Identifying Oak Galls: The "Hollow Bulbs" on Scrub Oak

Arizona resident finds 2-inch hollow bulbs on scrub oak twigs, each with a seed-like object suspended by fine hairs and an exit hole. They ask what these are, the insect species, and if there are multiple types.

What They Are

These are insect galls. Tiny wasps (cynipids) lay eggs on leaves or twigs and secrete chemicals that cause plant tissue to swell, forming a protective chamber. Larvae develop inside, then chew their way out – the hole is an exit, not an entry.

Impact and Management

• Galls cause no significant harm to the tree.
• No treatment is needed; populations fluctuate yearly with weather.
• Several gall wasp species exist; identification requires close examination of gall structure. See Penn State’s guide: Oak Galls.