Galls are abnormal growths that develop on the tissue of oak trees (Quercus spp.) and other woody plants. Oak tree galls can be caused by wasps that lay their eggs in plant tissue, which are called gallmakers, as well as by other insects and pathogens. Both groups of oaks – red oaks and white oaks – can suffer from wasp galls. Galls rarely pose a serious threat to the health of established oak trees, which is a good thing because treatment for gall wasps is rarely effective or practical.
Twig Galls on Oak Trees
Several species of wasps are responsible for twig and stem galls on oak trees. They include the horned oak gall wasp (Callirhytis cornigera) and the gouty oak gall wasp (Callirhytis quercuspunctata), both of which produce woody galls that can be heavy enough to weigh down branches and have the potential to kill an oak tree if present in very large numbers, though this is rare.
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These unsightly galls are produced when the wasps lay eggs in plant tissue that is actively growing. The wasps are most active in the spring during bud break. You can tell gouty and horned oak galls apart because of the distinctive protrusions on the latter.
Leaf Galls on Oak Trees
Other types of wasps produce galls on the leaves of oak trees. In fact, it is possible for a single tree to suffer from multiple types of leaf galls at the same time.
Wasps in the genus Neuroterus, for example, lay their eggs on the underside of white oak leaves, which can result in leaves turning brown and dropping from the tree in the spring. These particular galls are known as jumping oak galls because then the infested leaves on the ground may "jump" slightly as a result of the larvae moving around inside.
Other types of wasps produce what are known as oak apple galls, which are spongy, spherical galls measuring 1 or 2 inches in diameter. These galls grow from the leaves of oak trees rather than the twigs too. Specifically, they develop from the petioles, which are the stacks that attach the leaf blade to the plant stem. Seen from a distance, the galls resemble apples, hence the name of these lesions.
Managing Galls on Oak Trees
Twig and stem wasp galls can be more problematic than leaf galls. Because they siphon nutrients from trees, twin and stem galls are more problematic on young oak trees than established oaks. Once galls on trees form, however, there is no way to remove them except by pruning and destroying infected branches, which is not always feasible. Because some wasps that cause galls overwinter on plant matter, it is a good idea to remove and destroy any fallen leaves around oak trees in the fall.
Because the problems they cause are primarily aesthetic in nature, control measures for galls wasps are rarely recommended. Chemical sprays can be used to target these wasps and prevent them from laying eggs. However, timing the spraying in the spring can be tricky and usually calls for professionals. Insecticides can also kill off the beneficial insects that prey on gallmakers and keep them in check.