Insects and mites are the most common organisms that cause galls. Many leaf miners can produce cell proliferations that intergrade into true gall development. The attempt by plants to overcome injury by producing callous tissue is in a sense a kind of gall formation. The tulip spot gall, Thecodipolosis liriodendri , is only a slight thickening of the leaf while the tar-spot gall, produced by a species in the genus Asteriomyia has even less thickening and is little more than a leaf mine. The boxwood leaf miner, Monarthropalpus buxi , makes a leaf mine but the surrounding tissues proliferate.
Tannic acid is one of the main economic products obtained from galls.
A gall made by an Eurasian cynipid yields 65 percent tannic acid while American sumac galls yields 50 percent. Dyes are also obtained from galls. Natives of East Africa used to use galls as a source of dye for tattooing. However, galls were apparently not useful to American Indians. Pliny states that the Aleppo gall is able to dye hair black.