The organisms are usually small and difficult to identify. One of the first habits of insects observed by man, although most likely unaware of the origin, was that of gall making. Pliny, about 60 A.D., describes plant galls and their rapid growth. 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.