Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf and twig galls are most noticeable. The inhabitant gains its nutrients from the inner gall tissue.
Galls also provide some protection from natural enemies and insecticide sprays.
Important details of the life cycles of many gall-makers are not known so specific recommendations to time control measures most effectively are not available. Gall makers must attack at a particular time in the year to be successful. Otherwise, they may not be able to stimulate the plant to produce the tissue which forms the gall. Twig and Stem Galls Twig and stem galls, such as the gouty oak gall and horned oak gall, are solid, woody masses that can girdle branches or make them droop from the sheer weight of the heavy growths. The galls can grow to more than 2 inches in diameter.