The agamic form is a small gall on the underside of oak leaves, often with a squared shape and a depressed central area.  The surface of the gall may have a mottled or bumpy surface, and is soft and pale brown at fIrst, later becoming harder and reddish-brown. Inside are two chambers, with the wasp larva in the basal chamber.   

Provide photographs of the gall on the host plant and  a photo of the gall sectioned to show twin chambers. Note the host species.

English/sessile oaks.

See British Plant Galls (Redfern & Shirley, 2023).