Acorn Knopper Gall on Oak, caused by the wasp Andricus quercuscalcis laying eggs in the growing tissues of the tree. (1)

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.

Uninfested acorn without gall: Normal uninfested acorn on Oak, unaffected by the egg-laying activities of the Andricus quercuscalcis wasp. (2)

Galls on Oak trees: Acorn Knopper Galls on acorns: abnormal growths caused by the wasp Andricus quercuscalcis (3)

Acorn Knopper Gall on acorns: abnormal growth caused by the wasp Andricus quercuscalcis (3)

Acorn Knopper Galls on acorns: abnormal growths caused by the wasp Andricus quercuscalcis (5)

Acorns on an Oak tree; Acorn Knopper Gall on right side of image;  normal acorn on left (6)

COPYRIGHT JESSICA WINDER 2011

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