Amphibolips confluens (cynipid wasp)
There are 717 species of gall wasps attacking oaks in North America, many of which cause a variety of leaf galls. Leaf galls are caused by growth-regulating chemicals produced by the insects. These chemicals contain larvae.
The adults are small 4-winged, somber yellow to brown, ant-like wasps, their larvae are legless, without distinct heads.
The California live oak and valley oaks are favored and are found in the United States and California.
The symptoms include large, shiny, round gall below leaf petiole or mid-rib. Some galls as large as apples. When dry, spongy center is a mass of fibers radiating from larval capsule to thin, papery shell of gall. No apparent damage to tree, but fallen galls are damaging to lawn mowing equipment.
Their life cycles are not fully known. Two broods, one of only females and the other of both sexes. A single larva is found in each gall. Eggs hatch in late summer and larvae overwinter in young gall. Two-year cycle with adults emerging from gall the second summer.