No one likes Wasps! These sacks on trees are basically wasp eggs called wasp galls. Wasps will lay their eggs on the new leaves as they start to grow in the spring. Unless there is a major infestation of these galls, it is not a threat to the health of the tree. Use of pesticides is not recommended due to the fact it will do more harm than good.
Many different insects makes these galls on trees, but there are some things you should know about them. When cracking one open you might fined a strange gel that does not resemble any insect. This is because the eggs will feed on the gel until they hatch. Most of the 600 species found in North America are only between 6 and 8mm.
Next time you are walking around the woods be sure to look on oak trees. In a condescending manner be sure to impress your friends with your knowlege of these obscure deformities. No one likes wasps but everyone likes a smart ass.
Oak apple wasp galls
Oak marble wasp gall
Acorn plum gall (form on acorns)
spiny leaf gall
Knopper gall (These vary greatly in appearance)