Willow blister galls

Photo: John Mills of the community orchard group

These are blister galls  or red bean galls, very common on crack willow and white willow.  They are caused by a sawfly called Pontania proxima, a sort of primitive wasp, which lays eggs in the developing leaves.  The galls are formed by the tree so that the sawfly grubs feed on the extra material without affecting the function of the leaves much.  The galls are green at first and turn red in late summer when they become more noticeable.

The adult sawfly, measuring a mere 5mm from nose to tail looks like this.

photo: chovzvirat.cz