The larva of the gall midge Putoniella pruni causes a gall on Blackthorn and other Prunus species. The galls ca be found in 2 types: as a swelling on the midrib underneath, often reddish, with a slit-like opening above; or a hard, upward-rolled, pod-like leaf margin.  The larvae are orange.  

Gall  Adult

Look for it in places with a lot of Blackthorn.

Adult in April and May but the gall is more likely to be seen than the adult midge.

Uncommon to rare - only two sites in Britain previously listed, one in Surrey and one in the west.

First record for VC55 (Leicestershire and Rutland).