Plant Galls are abnormal growths on a plant caused by parasites, for instance other plants, fungi, insects or mites. The gall provides both shelter and food or nutrients for the invading organism.
Galls caused by insects
The galls shown below are mostly caused by gall wasps although other insects cause them too (the last two by gall mites). You are unlikely to find the very small gall wasps but some of the galls are very common and easy to spot. Many have complex life-cycles with two distinct alternating generations. Most of the galls shown below contain the larvae of the agamic or female-only generation. These don’t need to mate but will lay eggs that cause a different gall containing the sexual generation, ie males and females. The galls below all grow on oak trees unless stated.
Common Spangle Gall
Caused by Neuroterus quercusbaccarum this is the most common gall on oak trees in the valley
Silk Button Spangle gall
Caused by Neuroterus numismalis this fairly common gall gets its name from its silky button-like appearance.
Smooth Spangle Gall
Caused by Neuroterus albipes this relatively unusual hairless gall is usually cream but sometimes with patches of purple or pink.
Caused by Andricus quercuscalicis this fairly common gall develops on acorns and is rather sticky when fresh. Colour can vary from green to brown or red.
Caused by Andricus kollari this common gall starts off green and ends up brown and woody as shown.
Caused by Cynips quercusfolii this striking gall may have a smooth or warty skin.
Oak Apple Gall
Caused by Bhiorhiza pallida this gall appears in the spring and contains numerous male and female gall wasps. This gall is old and has lost its apple-like colouring.
Caused by Andricus fecundator this unusual gall contains an inner gall which falls out in the autumn leaving the outer scales on the tree.
Robin’s Pincushion Gall
Caused by Diplolepsis rosae and also known as the Bedeguar Gall, this is fairly common on wild roses in the summer.
Gall mites on Field Maple
Caused by the gall mite Aceria myriadeum there can be hundreds on a single Field Maple leaf.
Gall mites on Alder
Caused by the gall mite Eriophyes laevis these grow on Alder leaves and turn red as they mature.