These type of galls represent a very long co-evolution between an insect or mite species and a limited plant group. Usually, gall formation is induced when the insect or mite begins feeding at a critical time in the plant's development or growth. Powerful compounds within the saliva actually alters the genetic message in the individual plant cells and alters the message within the shoot, leaf, or other tissue from developing normally to that of producing the gall.
Galls always surround the gall-inducing organism offering protection and a food source.
However, there exists a number of parasites that specialize in attacking gall-inhabiting insects. Galls that are found on foliage and flowers are usually only considered to be an aesthetic curiosity in their importance. However, when galls are formed on the stems and branches of the host plant, death of those parts, and even the entire plant, may occur. Like leaf miners, gall-inducing insects and mites are extremely difficult to control once gall formation has occurred. Gall formation on individual plants may vary greatly from one year to the next.