Oak
Galls
By
Mike Stiles, Master Gardener
Our native oak trees, especially the Valley Oak, can sometimes be riddled with
large, often ping-pong ball sized growths along the stem, and other, smaller
growths on the leaves and stems of the tree.
Small wasps produce these "oak apples" or galls.
A tiny gallwasp or cynipid wasp pierces a twig or leaf with
its egg laying device and deposits an egg inside the plant tissue.
Fluids deposited with the egg, and produced by the larva cause the plant cell
multiplication process to begin. The
larva develops within a cavity inside the gall, feeding on material produced in
the cavity lining. At maturity, the
larva transforms into a pupa, and later becomes an adult that chews its way out
of the gall. By causing the tree to form
a gall, the gallwasp has provided food and shelter for its offspring.
Each gallwasp forms a gall of a particular size, shape and color. The large "oak apples," are induced
by the gallwasp "Andricus californicus". The pinhead-sized "jumping oak
gall" falls to the ground and jumps around for several days because of the
action of the wasp within. Other galls
look like pink stars, reddish cones or tan wheels. One particular twig gall resembles a tiny
loaf of bread.
Other insects can invade the gall. Some
are parasites of the gall wasp and others use the gall as a food source. Most gall wasps are not harmful to oak trees,
although several may cause scorching or spotting of leaves, and a few may
result in the death of the twigs they infest.
A healthy tree can support numerous galls without being seriously
harmed.
Galls have been used commercially in the preparation of gallic acid and tannic
acid, and are extensively employed in tanning and dyeing, and in the
manufacture of ink. It is thought that
medieval period ink was formed by boiling galls, and then
mixing the solution with oxidized iron.
The ink was rumored to actually darken with age. Galls have also been used medicinally, as an
astringent to treat a variety of ailments.