Cynipids are tiny wasps known for their ability to create plant galls. The wasp lays its egg inside the plant tissue. The larva secretes substances that drastically alter the plant tissue for the benefit of the wasp. The outer appearance of the gall is characteristic of the wasp species.
The gall wasp larva is located in a chamber in the center of the gall. The larva secretes signals that induce the plant to make a layer of nutritive cells that surround the larval chamber. As the larva grows and feeds on the nutritive cells, adjacent cells will grow into new nutritive cells. Typically, the larval chamber and nutritive cells are separated from the rest of the plant by a layer of tissue. The outer tissues of the plant surrounding the gall are high in tannins and other plant defensive chemicals. The hard surface is deterrent to many predators and interlopers. However, some parasitoids are able to penetrate the gall defenses.
The nutritive cells provide the gall wasp with the high protein it needs to rapidly grow and develop. Gall wasps are commonly found on oak trees. Look for them this summer.