Bob Sommer’s article about galls in our Fall 2013 issue of the PCT Communicator is a staff favorite. We’re always amazed by the variety and abundance of galls along the trail.
Red apples hanging down from an oak tree on the PCT? Doesn’t make sense until you realize that these aren’t really apples. They are galls. The same is true of the enlarged scarlet edges on some manzanita leaves.
Searching for a safe place to deposit her eggs, a tiny wasp may choose the leaf of a coast live oak (Quercus agrifolia). Eggs injected into the leaf cause the plant cells surrounding the eggs to become enlarged or multiply rapidly (as in a human cancer) to create a gall, which is the plant’s defensive reaction to the invasion.
Here are some extra photos that didn’t make it into the article due to lack of space.
Striking crimson galls produced by Leaf Gall Aphids (Tamalia coweni) on manzanita.
Floral bud galls induced by the Rosette Gall Wasp. In Spring the galls are green and later in summer will turn brown. Note the escape holes made my the grown larva and nearby presence of different types of galls.
Galls stimulated by the Twig Gall Wasp (Disholcaspis mellifica) which release honeydew that attracts ants (shown in photo) who keep predators away from the galls.
Oak Apples and leaf painted using oak gall ink.
Bob Sommer is Distinguished Professor Emeritus at the University of California, Davis. He is an amateur natural historian and writes the “Easy Edibles” column for Mushroom, the Journal and is co-author of Field Guide to Mushrooms of Western North America published by UC Press in 2013. He and his partner, Barbara Sommer, have hiked pieces of the PCT for three decades and have been PCTA members for 10 years. For the full article and more gall photos, see our Fall 2013 issue of the PCT Communicator.