Question
we live in a wet area of oregon . visiting a park by the illinois river ,we noticed oak leaves on the tree & ground that had round globe like balls attached. they appeared to be hollow. what in the world are they. have checked many sites, can't find answer......can you help?......thanks...michael rountree

Answer
They are called galls caused by an insect (small wasp) The wasp lays its eggs on the surface of the leaf or twig and secretes a chemical that causes the tissue of the leaf or twig to swell and cover the egg. the egg hatches and moves on. The galls themselves will not harm the overall health of the tree although they can look bad. Leaf galls may cause the leaves to drop prematurely. There are many types of these wasps some attack the twigs and others the leaves. Control is not usually needed and has to be timed with the wasp presence and not the galls themselves.

In general, most leaf galls on oak do not affect the health of the host tree. A few can cause leaves to drop prematurely, or distort them so that photosynthesis (the plants food-making process) is interrupted. Galls generally are aesthetically objectionable to homeowners who find them unattractive and fear that galls will cause damage to the health of their oak trees.

Chemical control is seldom suggested for management of leaf galls on oak. Cultural methods of control may be effective in reducing the impact of these insects. Some fallen leaves may harbor various life stages of gall-producing pests. Therefore, it may be useful to collect and destroy all infested leaves. Some of these pests overwinter in twigs and branches of oak. Where such woody galls are detected, prune and destroy the infested plant material when the galls are small and have just started to develop.

Once a gall begins to develop, it is almost impossible to stop or reverse its development. Unless registered insecticides can be applied when gall wasps are flying, they offer little or no effective measure of control. Lack of serious plant damage from leaf galls and the difficulty in proper timing of insecticide applications pose a strong argument against the use of insecticides to reduce galls on oak.

In short, the galls will not kill the tree and usually natural controls will keep the population from repeating next year.  


Here are a web links to a couple of sites on gall wasp that discuss the wasps and control measures.
http://entomology.unl.edu/ornamentals/pestprofiles/gallwasps.htm
http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note05/note05.html