Although they may look like fruit on oak trees, the small round galls that can be found are not actually grown by the tree.
You may have noticed when looking at an oak tree small round balls hanging on the branches much like acorns. These balls are known as galls and are not actually fruits.
Galls are caused by parasitic insects and are actually growths. There are many species of gall wasps in North America that need oak trees.
Each species of gall wasp produces a different type of oak gall. Different types of gall look distinct and can be found on different parts of the tree.
Hard galls can be found on the twigs, with galls with colors of red, brown, green, or white on the leaves, and currant galls on the catkins. Galls change color from green to reddish during summer and then darker brown or black in the cold, winter season.
How Do Galls Grow?
Depending on the species of wasp depends on where the female lays her eggs. When they lay their eggs they also leave a substance which is similar to the growth hormone of the tree.
This substance allows the plant cells to multiply. The eggs grow inside the gall, developing into grubs. As the gall gets bigger so do the grubs inside, with each grub living in a separate chamber. Marble galls only contain one larva in a central chamber.
Two Life Cycles
The female wasps burrow into the fine roots of the tree to lay their eggs once they have finished mating in summer. The larvae then grows inside the root galls but does not mature until the following year in autumn or winter.
The young then come out of the gall between November and January. The young are always born female and then lay their eggs in the buds of the oak. The eggs do not need a male to fertilize them and can hatch on the twigs of the oak tree.
When spring comes around the galls containing the eggs grow. The galls are soft and spongy and contain lots of chambers where the larvae develop. The larvae grow into adults before they come out in late summer, effectively being born again from the gall. Small holes can be seen in galls when the insects have emerged.
The marble gall is a species of wasp which lay their eggs to mature on twigs of scrub oak. Marble galls only contain the larvae of one female wasp, which then lays its eggs in the buds of the Turkey oak. The galls develop over the winter and can be seen in March and April. When the wasp emerges they can be male or female, leaving a small hole in the gall.
Currant And Spangle Galls
Although they may look like redcurrants, the currant galls that can be found on male catkins are actually currant galls. Both male and female wasps grow in these before coming out to mate in June. The female then lays their eggs on the underneath of oak leaves.
Each egg causes a spangle gall to form, and as many as 50 can be seen on a single leaf. The galls fall to the ground in Autumn and with the protection of dead leaves, the young grow inside them. In April females emerge and crawl up the trees to lay eggs, causing the currant galls to form, starting the process again.