Galls on Oak There are a variety of gall-forming species of
small wasps that commonly infest oak, Quercus spp ., trees in
Pennsylvania. Most leaf galls on oak cause little or no harm to
the health of a tree. However, twig or branch galls may cause
injury or even death to a heavily infested tree. Two common
species of twig gall-producing insects are the horned oak gall
wasp, Callirhytis cornigera (Osten Sacken), and the gouty oak
gall wasp, C. quercuspunctata (Bassett). These species are in
the insect family Cynipidae. Both the horned oak gall wasp and
the gouty oak gall wasp are known to occur from southern Canada
to Georgia. Each of these galls may be diagnosed by their unique
characteristic size, shape, and color. Figure 1. A horned oak
gall on a twig. Galls are abnormal plant growth or swellings
comprised of plant tissue. Galls are usually found on foliage or
twigs. These unusual deformities are caused by plant growth-
regulating chemicals or stimuli produced by an insect or other
arthropod pest species. Th