Gall-inducing insects include gall wasps, gall midges, gall flies, Agromyzidae aphids (such as Melaphis chinensis, Pemphigus spyrothecae, and Pemphigus betae), scale insects, goldenrod gall fly, and psyllids.Various galls produced by insects and mites are listed below: Many rust fungi induce gall formation, including western gall rust, which infects a variety of pine trees and cedar-apple rust.
They are plant tissue which is controlled by the insect.
Nitrogen-fixing bacteria (Rhizobium species) cause swellings on the roots of most legumes (such as clover, peas and beans).
These swellings, called nodules, are easily distinguished from root-knot galls by differences in how they are attached to the root and their contents.
Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified.
This applies particularly to some insect and mite plant galls. In human pathology, a gall is a raised sore on the skin, usually caused by chafing or rubbing.