Tree Galls: Unusual Growths on Trees

Unusual is one of the best way to describe tree galls. You can find these abnormal plant growths on leaves, twigs, and branches. They come in a range of different shapes and sizes. One could be a large, tumor-like outgrowth high on a tree’s steam. Another could be small bumps on the leaves of a tree. Most knobs, lumps or wart-like growths on plants are usually galls and even come in a variety of colors. Often, these growths are formed by insect feeding or egg-laying activity. The tree will respond to the activity by stimulating growth specifically in that area. However, tree galls sometimes result from the presence of fungi, bacteria and viruses.

The appearance of galls is varied, and so are the causes.
The appearance of galls is varied, and so are the causes.

Types of Tree Galls

There are thousands of insects, mites and other pests that produce galls. Their presence stimulates growth of the plant tissue where the eggs or larvae reside. The gall can then shelter, protect, and feed the insects. Some of the most common gall-making insects are gall wasps, gall midges, psyllids, and adelgids.

One example is the maple bladder-gall. You’ve likely seen this and not even known! Eriophyid mites cause small, bead-like growths on maple leaves that start out green, but turn bright red by the end of summer. Further, oaks are often impacted by growths such as horned oak galls. Caused by a wasp, the resulting galls are blister-like and appear along veins on the undersides of leaves.

Additionally, insect galls impact many coniferous species as well. Spruce galls are a serious pest that can seriously damage trees. Eastern spruce gall primarily attacks Norway and white spruce while Cooley spruce gall infests blue, Englemann and Sitka spruce as well as Douglas fir.

Unlike the above cases, crown gall is not the result of insect activity, but rather a disease vectored by bacteria. Hundreds of species are susceptible and the resulting growths can stunt plant growth.

Impact on Trees

Many galls cause no serious harm to the tree, though they are often unsightly. A heavy infestation could result in branch dieback and decline in health of the plant. However, there are some that indicate a more serious issue is present. Sometimes, the growths can weaken trees, interrupting the flow of nutrients and water and reducing overall health.

Depending on the cause of the gall and the level of severity, treatment may or may not be needed. Pruning out of the galls can help from both an aesthetic and tree health standpoint.

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