As Clarence and I finished our 10.8 mile hike on Rich Mountain, we found a curious object that raised lots of questions for which we had no answers. It looked like a 2-to- 3-inch diameter dense cotton ball with little russet-colored bumps dotting the surface. There was a hole on the underside with several seeds inside and part of a twig buried firmly in the mass.
Emailed photos produced immediate identification responses from botanist friends – an oak gall. A quick internet search revealed that it is a Wool Sower Gall caused by secretions of grubs of Callirhytis seminator, one of the many gall wasps (Cynipids). This particular wasp is specific to white oak trees and the galls only occur in spring. This gall is sometimes called an Oak Seed Gall because of the seed-like structures I saw inside which actually contain wasp larvae.
The wasp is a tiny creature just a few millimeters in length with a humped back, and she lays her eggs on a white oak buds. The eggs hatch in early spring just as new tree growth is beginning. Chemical secretions from the young grubs stimulate the plant to develop the gall tissue which provides protection and nutrition. Some gall wasps have alternate generations that develop on a different part of the plant, and the Wool Sower Gall wasp is probably one of those. One generation is sexually produced and the second is parthenogenetic (from females only). The offspring in any one generation will most closely resemble and produce galls like their grandparents.
An article on oak galls by Tony Bratsch, University of Illinois Extension, says over 700 species of gall wasps have been documented in North America. Galls can also be caused by mites, midges, and a few other insect types. Oaks seem to be favorite host plants for many. I’ve spotted Oak Apples, 2-inch round tan balls, and Acorn Plum Galls, smaller smooth greenish, mottled balls the size of a large marble, on trails in recent months and now I can add the Wool Sower Gall.