Ever wondered what those colorful growths on your oak tree were? A variety of wasps, including the most common Cynipid wasp, lay their eggs on twigs and leaves and secrete a growth regulating chemical during the process. These chemicals interact with chemicals in the plant tissue (auxins, cytokinins, gibberellins, etc.), which results in swelling, or galling. When the eggs hatch in twigs, the larvae can cause additional swelling for up to two years before the adult wasps chew out of the gall, leaving exit holes as evidence. There is a large variety in types of galls, from dense woolly/fuzzy galls, to bullet and horned galls, and over 80% of those reported in the US are hosted on different Oak species.