Folks have been bringing in oak leaves with lumps, bumps, and fuzzies, wanting to know, what is this and what do I do about it? Several different insects can cause galls, but predominantly it is different species of small wasps. I collected these three most common types of oak leaf galls off just one branch of my tree. The wasp flies in and lays eggs on the leaf or stem, in a fresh and growing portion of the tree. When the larva emerges from the egg, it starts secreting a chemical that causes the oak to grow abnormal tissue in which the larva shelters and feeds. It is usually just a cosmetic issue, not a threat to tree health, and other than the stem galls, other galls fall away as the leaves drop, with seemingly little carryover from one year to the next. Insecticide treatments are not recommended in home landscapes, only in commercial oak nurseries where the trees are relatively small and in dense blocks, and aesthetics are paramount to salability. Factoid- in medieval times, ink was made by crushing oak galls to release tannic and gallic acids, in which iron scraps were dissolved, the resulting mess, after filtering, and gum arabic added, making black ink. This was the standard writing ink for over 1400 years, and is still manufactured for use by some artists.
–James
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