The plant actually closes itself off from the gall as a way of protecting itself from further damage. Insect becomes enclosed by gall growth and feeds only on gall tissue during its development. Small holes on outside of gall indicate adult insects have left. Foliage, branches, stems, roots. Many nematodes seriously weaken plants by damaging roots and causing knots on them.
Some nematodes are beneficial and prey on harmful insect larvae.
Oak gall, caused by gall wasps specifically on oak trees. Oaks can get 805 different galls. Leaf curl and leaf gall, evident when leaves become puffy and tissue thickens, puckers and becomes brittle. Often caused by fungus which can remain on plant and multiply. Use dormant oil spray before bud breaks to branches and trunk.