DESCRIPTION: A small variegated Psyllid Bug is the causal agent here with the affected leaves becoming thicker, curling under and eventually bearing a fine red reticulation on a green background.  The exact species that causes the galling is a tricky thing to determine with 3 likely bugs very similar in appearance.  The swelling and distortion of the leaves arises when the larvae leave their eggs and release certain gland-based toxins.  DISTRIBUTION: Common throughout the UK.  HABITAT: On the leaves of Ash trees (Fraxinus excelsior).