Sometime during the fall, as everyone is exclaiming over the beautiful fall foliage of the maple trees, the Oregon White Oak leaves discretely turn dark brown and fall to the ground. Once their leaves begin to drop, the oak galls which have been forming in the trees, become visible and drop too.
One oak tree can have many different kinds of galls, each with a distinctive shape and color, and each caused by a specific type of insect. The galls in this blog post all came from the same tree, and another type of gall was clearly visible growing on the branches. At the same time, I couldn’t find a single gall on a tree growing right next to this one.
One of the most common insects to form galls is the “gall wasp” which lays its eggs on new leaves and twigs in the spring. The egg and wasp larva exude a specific type of growth hormone which causes the oak to grow the gall around it, providing both food and protection. While it is a parasitic relationship, it causes little harm to the tree and the wasps are also harmless to humans, so if you have oak galls in your trees there is no reason for concern.
As far as I can determine, the “Oak Apple Wasp” forms the large apple-like galls on the twigs and branches, while the “Speckled Gall Wasp” forms the smaller, speckled galls on the leaves.
As a kid, we loved finding the large galls. We’d stomp on them to watch them “smoke”. Of course, it wasn’t really smoke, but the fine dusting of what appears to be a molding mass inside.