wasps on white oak?

Hello all, I have a 14 foot swamp white oak that is covered with galls. Flying and landing on the galls are hundreds of different kinds of wasps. They seem to be landing only on the galls but not doing anything to them. Inside are larva of another insect but the wasps do not seem to be biting their way into them. There are no holes anywhere. Why are there so many wasps congregating in this tree?
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check this out: http://entomology.unl.edu/ornamentals/pestprofiles/gallwasps.htm
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