Fagaceae (beech family)
The largest category of galls exist on oaks. Gall producers are attracted
to oaks because they are long-lived, and numerous species are widely distributed
in North America. Galls can occur on all parts of an oak, including the
fine roots, old root bark, bark of trunk and large branches, leaves, buds,
acorns, and fruits.
Different types of oak galls:
Root or Subterranean Galls
Root galls appear right below the surface of the ground or at the crown
between the ground and the root.
Branch and Twig Galls
Most of these galls are gall wasp cells. The irregular swellings on the
branches develop from the tender inner bark and soft outer wood. These
galls are compound and multi-cellular. Some twig and branch galls, like
the gouty oak gall, can kill the structure. Others split the bark and
produce bud-like structures. Some galls even secrete sweet substances
to attract bees, and use that as a way to spread itself.
Oak leaf galls are large and highly variable. They can appear on stems, petioles,
bases, midveins, lateral veins, and even in between veins. There are two
groups of leaf galls, the spring/vernal galls and the fall/autumn galls.
The spring/vernal galls attack the soft tissue of the leaf and the gall
shrivels as the wasps leave in early summer. These insects reproduce sexually.
The fall/autumn galls appear in midsummer on full-grown leaves. These
galls develop slower, and are on thicker, firmer plant tissue. There is
also more nourishment for the larva. Eventually, these leaves fall to
the ground, and adults emerge in the spring. This type of insect reproduces
asexually. Although leaf galls are numerous, the position, type, and method
of production helps identify the insect inhabiting the gall.
These are a specific type of leaf galls that appear like oak apples, but
are smaller. The center of this type of gall becomes free after awhile
and is able to roll around inside the thick outer walls of this gall.
The gall usually forms around summertime. Upon maturity, it drops to the
ground. There, it is nourished by osmosis from the thick nutritive inner
Oak buds are convenient places for galls. There is no need to change its
structure too much to change the point of origin of the gall.
Oak apples are common structures seen on oaks. They emerge directly from the
bud. Although it looks like a fruit, the oak apple is actually a deformed
leaf hanging by its stem or petiole. Gall wasps form oak apples. Inside
the oak apple is a central cell, spongy nutritive fill, and the outside
shell. They usually range from 1 to 2 inches.
Flower and Catkin galls
Some oaks flower for a very short period of time. Some galls time their
life cycle in a way so that when they emerge from their gall, they have
enough time to drop their eggs into the blossoming oak flower and grow
up before the catkins drop.
Usually plum and pip galls appear on acorns.
Click 'compare galls' link to see more oak galls and to compare them with other galls.
Below are some examples of oak galls: