Strange Fruit: Oak Galls

Native oaks (Quercus spp.) are keystone species for insects and the ecosystems in Georgia. Over 550 species of Lepidoptera (butterflies and moths) use them as a host plant, and their larvae (caterpillars) are important sources of food for birds and other predators. Other insects rely on oaks too and sometimes their use of oaks creates growths that might appear to be flowers or fruit. These growths are the plants’ response to the egg-laying activities of small wasps or midges. According to this article, there are around 800 different types of galls to be found on oaks in the US/Mexico/Canada areas.

I have seen a number of different types of oak galls in my area and I have pictures of some but not all of them. I will supplement this post with photos from others (each image is credited unless it is mine). Perhaps the most noticeable of the oak galls in my area is the Wool Sower Gall. When it is at its best, it is a fluffy white ball with red spots created at leaf buds. Identification groups get dozens of requests to identify “this tree” when the galls are prolific.

Wool Sower Gall on white oak (Quercus alba)

Some galls are leaf galls – meaning the tiny insects lay eggs in the leaf – while others are stem galls. A friend of mine recently found an excellent example of a mid-rib type of gall, perhaps the Clustered Midrib Gall Wasp (Andricus dimorphus).

Mid-rib leaf gall (D. Lane photo)

Another fairly noticeable type is the Oak Apple Gall. By the time most people find it on the ground where it fell, it is a lightweight, papery husk.

Oak Apple Gall (Joe Boggs photo)

One year there was an abundance small, hard red galls that look like aborted fruits. The identification groups in Georgia got numerous questions on what tree produced these.

Small galls that look like fruits (wasp larva in center)

In researching for this article, I found great information on an Ohio website. It contains a number of articles on the different types of oak galls. It is one of the few sites that presents information without trying to give guidance on how to kill them. 

Most galls seem harmless to the tree, but some can cause minor twig damage. None are fatal to the host plant. The diversity of shapes and sizing is incredible. Look out for galls on your oak trees and be amazed at what nature can do.