People are often intrigued by the discovery of galls, peculiarly deformed growths found on many trees. Most are caused by gall wasps, a group of insects most of which are tiny and hence easily overlooked. They lay their eggs on tree buds, leaves or flowers (according to species), and galls form from plant tissue in response to chemicals produced by the gall wasp larvae, which live inside the gall. A gall’s appearance is unique to each gall wasp species. Many species have two entirely separate generations, each one producing distinct galls.
Pedunculate Oak hosts a particularly large quota of gall wasp species, not to mention some striking galls. Among the more familiar are the following: Marble Galls, spherical and woody galls, up to 25mm across, caused by Andricus kollari; Oak Apples, spherical but knobbly galls, caused by Biorhiza pallida (the second generation form galls on oak roots); Artichoke Galls, where buds become swollen and artichoke-like, caused by Andricus fecundator; Spangle Galls, silk-button or flat button galls on the underside of leaves, caused by wasps of the genus Neuroterus; Cherry Galls, caused by members of the genus Cynips; Knopper Galls, distorted outgrowths on acorns, caused by Andricus quercuscalicis.
Although insects are the main causal agents of galls on trees in the British Isles, they are not alone in this ability. Many people will be familiar with the twiggy masses known as ‘witches’ brooms’ that form on birch trees. These are caused by the fungus Taphrina betulina. A closely related species, T. pruni, causes Pocket Plum galls on Blackthorn, where ripening sloes become distorted to resemble miniature runner beans.
Marble Galls.
Oak Apples.
Artichoke Gall.
Derick Bonsall
Knopper Gall.
Nail galls on Lime, caused by the mite Eriophyes tiliae.
Wasp of genus Cynips, causal host of Cherry Galls in oak.
Witches’ brooms are most evident on leafless birches during the winter months. A large specimen can measure 40cm across or more.
Cherry Galls.