Tree insect problems

Gall insects – oak leaf gall

3. Oak leaf gall

Information on the identity, life cycle, and control of most of these wasps is sketchy. Numerous species of cecidomyiid and cynipid wasps cause the formation of galls on the leaves, leafstalks, and stems of oaks. Oak-apple galls, which are found on midribs and petioles of leaves, are typical. They are caused by several species of very small wasps, the most common being A. confluenta.

Life cycles of these insects probably involve alternate generations of different parts of the oak. In spring, oak-apple galls are spherical, green, and vary from 12 to 50 mm in diameter. A single white, legless, wasp larva can be found in the center of the gall, surrounded by a hard capsule which in turn is surrounded by a spongy mass. As the gall matures the spongy mass dries into fibres and the shell becomes light brown, thin, and papery.

Although leaf, leafstalk, and stem galls are common and sometimes unsightly, most do not seriously damage the host.