Gouty oak gall on pin oak (Quercus palustris)
A large specimen of gouty oak gall on shingle oak (Q. imbricaria)
Heavy infestation of gouty oak gall on pin oak (Q. palustris)
Horned oak and gouty oak galls are abnormal growths or swellings comprised of plant tissue found on the twigs or branches of oak trees. These deformities are caused by a tiny, non-stinging, wasp which produces a chemical or stimuli inducing the plant to produce large, woody twig galls which can reach almost 2 inches in diameter. Horned oak galls are induced by Callirhytis quercuscornigera, while gouty oak galls are caused by a related wasp species, C. quercuspunctata. Horned oak galls can be distinguished from the gouty oak gall by the small "horns" that protrude from the surface, and can be found on black, blackjack, pin, red, shingle, and water oaks. The gouty oak gall has a smooth surface and can be found on black, pin, red, scarlet, shingle and water oaks. Both species are often found in the same geographical area.
Light to moderate infestations of horned/gouty oak galls are generally an aesthetic issue. However, severe infestations can bring about the decline of the tree. The gall structure is integrated with the stem and can cause girdling. This can reduce the flow of nutrients and water to the end of the twig, often causing dieback from the point of attachment. Occasionally, these small sections of twigs may break off on windy days or in storms, leaving minor debris around a landscape. Additionally, squirrels have been known to chew on the galls and galled twigs, also contributing to gall litter. An effective chemical control for management of horned/gouty oak galls has not been found. Symptoms and Diagnosis
Young galls are slight, tumor-like swellings on twig growth. Bark color of young galls are a greenish-brown and bark surfaces are smooth on gouty oak gall and horns on horned oak gall. As the gall matures the tumor-like swelling will enlarge to as much as two inches in diameter and becomes woody and discolored. At first there are just a few galls throughout the tree. Repeated lifecycles of the wasps can result in areas of the tree being covered with galls.
Life Cycle
The life cycle of the wasps that causes horned/gouty oak galls is complex, as it involves alternating generations or separate sexual and asexual reproductive stages. In early spring a tiny wasp of the cynipidae family emerge from woody stem galls. The females lay eggs on the veins of the oak leaf buds. Male and female wasps emerge from these tiny, blister type galls found along the veins on the underside of the leaf about mid summer (sexual generation). Mated females deposit eggs in young oak twigs. The next spring small swellings develop on the twigs and enlarge over the next two or three years. The galls provide protection, food, and shelter for the larvae developing within the gall. When the larvae near maturity, the characteristic horns emerge from the gall structure. Once the horn is fully extended, an adult wasp emerges from each horn (asexual generation) and another life cycle of the wasps begins.
Integrated Pest Management Strategies
1. Live with a minor infestation. Galls are aesthetically unattractive, but trees can still thrive with a minor infestation and proper care. There are insects that are parasitic and predators that prey on the wasp larvae, such as the oak gall borer (Synanthedon decipiens). These natural enemies can help to reduce wasp numbers and provide some control of the wasp.
2. Use cultural methods of control to help reduce the wasp. When the galls are small and just developing on twigs and branches, prune where possible and destroy the infested plant material. Rake and destroy all infested leaves and gall debris. Maintain the health of the tree by proper watering, especially during dry periods.
3. Select the right tree for your landscape. To reduce stress on trees, select species that are adapted for your landscape's soil and moisture conditions. Oak species such as pin oaks (Q. palustris) prefer acidic soil (pH < 7), and often become chlorotic in the alkaline soils (pH > 7) of the Midwest. 4. Contact a certified Arborist. Trees experiencing severe gall infestations, canopy dieback or thinning, or limb drop should be assessed by a certified arborist. 5. Wasp infestations are unlikely to be controlled by chemical treatment. Therefore, there is no effective way of controlling horned or gouty oak gall.
Organic Strategies
Strategies 1 - 3 are strictly organic approaches.