Cynips divisa is a Cynipid gall wasp that causes galls to form on Pedunculate and Sessile Oak. These galls are more likely to be seen than the adult wasps.

The Red Pea Gall is caused by the agamic generation on the underside of the Oak leaves.  The point of attachment may show as an indistinct spot on the leaf upperside. It is spherical, up to 6mm diameter, has a smooth surface, and is hard (not succulent).   The gall is thick-walled and hard to crush, with a circular chamber inside.

The gall can be similar to those caused by other species such as Neuroterus anthracinus, Cynips longiventris and Cynips quercusfolii. 

English/Sessile oaks

See British Plant Galls (Redfern and Shirley 2023).