The nodule on this valley oak tree (quercus lobata) is not an oversized acorn or fruit, but is an interesting case of parasitism. A wasp larvae induces the tree to make the nodule — called a gall — thus providing food and shelter during the wasps’ larval stage. The fascinating book An Island Called California, by Elna S. Bakker, contains a short description of how they are created.
Two generations of the cynipid wasp are necessary to form the “apple gall” like the one in the photograph. In the first generation, adult female wasps lay eggs on leaf buds, small twigs, acorns or other young parts of the tree. The eggs hatch, and the larvae begin to feed on the tree’s tissue. The insect larvae secrete an irritating substance which causes the tree to produce the apple-sized galls, thus providing a source of food and shelter for the larvae. Eventually, the larvae develop to adulthood inside or near the gall, but only female wasps emerge from the apple galls in the spring.
The females lay their eggs on leaf buds. The resulting larvae produce a differently shaped gall, with shapes like mushrooms, champagne glasses and more (see this video about galls from KQED Deep Look for examples). After a time, males and females emerge and mate. The females from this generation lay eggs to produce the apple gall, and the cycle continues. Generally, galls are not harmful to the tree, but there is a limit to how many a tree can produce without draining its vitality.
Galls are one of the raw ingredients for iron-gall ink. For centuries, this ink was made by combining ferrous sulfate (Fe2SO4) with an extract from an oak gall (gallo-tannic acid). The resulting ink was permanent and water-resistant, and was the standard for writing and art for 700 years. Many of the greatest European artists of the period — Leonardo da Vinci, Titian, and Rembrandt van Rijn, for example — executed their drawings using iron-gall ink.
Note: the photograph was taken at the quietly remarkable Cosumnes River Preserve in Galt, California (near I-5 between Sacramento and Stockton).
Other writings and references
Unmitigated Gall – a philosophical look at oak galls.
Sketches from a Tavern: Woman Standing and Two Men Seated, attributed to Rembrandt van Rijn, National Gallery of Art, open access.