Q: I found these unusual growths on my oak trees. What are they and will it cause my oak tree to die?
A: Thank you for bringing this to the Extension office; I have seen this gall only one other time in my thirteen years of being a Horticulture agent in Nassau County. I believe the gall is called an oak apple gall. Galls occur on a wide variety of plants.
These growths may be the result of fungi, bacteria, nematodes or mites, but insects are the prime cause. Gall-forming insects include aphids, phylloxerans, psyllids, midges (gall gnats) and cynipid wasps (gall wasps). Of the more than 2,000 gall-producing insects in the United States, l,500 are either gall gnats or gall wasps. About 80 percent of the gall wasps produce galls specifically on oak trees. In fact, 60 percent of all known insect galls occur in the oak family and 30 percent occur in the daisy, rose and willow families.
These growths are called galls because they contain large amounts of tannin, which has a very bitter taste. Long ago, they were known as “gallnuts” because they tasted as bitter as gall. Plant galls are abnormal growths of plant cells formed as a response to the insect’s stimulus caused by egg laying, or larvae or nymphs feeding. Galls seem to cause a lot of concern to the general public. Generally they do not seriously harm the plant. Most ornamental plants and trees are not apparently injured even by relatively large numbers of galls. Attached is the publication for more information on insect galls: http://edis.ifas.ufl.edu/pdffiles/IN/IN02200.pdf