A gall is a growth left behind by a gall insect, any species that lays eggs underneath plant tissue. As the larvae develops under the plant tissue, a portion of raised tissue called a gall takes shape on the plant, leaving behind an unsightly growth even after the larvae has left. If a gall-producing wasp has left a gall on your oak tree, you can treat the tree for the presence of galls in a few simple steps.
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Prune off gall-infested parts of the tree using pruning shears or a small saw. The effectiveness of this step depends in large part on the degree to which the tree is infested. For large trees with overwhelming infestations, it will be time-inefficient to prune off all the galls. But if pruning off gall-infested branches can still leave significant foliage on the tree, pruning can be an effective means for control.
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Clean up all leaf and twig debris near the base of the tree. Defoliated leaves, dead branches and other debris very often serve as over-wintering sites for gall-producing insects, so eliminating these sites can compel the wasps to seek another overwintering site away from your oak tree.
Apply a wasp insecticide to adult gall-producing wasps near the tree. Once wasp larvae are inside the gall, they are protected from contacting any insecticides. Therefore chemical control of gall-producing insects should be focused on killing adult wasps before they have a chance to breed. Timing such a spray application can be difficult; if possible, identify the species infesting your oak and learn the time of year when the wasps tend to reproduce so as to better time your spray application for periods of high wasp activity near the tree. Always follow labeled directions.