Galls are growths that occur on leaves and/or twigs of oak trees with great regularity. There are many, many different types but they all have a certain number of things in common:

1. Cause - The vast majority of oak galls are caused by the feeding of insects (often types of wasps) and spider mites. When these critters chew or suck the juices from the plant, their saliva contains chemicals that stimulate the formation of these galls which are actually plant tissue. Galls are somewhat comparable to tumors in animals. Often, the insect or mite insert their eggs into the plant tissue such that the gall forms around the eggs. When they hatch, the larvae feed on the inside of the gall and eventually eat their way out later.

Note: There are a few galls such as the forsythia gall and the euonymus gall that are caused by bacteria.

2. Damage - Almost without exception, oak galls are harmless to the plant. They may reduce the photosynthetic surface of the leaf by a minute amount but this does not even register with the tree. One exception may be galls that form on the stems and twigs at times in great numbers. They may girdle the twig or branch and cause its death.

As mentioned above there are dozens and dozens of different galls that are commonly found on oak trees. Here are just a few of them:

* Oak apple galls are light brown balls up to about 2 inches  in diameter that form in oak leaves. Inside contains a single larva of a type of wasp.

* Oak bullet galls cause growths on twigs most commonly on bur oaks (Quercus macrocarpa).

* Bullet galls are small, hard round growths that appear in August.

* Jumping oak galls are small, hard growths on leaves. Inside each gall is the larva of the insect which, when they drop to the ground, begin to jump around resembling Mexican jumping beans.

* Oak pill galls form irregular, hardened growth that are up to 1/4 inch in diameter on the upper surface of the leaf.

* Woody oak galls resemble a dense wad of wool which is attached to the midvein of the leaf.

* Horned oak galls form on the stems of several types of oaks.

There is no practical option for preventing galls.

Technically, galls could be treated by spraying the trees with insecticides before the adult insect deposits the eggs in the leaf or stem tissue. Since the galls do no serious damage to the trees, this would not be recommended...ever.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

Types of Insects

"Name That Bug Page"

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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