Gouty oak gall on pin oak (Quercus palustris) caused by a wasp (Hymenoptera)

Gouty and horned galls are abnormal growths or swellings comprised of plant tissue found on leaves, twigs, or branches. 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. Most galls are aesthetically not pretty, but normally cause little damage to tree. However, severe infections may bring about the decline of the tree. Chemical control is seldom suggested for management.

The horned oak gall has small horns that protrude from around the circumference of the gall. It can be found on pin, scrub, blackjack, and water oaks. The gouty oak twig gall is smooth and can be found on pin, scarlet, red and black oaks.

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. Wasp can spread to adjacent trees causing them to become infected.

Life Cycle

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 on the leaf vein about mid summer. 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 developing larvae. When the larvae reach maturity, the horned galls developed small spines or horns. An adult wasp emerges from each horn and another life cycle of wasps begins.

Integrated Pest Management Strategies

1. Live with a minor infestation. Galls are aesthetically not pretty on a tree, but mild infestations won’t hurt the health of the tree. There are insects that are parasitic and predators that prey on the wasp larvae. These natural enemies help to reduce the wasps. This could give some natural control of the wasp.

2. Use cultural methods of control to help reduce the wasp. These pests may over winter in twigs, branches, and leaves. When the galls are small and are just developing on twigs and branches, where possible, prune and destroy the infested plant material. Rake and destroy all infested leaves. Maintain the health of the tree by watering during dry periods and fertilizing if needed.

3. Wasp infestations are unlikely to be controlled by chemical treatment. Therefore, there is no effective way of controlling horned or gouty oak gall.

4. Heavily infested trees may benefit from applications of fertilizer. See the University of Missouri Extension Guide No. 6865 Fertilizing Shade Trees.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches. Strategy 4 could be considered an organic approach if an organic tree fertilizer is used.

More images:

Heavy infestation of gouty oak gall (Hymenoptera) on oak (Quercus)	Heavy infestation of gouty oak gall (Hymenoptera) on oak (Quercus)
Horned oak gall on pin oak (Quercus palustris) is caused by a type of wasp (Hymenoptera)	Gouty oak gall on oak (Quercus) caused by a wasp (Hymenoptera)
Gouty oak gall on oak (Quercus) caused by a wasp (Hymenoptera)	Gouty oak gall on pin oak (Quercus palustris) caused by a wasp (Hymenoptera)
Like gouty and horned oak galls, this twig gall on white oak (Quercus alba) is probably caused by a wasp (Hymenoptera)	Like gouty and horned oak galls, this twig gall on white oak (Quercus alba) is probably caused by a wasp (Hymenoptera); note exit hole
A particularly large specimen of gouty oak gall (Hymenoptera) on shingle oak (Quercus imbricaria)	Heavy infestation of gouty oak gall (Hymenoptera) on pin oak (Quercus palustris)
Heavy infestation of gouty oak gall (Hymenoptera) on pin oak (Quercus palustris)	Feeding of cynipid wasp larvae (Hymenoptera) caused this cluster of wedge-shaped galls on oak (Quercus), called pine cone oak gall or lobed oak gall (Andricus quercusstrobilanus). This gall goes through a color change from pink or red to yellow and finally to brown.
Pine cone oak gall (Andricus quercusstrobilanus) on swamp white oak (Quercus) caused by cynipid wasp larvae (Hymenoptera)	Pine cone oak gall (Andricus quercusstrobilanus) cut open to reveal a cynipid wasp larvae (Hymenoptera)
Gouty oak gall (Hymenoptera) partially eaten by a squirrel