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These are oak leaf galls, and although they may look really weird and kind of scary, they're actually very common and there's no need to worry. Thanks to MaryAnn and Danielle for sending us pictures from their trees!
This year we've seen a few more oak leaf galls than in the past, thanks to the mild winter and the late spring rains. The galls are actually produced by the tree itself, in response to a pest, usually an insect or a mite. Depending on the offending pest, the galls may be many different shapes, sizes and colors. They might even be fuzzy.
The small, reddish galls that we're seeing on the undersides of our oak leaves right now are caused by tiny wasps. The wasps lay their eggs on the leaves and the tree responds by forming a protective structure, the gall, to contain the wasp eggs while the insect larvae grow into adults. As adults, these tiny wasps do not sting. In fact, you probably won't ever see them.
The good news : this process causes no noticeable damage to the tree. Actually, we have them every year; you just usually don't notice them. So there's no reason to treat the tree or try to remove the galls. Your live oak will drop these leaves in a few months and put on new ones, and next year we most likely won't see as much of an infestation as this year.