August 17th, 2021
Walking in the Willits Valley when the sky has returned to its beautiful blue color and the temperatures have decreased enough to be tolerable allows me some time to observe closely at what is going on out there. The dry grasses and ground are still startling to my eyes but as I take some respite out of the bright sunlight under one of the magnificent Valley Oak trees, Quercus lobata, I notice thatthis tree has many different and strange galls on its leaves. They are brightly colored in some cases and in many shapes from little pinkish red cones to giant hand-sized apple looking growths.
A gall is a growth induced by creatures such as midges, mites, aphids, flies, and even bacteria and viruses, though the number one gall producer on oak trees are the tiny mosquito-sized wasps known as the Cynipid wasps. These tiny wasps induce the tree to produce a growth by either depositing their eggs inside the plant tissue or by eating the tissue and causing the tree to release an auxin plant growth hormone. This gall, which is filled with soft tissue created by the tree, then provides a nursery, home, food, and protection for the larvae of the wasp. The study of plant galls and their inducers is called Cecidology. Looking at all the various forms galls can be found in and realizing that each one if from a different species of wasp, I think must be an interesting study for sure.
The largest gall found in the western United States is the Oak Apple Gall. It is found on Quercus lobata, the Valley oak, the trees I am inspecting now. This gall is induced by the Cynipid wasp, Andricus quercuscalifornicus. Scientists have only ever found females of this species and state that they reproduce parthenogenetically, laying eggs without mating. They lay their eggs in the fall and deposit them in the stem of the oak, adding a chemical that is growth inducing to the tree. These oak apples are not good to eat but have been used in many ways from making ink and dyes to astringent tonics for wounds. When I see a Valley oak with what looks like hundreds of galls it is hard to imagine it doesn’t hurt the tree, but the research has shown that galls do not hurt or interfere with the host plant’s processes.
A bizarre twist to this seemingly perfect picture of plant helping wasp is that there is a species of wasp called “inquilines” that invade the galls and steal the food from the Cynipid larvae. Then there are wasps called “parasitoids” that inject their eggs into the gall so its offspring can eat the plant tissue and sometimes the Cynipid larvae also. Finally, there are other wasps called “hyper-parasitoids” which then go after the parasitoid wasps and eat their larvae. Phew! I will never look at a gall again without thinking of the drama that may be going on in each one no matter the size!
Besides these large galls, there are Spined turban galls induced by the Cynipid wasp Antron douglasii and red cone galls induced by Andricus kingii. There are others so if you would like to learn more about these fascinating structures and the creatures that make them, I recommend getting the Field Guide to Plant Galls of California and the Western States by Dr. Ron Russo, or you can go to Joyce Gross’s web page for great photos, http://joycegross.com/galls_ca_oak.php
Not only was it fun and interesting finding so many different galls but spending time under the magnificent valley oaks I noticed that most of them were incredibly heavy with acorns. It looks like it will be a mast year, a bumper crop of these nutritious and important nuts. This happens every few years. Look around and see if your oaks are producing abundant acorns, a hopeful sign for us and the animals that depend on this food source.