a small hole beneath the leaf which it had originally entered by in autumn. It sees the winter through in empty cones and bark crevices.
Seen summer to autumn, it is common and widespread. Native to Britain.