I have found that in most cases it is relatively easy to key out a gall to species level, assuming you have collected a specimen. It is sometimes necessary to open up the gall to view the contents, but in many cases the external appearance of the gall will suffice. However, some mite galls are very tricky to identify (gall mite taxonomy appears to need further study) and oaks and willows both support a huge number of gall species, which can make identification rather taxing.
Looking at galls has also given me an introduction to the fascinating biology of gall-causers, some of which, like rust fungi and aphids, have alternating generations on different hosts at different times of year. I was surprised to learn that the wasp responsible for the familiar oak apple, Biorhiza pallida, also has a non-sexual (agamic) generation on the roots of oak trees.
|Oak apple gall|