Andricus quercuscalifornicus

The adult wasps are small, stout and brown, about 5mm long with clear wings nearly twice the length of the body. The antennae are filiform and about the same length as the legs.[2] Despite their size they are one of the largest species of Cynipid wasps.

The gall itself is a typical oak apple gall in appearance, roughly spherical and varies from greenish to reddish or orange depending on host, age and environmental conditions. The galls range in size from a 2–14 centimeters across and often contain multiple larvae as well as parasites and other species that form a mutual relationship by feeding off the galls themselves. The outer galls are hard in texture and are not easy to penetrate.

Andricus quercuscalifornicus can induce and inhabit the galls of a variety of oak species, but they are primarily found in the galls of the Valley Oak. Due to this, they are usually found in areas such as riparian forests, open foothill woodlands, valley savannas, plant groves, and suburban areas. The wasps typically induce galls on the twigs of the Valley Oak, but the inter-tree distribution of these galls can be influenced by a multitude of factors including shoot vigor and the genotype of each tree.[3]

Andricus quercuscalifornicus is believed to reproduce strictly by parthenogenesis, and no male specimens have ever been recovered.[4] The adult female lays eggs in the cambium layer of oak twigs during the fall using her ovipositor.[4] More vigorous twigs will have more galls. The eggs overwinter on the twig, and then hatch in the spring, usually in early April. The resulting larvae induce galls immediately, where they can seem to balloon almost overnight onto the tree. This is the point where most of the parasitoids enter the gall, while it is still soft and small enough for their ovipositor to reach the larvae.[3] After anywhere from a few weeks to two months,[4] the gall stops growing and begins to desiccate, turning tan or brown. The larvae pupate and then bore their way out of the gall in late summer or early fall and fly off to lay eggs in other trees. A few larvae overwinter inside the gall and don't emerge until the following fall. The reason for this is unknown.[5]

These wasps form an important role in the ecosystem, with more than 20 known species that are parasitoids, inquilines, and hyperparasites that live on its life cycle, while the galls form a persistent shelter for various forms of fungi as well as many other insects. Several birds are also known to feed from the galls and their inhabitants.