Sometimes the things we find in nature can be so much fun! For example, have you ever heard of an Elm finger gall? It looks like little green fingers coming out of the leaf. Or how about gouty oak gall that causes round galls on oak twigs?
Finding several round galls on your oak twigs or even fuzzy balls on the backs of oak leaves can be disconcerting! It looks like the tree is growing strange things, and something terrible must be happening.
These funny looking galls are formed from insects. There are hundreds of gall-forming wasps, midges, phylloxerans, and gnats in North America. We also have galls caused by nematodes, fungi, and bacteria. Usually, the organism that causes a gall is specific to a plant host.
Galls usually occur on leaves and stems of plants, but they can also be found on flowers, fruit, twigs, branches, trunks, and roots. They have fun terms like blister galls, bullet galls, pouch galls, and roly-poly galls.
These insects or gall-forming organism inject a chemical into the tree either when they lay their eggs or during the larval stage. The tree responds to the chemical with plant hormones, which triggers profuse plant tissue growth and the gall is formed. The inside of the gall is a source of food for the insect larvae, and the outside is a protection from parasites, predators, and insecticides.
In Texas, we have a lot of galls that form on oak trees, but you can also find them on pecan, apple, elm, grape, hackberry, and cottonwood trees. Fortunately, galls are generally harmless. They will not affect the tree other than potentially looking ugly if you have a large infestation.
Treating galls is very difficult to do because you have to time the insecticide just right to target the wasp or other insect before it lays eggs. Once the gall is formed, there is nothing that can be done. I would advise just taking time to marvel at the way that God put together a tiny insect that can inject a chemical to make a tree form a perfect house and food for its young. Nature is neat!
For more horticulture information, contact us at the Williamson County AgriLife Extension Office at 512-943-3300.
