Galls are formed when a plant is stimulated to develop irregular growths, caused by growth regulating chemicals produced by some insects or mites, or in some cases a fungus. Examples of galls include cedar apple rust and oak apple gall. These growths can occur on any part of the plant, including the leaves, bark, flowers, buds, acorns, or roots, however leaf and twig galls are often the most noticeable. In the case of an insect induced gall, the irregular growth produced by the plant provides both nutrients and protection for the insect. 

In order to take full advantage of the plant’s growth capacity, gall makers must act at a particular time of year. In the case of leaf galls, such as the wool sower gall pictured here, gall initiation begins just as new leaves are beginning to unfold in the spring. These pink-spotted, white cottony wool sower galls are about the size of a ping-pong ball and are produced by a tiny species of Cynipid wasp, Callirhytis seminator, and are most commonly found on white oaks. 

C. seminator eggs, which are laid in winter, hatch just as new leaves are appearing on the oaks in the spring. The newly emerged larvae secrete chemicals that stimulate the plant to develop the gall tissue. Interestingly, each puffy gall is actually a conglomeration of small hairy galls, each produced by an individual grub, joined at a common spot on a twig. Although I chose to let these galls remain undisturbed, if you pull one of these gall apart you will see small seed-like structures, each containing a developing wasp.