california gall wasp

Andricus quercuscalifornicus

Caution: There may be other wild species of this genus not yet included in this guide.

forms on branches of white oaks (section Quercus), usually valley oak (Quercus lobata)
a large tan gall that looks similar to the fruit of california buckeye (Aesculus californica)

oak gall is attached to the side of a branch, may be on a tree with leaves, may be lumpy, has no lines on which to split
buckeye fruit hangs from the tip of a drooping twig, is typically seen only on leafless trees, and may be partially split in half to show a shiny brown layer beneath

Chris’s observations: 28 (23 are research grade)

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Bay Area species: iNaturalist