More about oak galls

Yesterday’s post about oak apples prompted questions. Here is more information about some of the oak gall wasps that induce oak trees into producing such strange growths.

At this time of year, the park’s many oak trees have shed not only their acorns but a variety of galls. These are the trees’ responses to the egg-laying habits of oak gall wasps which lay their eggs in the leaf buds, the acorn buds, the flowers and stems of oaks. There are approximately seventy species of oak gall wasp in Britain. They are very small and unobtrusive, leading mysterious and complex lives among the leaves of our oak trees.

Oak gall wasps

It used to be thought that the galls were a defence mechanism on the part of the tree but we now know that the galls are to the benefit of the wasp, not the tree. The shape and outgrowth of the gall is dictated by chemicals produced by the wasp’s egg or larva. The chemistry of the egg and larva forces the host tree to provide them with appropriate nutrition by concentrating photosynthetic resources into the tissues of the gall. Think of it as hacking: the egg and the larva have hacked into the chemistry that controls the tree’s growth programme.

Three kinds of oak gall seen and photographed in the park. From left to right: knopper, marble and artichoke galls

The gall’s structure, individual to each species of wasp, provides a microclimate for the incumbent larva. Some authorities believe it also provides tailor-made protection from the enemies of that particular species of wasp.

By the end of the summer, all three of the above galls have become hard and woody, a process known as lignification. The timing of lignification is also under the control of the gall wasp larvae; deciduous galls, such as knopper and artichoke, fall from the tree and overwinter on the ground. These galls fall slightly before leaf-fall in the autumn, ensuring a covering of leaves and a protected environment in which the larvae can pupate, to emerge as an adult in the spring.

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