Acorn Knopper Gall

A fairly common sight on oak trees in Britain are odd growths or deformations on the acorns and their cups. These are a type of gall which is caused by a small wasp laying its eggs into the young catkins in spring. The action of inserting the eggs, and the effect of the presence of the developing grubs or larvae that hatch from them, stimulates abnormal growth in the plant. These strange growths are called galls. The shape and position of the gall on the plant is specific to just one particular insect. The insect which caused the Acorn Knopper Galls shown in these photographs is the Hymenopteran Andricus quercuscalcis.

The gall growth is a mass of ridged plant tissue – a simple form when just one or two grubs are present but much more complex in shape when more larvae are involved. The gall can grow so much that the acorn and its cup are completely hidden from view. The gall changes colour from red, to green, to brown as it develops. When the larvae become winged insects inside the gall, they escape from an opening in the gall and fly away.