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Although they may look like fruit on oak trees, the small round galls that can be found are not grown by the tree.

When looking at an oak tree with tiny round balls hanging on the branches, you may have noticed it, much like acorns. These balls are known as galls and are not fruits.

Galls are caused by parasitic insects and are growths. Many species of gall wasps in North America need oak trees.

Each species of gall wasp produces a different type of oak gall. Other types of gall look distinct and can be found in other tree parts.

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 darker brown or black in the cold winter.

Galls on oak trees infographic

Which wildlife lives in a birch wood?  Find out here in this article I wrote.

How Do Galls Grow?

The species of a wasp depends on where the female lays her eggs. When they lay their eggs, they also leave a substance similar to the tree’s growth hormone.

This substance allows the plant cells to multiply. The eggs grow inside the gall, developing into grubs. As the gall gets bigger, so does the food inside, each in a separate chamber. Marble galls only contain one larva in a central room.

Two Life Cycles

The female wasps burrow into the tree’s fine roots to lay their eggs once they have finished mating in summer. The larvae then grow inside the root galls but do 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 hatch on the oak tree’s twigs.

When spring comes around, the galls containing the eggs grow. The galls are soft and spongy and have many 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.

Want to know which wildlife you can see in coniferous forests?  Find out here.

Marble Gall

The marble gall is a wasp species that lays its eggs to mature on scrub oak twigs. 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, it can be male or female, leaving a small hole in the gall.

Current And Spangle Galls

Although they may look like redcurrants, the currant galls found on male catkins are currant galls. Both male and female wasps grow in these before coming out to mate in June. The female then lays their eggs on underneath 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.

Want to know what lives in tree holes?  Find out here.