Most species of gall wasps attack specific tissues of the host plant, forming galls, a type of defense response, on roots, stems, twigs, leaves, buds, or flowers. Salivary secretions from the wasp larva stimulate the plant to produce the gall, so the insect actually modifies the plant's normal response to injury, resulting in food and shelter for the larva. Many leaf galls are red and white and resemble miniature stars, sea urchins, cones, cups, or saucers.
The first description of a gall wasp's life cycle was published in 1675 by Italian physiologist Marcello Malpighi, Pope Innocent XII's personal doctor. Life cycles of these wasps can be quite complex, and two alternating forms in the life cycle may be produced that differ widely in appearance. In species that exhibit such alternation of generations, different types of galls are formed on different parts of the host plant.
The California oak gall wasp forms "oak apples" on twigs and branches of Oregon oak and several other species of white oak. This wasp is very common from Mexico north into Washington. The adults are large for gall wasps, 3 to 4 mm (0.12 to 0.16 in) long, and brownish-red. The large galls can be seen on trees year-round. Oak apples are green when they form in the spring, and they resemble an apple: smooth outside and spongy inside. When cut open, several larvae can be found developing in separate, hardened chambers. By summer the galls become light brown. Old galls become blackened with sooty mold. These roundish galls may be as large as tennis balls. They form in clusters or individually and may remain on stems and twigs for several years. Round emergence holes can be seen in older galls that have fallen to the ground.
The California oak gall wasp lays unfertilized eggs and produces only female offspring. The adult wasps emerge and lay eggs in the fall. These eggs produce galls the following spring or summer, although some individuals may take more than a year to develop.
The galls made by gall wasps provide resources and refuge for many other creatures. These include parasites of the gall wasps themselves, weevils and their parasites, solitary bees and wasps, spiders, and other small arthropods.
Scientific classification: The gall wasps are classified in the family Cynipidae, order Hymenoptera. The California oak gall wasp is Andricus californicus.