What causes a plant gall can often be known without the actual agent being there. Many types of gall have typical structures. This is true of many galls produced by insects and mites.
Insect galls on plants give some herbivorous insects their own microhabitat. They are plant tissue which is controlled by the insect. Galls are the habitat and food source for the maker of the gall. The gall may contain nutritious starch and other tissues. Galls often give the insect physical protection from predators.
Insect galls are usually induced by chemicals injected by the larvae or the adults of the insects into the plants, and possibly by mechanical damage. After the galls are formed, the larvae develop inside until fully grown, when they leave. In order to form galls, the insects must seize the time when plant cell division occurs quickly. The growing season, usually spring in temperate climates, is longer in the tropics.
-  Merriam-Webster Online Dictionary, accessed Nov. 16, 2007 ("an abnormal outgrowth of plant tissue usually due to insect or mite parasites or fungi and sometimes forming an important source of tannin")
- Larson K.C. and T.G. Whitham 1991. Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions. Oecologia 88, 15–21.
- Weis A.E. and A. Kapelinski 1994. Variable selection on Eurosta’s gall size, II: a path analysis of the ecological factors behind selection. Evolution 48, 734–745.
- Stone, Graham N. and Schonrogge, Karsten 2003. The adaptive signiﬁcance of insect gall morphology. Trends in Ecology and Evolution 18(10): , 512-522