Question
My two year old oak trees have galls on various limbs. Are these caused by a tiny wasp called a cynipid? Will the galls hurt the trees? Is there a recommended treatment or do you just ignore the problem?

Answer
Galls are irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites. 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. Generally, initiation of leaf galls occurs around "bud break" or as new leaves begin to unfold in the spring.

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. Horned oak galls can be found on pin, scrub, black, blackjack, and water oaks while gouty oak galls occur on scarlet, red, pin or black oak.

These galls have a long and complex development that takes two or more years to develop. The first stage is a blister-like leaf gall that occurs along larger leaf veins. The second stage is a knotty twig gall that is started in mid-summer and becomes fully mature in 1 to 2 years. Adults emerge in the spring. Gouty oak twig galls are smooth; hormed oak galls have horn-like projections. One female wasp can emerge from each horn.

Generally, insecticidal control is not satisfactory because the wasps are physically protected within the galls. Correctly timing applications to provide effective preventive control is difficult. Where practical, pruning of infested twigs may help to reduce the problem on lightly-infested trees. However, pruning is impractical if large trees are heavily infested.
I would not be too concerned with the galls they really do little to harm the tree. Here is web site that gives more information on oak galls. http://www.ento.psu.edu/extension/factsheets/gallsOnOak.htm