Andricus legitimus f. agamic

The larvae cause asexual galls within the acorns of some Oak species. The degree of suppression of the development of the acorn varies, but it often remains completely enclosed by the cup. The gall may not be evident unless there are exit holes, but may be found by collecting and sectioning stunted acorns that still held on the tree in winter.

The larva occupies a single chamber in the acorn, but multi-chambered galls are common; this is said to be caused by the inquiline wasp, Synergus clandestinus. (ref: Redfern & Shirley 2023). 

[Note: The sexual generation gall is unknown.] 

Unless identified by a recognised expert, a photo is required of the sectioned acorn, showing larvae inside. Note the host species

native oaks

common (British Plant Galls, Redfern & Shirley, 2023)

rarely recorded

See British Plant Galls (Redfern & Shirley, 2023)