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 small parasitic wasp which deposits its eggs onto the leaves and twigs. Chemicals found during this deposition cause the leaves and branches to swell and form the large, woody galls. Most researchers say that the while the galls are not attractive if only found in small numbers they may not have serious health risks for the tree. However if the galls become too severe they may girdle the twigs stressing the tree and ultimately cause stem dieback. Large infestations of galls are causing major dieback and tree death.
The horned oak gall forms small horn-like projections on the galls during the second year of its formation. It can be found on pin, scrub, blackjack, and water oaks. The gouty oak twig gall lacks the horns and has a smooth surface and can be found on pin, scarlet, red and black oaks.
Squirrels have increasingly been found chewing on the galls causing much twig and leaf ground litter from the canopy above.
Life Cycle
The gall forming wasp has a complicated life cycle and a very narrow time-frame of emergence. It begins in the early part of spring as the tiny wasps emerge from woody stem galls. 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 following spring and over the next two or three years the galls begin to enlarge and reach maturity until they hatch out and breed.
Prevention and Treatment
There are treatments but timing is critical. Each tree should be evaluated and assessed by a certified arborist that is experienced and understands tree biology. This insect has a complex life cycle (3 years), so the treatment is ongoing. As always the best treatment is proper care and STRESS MANAGEMENT. Keep trees free from stress (drought, nutrient deficiencies, soil compaction, mechanical damage, etc…). Water deeply during extended periods of dry weather. Fertilization helps to supplement essential nutrients and condition the soil. Practice good sanitation by raking up and removing infested leaves and fallen galls should alleviate some future infestations. Some galls could be pruned out but may not be realistic on larger trees with larger infestations. NEVER remove more than 10% of live green tissue from a mature tree! Trees need foliage (leaves) to live. Topping and Lion-tailing are extremely harmful for healthy trees, but over pruning an already stressed tree is a death sentence. See our post on Lion-tailing here.
Additional Informational Sites:
University of Kentucky article on galls
