Rather than being a fungus, oak trees with balls on the leaves are inundated with what is known as gall. These structures are caused by various types of wasps or flies to house their eggs and larvae. Rather than building the galls themselves, the insects force the oak tissue to do it for them.
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What looks like balls on the leaves of an oak tree are commonly referred to as "galls." This growth is caused by smalls wasps known as cynipids, also called gallwasps. These brightly coloured swellings develop on both the twigs and foliage of the oak tree as the gallwasp pierces the oak with its mouthpart and lays an egg inside the plant tissue. Fluids deposited by the wasp encourage the cells of the tissue to begin to multiply and grow.
The result is a gall, a sort of temporary housing and buffet for the egg and the larvae when they hatch. The larvae grow inside the gall, feeding on the lining. Upon reaching maturity, the adult gallwasp chews its way out of the gall. Dependent on the species, galls vary in size, shape and colour. Other insects also inhabit the gall, either during or after the residency of the gall wasp. Some are parasites of the gallwasp, while others feed on what has been left behind.
The gallwasp Andricus californicus causes a large gall to form that is commonly referred to as an "oak apple," due to its size and red colour. Another type of gall is the minute "jumping oak gall" built by the gallwasp Neuroteras saltatorius. This gall sparks a great deal of attention because it falls to the ground and the larvae inside cause the structure to actually jump up and down along the earth for several days. According to the University of California at Davis, another gall, built on twigs of the oak, resembles a tiny loaf of bread.
The primary damage caused by the galls is aesthetic. They rarely cause significant injury, although some spotting or scorching of leaves and twig dieback is possible. Healthy oak trees generally withstand an infestation of gallwasp, with no significant difficulty. Additionally, it is relatively difficult to attack the larvae inside the gall with either insecticide or oil, because the gall structure itself serves as an effective protection against such measures.
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