What are galls for?

A good question, and still a debated one! Galls have a range of properties - shapes, size, position on the tree, timing of development, host plant - some or all of which may be the result of natural selection. As far as is known, the gallwasp controls most of these gall properties; the mother controls when and where on the oak tree the egg is laid, and also whether a gall will contain one or many developing larvae, while the larva controls the structure and perhaps also the rate of development of the gall. The oak host does have some impact on gall structure, but these are usually minor. An example is provided by the surface texture of galls induced by the asexual generation of Cynips quercusfolii: On Quercus robur, these galls usually have a smooth surface and no spotted markings, on Quercus petraea, the gall is covered with raised white spots or tubercles, and on Quercus infectoria (in Turkey and Iran), the same gall has white spots, but also a relatively smooth surface. No-one knows whether these minor differences matter to the gallwasp!


Host effects on Cynips quercusfolii gall morphology: LEFT: on Quercus infectoria in Iran RIGHT: on Quercus robur in Britain

Because gallwasps are in control of gall development (see How are galls formed), current explanations centre on the value of the gall to the gallwasp. Three general hypotheses for the adaptive significance of insect galls have been proposed:

  1. The nutrition hypothesis: that the insects manipulate the nutritive value of gall tissues to their own benefit.
  2. The microenvironment hypothesis: that galls protect the insect from unfavourable external conditions, and
  3. The enemy hypothesis: that galls have evolved to protect the insect from attack by natural enemies.

Of these three, both the nutrition and enemy hypotheses probably apply widely. Of these two, only the Enemy hypothesis can explain the evolution of gall structural diversity. To learn why gallwasps need protection, go on to the Gall communities page, or download our recent review (Stone & Schönrogge 2003)