Some of the more unusual natural objects in Missouri's forest are the insect galls that are found on oak trees. I've had numerous questions from persons who wonder what kind of tree they saw that looked like an oak but had fruits that were spherical structures with spongy tissue inside. It reminds me of my botany professor who said that, in identifying plants, we should adopt the attitude that plants are always trying to fool us. Many observers have been fooled by galls on oak trees.
Galls can form on plants for a number of reasons but insects are the most common cause, especially small wasps of the family Cynipidae, called "gall wasps." In Missouri, most galls are found on oak trees but they also occur on other trees , some roses, and members of the sunflower family, such as goldenrods. While the tiny wasps are rarely noticed, the galls that they induce on the tree can be real eye-catchers .
The gall develops because the insect lays its eggs in the plant tissue or its larvae or nymphs are feeding on the plant. The gall itself is abnormal tissue that is produced by the plant as a way to isolate the insect. Galls contain plant tannins and the bitter taste of galls led to their naming after the bitter nature of the gall, or bile, produced by the human liver.
If you find an insect gall on the ground or on a low tree stem or leaf, you can cut it open and usually find one or more larval forms of the small wasp or other gall-forming insect in the center of the spongy tissue. The larva will live inside and feed on the tissue of the gall until it eventually creates an exit hole and leaves the structure. Galls usually have little or no impact on the health of the tree on which they are found. If you see a strange-looking "fruit" on a tree twig or leaf, remember insect galls and don't let that tree fool you.