There is a young Bur Oak (Quercus macrocarpa) growing outside the front door of Willow House. I was examining it recently to see how the acorn crop was shaping up. I did find acorns, such as that pictured above, but I also found clumps of small, hard round growths along the tree stems. Being familiar with the various and sometimes puzzling forms galls can take, I suspected I was looking at a new gall that I was unfamiliar with. Sure enough, a quick bit of research proved this suspicion to be correct.
I was surprised to learn that oaks host more types of gall-makers than any other plant. Of the nearly 1500 North American galls identified to date, more than half of them are associated with oaks. There are galls found on all parts of the oak tree: leaves, twigs, roots, flowers and even fruit. The largest number of varieties form on the leaves. Twigs come in second.
Most oak galls are formed by the Cynipid family of wasps, sometimes called gall-wasps. These cool wasps are known to have alternating generations, one generation without males, the next with both sexes. Dr. Alfred Kinsey, the sex researcher, began his career studying cynipids.
Similar galls can be formed by a variety of species. The hard, round galls that I noticed along a twig are called bullet galls. Over fifty types of bullet galls are known. Among the most common species forming bullet galls are Oak Rough Bulletgall wasps (Disholcaspis quercusmamma). The bullet galls are pictured below.