The most common gallmakers are tiny, dark wasps called cynipids or gallwasps. The larvae of certain moths, beetles, flies and a few other insects and mites also form galls. Galls consist only of plant tissue. In most cases, normal plant cells have been stimulated to multiply at an unusually high rate by the activity of a gallmaker. To successfully form a gall the insect or mite must begin it’s attack at a very precise moment in the plant’s growth cycle.
A gallwasp initiates the process by piercing a selected plant part with its egg-laying device and depositing an egg inside the plant tissue. Fluids deposited with the egg cause the plant cell multiplication process to begin. The larvae that hatches from the egg produces additional substances that maintain and control cell division. The larvae develops within a cavity inside the gall, feeding on material produced on the cavity lining. At maturity, it transforms into pupa, and later it becomes an adult that chew its way out of the gall. By causing the plant to form a gall, the gallmaker has provided food and shelter for its offspring.
Each gallmaker forms a gall of a particular size, shape and color; no other species forms one quite like it. Gallmakers are specific about the types of plants and the plant parts they attack. Some galls contain more than a single immature gallmaker, but usually each lives within its own cavity.
Most insect or mite caused galls in California are not harmful to the plant. Several cause a scorching or spotting of leaves and a few result in death of twigs they infest. In nearly all cases prevention of gall formation is exceedingly difficult and is not considered practical. For many insect and mite species that cause gall, means of prevention or control are unknown.