Galls on Oak Trees
Over the course of my career I have had many questions from folks wanting to know what those little balls are on their oak tree. And the second question is how to keep them from killing the tree! How do you tell someone that the damage they see is really not bad at all and in fact these balls are part of the normal eco-system? Well, I can honestly say that most people need more of an explanation than that and here is the answer. The Mealy Oak Gall has been around as long as the live oak tree. What is a gall, in particular the mealy oak gall? Galls are abnormal swellings of plant tissue that can be caused by insects, bacteria, fungi, mites and nematodes. In our area the galls of most concern are those caused by insects and few by mites. Basically the insect, through many different processes depending on the insect, causes the plant to form or grow tissue that surrounds the insect. As the tissue grows the insect is surrounded and protected within a gall and this gall is the food source for the insect. Once the gall is mature it quits using the plant nutrients but the insect inside will continue to feed through its life stages. This gall making and insect feeding may sound very harmful to the plant but overall gall-making insects are not considered problems and even some galls are attractive and used in crafts. In the case of the Mealy Oak Gall most people assume that these round galls on the stem are harmful and so want to spray them. Of course it is impossible to get an insecticide to the insect since it lives inside the tree and remember they are not harmful to you or to the tree.
The Mealy Oak Gall is caused by the gall-making cynipid wasp. There are over 1,000 species of gall-making wasps world-wide but in our area this is the primary one. This cynipid wasp has a unique life cycle that includes two generations with one being asexual and one being sexual. The generation we are most concerned with is the asexual when the galls are formed. You can begin seeing them on the stems by late summer and really notice them in late fall. Sometime in January the adults will emerge from these galls and the sexual generation starts but only with a very tiny gall structure.
A second gall-making insect is the Oak Apple Gall. This is a really big gall that looks like a green apple on a red oak tree. This gall is basically hollow but if you can imagine as many as a hundred “apples” hanging on your red oak then you understand why people get concerned. Again this is a harmless pest but the gall makes for really interesting conversation.