ON THE GROUND, AGAIN
Carol Ann McCormick, UNC Herbarium assistant curator and keen observer of nature, has been looking at the ground, too…
As I walked through an oak-hickory forest in southeastern Alamance County last weekend (October 22, 2011), I noticed some leaves with interesting … well, for lack of a better description, “chain thingies” on them. Inspired by Carrboro Citizen FLORA columnist Ken Moore to “take a closer look,” I picked up a few of the leaves, to make more observations as I continued my walk, and to seek expert assistance in identifying the cause of the chain thingies.
My first observation is that I found chain thingies only on red oak leaves—which is not to say that they do not occur on white oaks, only that I did not observe any on my 90-minute walk. Second, the number of chain “links” varied from leaf to leaf: some had two dozen, some had closer to a hundred. Lastly, the chain thingies were always along the veins on the underside of the leaf.
I brought my leaves into the UNC Herbarium, and volunteer Rob Coffin opined that the chain thingies are galls—abnormal growths of plant tissue caused by a parasite (insect, fungus, or bacteria). Using the scholar’s tool of choice (Google™), I found David Stephan, Extension Specialist in the Department of Entomology at N.C. State University, and the excellent “Plant Disease and Insect Clinic” website (http://www.cals.ncsu.edu/plantpath/extension/clinic/#).
Steve Seiberling, photographer extraordinaire in the Herbarium, took photos of the leaves I’d collected, and I emailed the images to David Stephan. Within minutes, Mr. Stephan replied: “Although most of the critter galls you find on oaks are caused by cynipid gall wasps, there are some caused by species in other groups. Your galls are from cecidomyiid gall midges, either Contarinia sp. or Macrodiplosis sp. An uncertain number of species in both genera produce galls on red oaks. In addition, gall midges of the genus Polystepha also are found on red oaks in NC, but their galls don’t have the slitlike opening that distinguishes the other 2 genera.”
A Google image search on Contarinia yielded Pear Midge (Contarinia pyrivora, Pyrus being the genus name for pear trees), bottom left photo above, and Pea Midge (Contarinia pisi, from Greek pisum= pea), bottom right photo. While neither is the midge that causes oak galls, you get the general idea that the critter is winged and tiny.
Be on the lookout for these and other galls as you enjoy winter woodlands and fields!
—Carol Ann McCormick, Assistant Curator, UNC Herbarium
Oak leaves with galls photo by Steve Seiberling, UNC Herbarium
Contarinia pyrivora photo by Magnus Gammelgaard: http://plante-doktor.dk/skadedyr.htm
Contarinia pisi image: http://www.agroatlas.ru/en/content/pests/Contarinia_pisi/