Question
one of our oak trees has little green balls on the underside of the leaves. we can count at least thirty,do you know what they are ? this tree is six years old and never had acorns. thank you Doug

Answer
These are leaf galls formed by a tiny insect.

Insects or mites that cause galls to form are called gallmakers. Galls grow in response to either feeding by the gallmaker or as a result of egg laying on or within the plant tissues. Galls grow to surround the tiny insects and mites that form them, providing some protection from adverse weather, predators, and parasites. In addition, the gall is a ready source of food for the gallmaker, rich in protein and carbohydrates.The insect or mite develops and grows inside the gall during the summer and emerges as an adult either in the summer or the following spring. Each species of gallmaker causes its own distinctive gall that is different in appearance from galls caused by other species.

Most insect galls do not seriously affect the health and vigor of healthy, well-established trees and shrubs. Occasionally, a heavy gall infestation causes severe leaf or stem deformation and prema-ture leaf drop. However, these annoyances usually do not cause long-term damage to the tree. Leaf galls may be especially aesthetically displeasing but they do not directly kill the plant.

Because most galls do not seriously affect trees,controls are not usually necessary. Also, galls cannot be 揷ured?after they have formed and spraying the galls does not make them go away.

Here is a web link to more information on leaf galls.
http://insects.tamu.edu/extension/bulletins/L-1299.html