Knopper gall wasp
Scientific name: Andricus quercuscalicis
The knopper gall wasp produces knobbly red, turning to brown, growths, or 'galls', on the acorns of Pedunculate Oak. Inside the gall, the larvae of the wasp feed on the host tissues, but cause little damage.
StatsLength of gall: up to 2.8cm