Tree Galls – What Are They?

Abnormal growths on trees are always a cause for concern for homeowners, leaving many wondering if their tree has a disease, or if the growth will harm the tree and needs to be removed. These growths are often referred to as galls, and they come in all shapes, sizes, and colors. What exactly causes these growths, and are they harmful to the tree? Read on to find out!

Gall on an Oak Tree

The tricky thing about galls is that there are so many different types, depending on the species of tree and the cause of the gall. Insects and mites are the most common cause of galls, though they can also be caused by fungi, bacteria, and viruses. The formation of the gall is the trees’ hormonal response to being infected or injured by a pest. In the case of insect galls, the gall acts as a home for larvae to develop until they emerge. Each gall-producing pest is specific to one type of tree or plant, and often affects only a certain part of the plant, such as flowers, leaves, or stems.

Galls can come in all shapes and sizes – some are small clustered bumps on leaves, while others grow along stems or twigs, and still others can appear to hang from the tree, similar to a fruit. Many have a hard shell, but others can appear soft, such as the one pictured above. One of the best places to look for galls is on oak trees, as oaks are a host for around 500 different types of gall-producing pests and diseases. Oak Apple Gall, pictured below, is a common one found in New Jersey. These galls are 1-2 inches large, and often resemble an actual apple. Various species of wasps lay their eggs on leaf buds, and the tree forms a gall in response to chemical secretions from the larvae. The larvae feed on the gall tissue and remain inside while they undergo metamorphosis. Gall-producing wasps do not sting humans, so no need to worry about that!

Oak Apple Gall

While the formation of galls is interesting, the question most homeowners have is whether or not they’re harmful to the tree. The good news is, they generally are not! Extreme amounts of galls along twigs and branches, such as gouty and horned oak gall, may eventually cause weak points in the tree, but the galls themselves do not bring any harm to the tree and do not need to be removed. Galls are usually not present in such a large amount that there is a significant impact on tree growth, although they can be unsightly and cause leaf distortion or even drop in some cases. However, the trees should recover from this and re-leaf the next year. 

Another Oak gall – so much variety in appearance between galls!

If you are concerned about galls developing, the only surefire way to prevent them from forming is to avoid planting species that are susceptible. This can be difficult as there are over 1500 different gall-producing species, and most tree species have the potential to be affected. You can prune off affected parts of the tree as galls appear, but it isn’t necessary. To help prevent gall-producing pests from returning the following year, be sure to rake and dispose of all leaf litter over the winter. Galls are harmless, and the pests that produce them are an important part of the ecosystem. Gall-producers have a number of predators, such as birds and insects, that help to keep them under control. So if you are fine having some strange looking ornaments on your tree and want to provide food for other creatures instead of removing the galls, that’s perfectly okay!