ash midrib gall midge (Dasineura fraxini)

The larvae of this midge cause a gall on the midrib of ash leaflets (ie. not the rachis), consisting of an elongate swelling visible on both the upper and lower sides of the leaflet. The gall blackens with age without causing much distortion of the leaflet.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
Read the preface. Order the book.