Another Gall

The Robin's Pincushion Gall, is caused by the gall wasp, Diplolepis rosae.  It is probably one of the more widely recognised galls found on wild dog roses, Rosa canina and on field roses, Rosa arvensis.

The largest of them form around buds, smaller ones from leaf axils and the smallest of all on individual leaflets, as here.  When an egg is laid in a developing leaflet, it grows out of both the upper and lower surfaces as can clearly be seen in the 'extra'.  On this particular leaf, four of the seven leaflets were sporting growths.

Google says these galls are more common on roses that are stressed, potentially due to factors like poor soil conditions or inadequate sunlight. 
Many of the rose bushes along the coastal path are infested.  I find them there every year but this year they seem to be even more prolific.  Salt spray may be one factor but this spring has been particularly dry, so that may explain the increase.  Plants that have them one year will invariably have them in successive years.