Oak Bullet Gall. This is only one of hundreds of stem galls found on the twigs of oak trees. Bullet galls are hard, pea-sized spheres clustered along the side of the twig. Horned Oak Gall. This very hard, woody expansion of the oak twig is irregular in shape and as large as a golf ball.
Tubular projections grow from the mature gall when the gall-maker wasps are ready to emerge.
This common gall from the eastern United States almost changed the course of world history. The irregular wartlike galls often cover the leaves of grapes. No harm comes to our native, wild grapes. However, the phylloxera insect was accidentally introduced from the United States into the grape growing regions of France in 1859 and by 1870 had brought near-collapse to the French economy. Americans eventually provided the practical solution that rescued the French wine industry from crushing disaster.