A gall wasp’s larvae live inside plant-made growths called galls, including the round galls commonly found on oak trees. The gall is both a nursery and a food source for the developing larva.
A female gall wasp uses her ovipositor to place an egg in oak tissue. Chemical signals from the egg and young larva redirect the plant’s growth, causing surrounding cells to form a protective chamber. The larva feeds on nutrient-rich tissue inside this chamber before pupating and emerging as an adult.
Different gall wasp species create distinctive structures on leaves, buds, stems, roots, or acorns. The common oak gall wasp, Cynips quercusfolii, produces spherical galls about 2 centimetres across on the undersides of oak leaves. Other species produce woody, marble-like, or fuzzy growths.
These galls are often mistaken for plant diseases or insect eggs, but they are carefully engineered extensions of the oak itself. Some other insects, including parasitoid wasps, can invade the gall and attack the resident larva. Oak galls also have a surprising human connection: they were once a key ingredient in iron gall ink.