This is the 5th BYGL Alert this season dedicated to wasp galls on oaks. That may seem excessive, but it’s estimated that around 800 types of galls may be found on oaks in the Nearctic which is the part of the world that includes all of the U.S. and parts of Mexico and Canada.
Of that number, it’s further estimated that around 700 are produced by tiny wasps (order Hymenoptera) belonging to the family Cynipidae. So, if we remain focused on gall wasps, we’ve only scratched the surface.
The galls I’m highlighting in this Alert are produced by cynipid wasps belonging to a single genus, Andricus. The galls began their development some time ago this season; however, they are now becoming evident as they reach their maximum size, and in some cases, are already starting to change colors as they decline.
A Gall Gallery
Oak Rosette Gall (A. quercusfrondosus)
During a diagnostic walk-about training earlier this month on the OSU Columbus Campus, Dave Shetlar (Professor Emeritus, OSU Entomology, the “Bug Doc”) showed the participants these galls he collected from a chinkapin (chinquapin) oak (Quercus muehlenbergii). Some sources indicate the galls are specific to this oak species which is supported by Dave’s find as well as some of the pictures illustrating this Alert that were taken on chinkapins in southwest Ohio.
The gall-making wasp hijacks a bud to direct the oak to produce a dense proliferation of tiny leaves. I’ve always thought the galls look a bit like tiny pineapples. The miniature leaves change colors as the galls “mature” until they become dark brown to black. The galls remain attached to the tree throughout the winter and the gall wasps emerge in the spring.
Occasionally, we find several types of galls on the same oak tree. The image below shows an oak rosette gall with a Round Oak Bulletgall, induced by the cynipid wasp Disholcaspis quercusglobulus, sprouting from another bud making the bulletgall look like it’s part of the rosette gall structure. You can read more about this bulletgall and what happens when a gall maker fails to “pay” for protection by clicking on this hotlink: https://bygl.osu.edu/index.php/node/2034
Tentacled Oak Acorn Gall (A. incertus, formerly A. fimbriatus)
Few galls have a common name approved by the Entomological Society of America (ESA), and this gall is no exception. I’ve given this gall this common name based on the tentacle-like growth that appears on oak acorn caps. Since the gall growth is confined to acorn caps, these galls cause no harm to the overall health of their tree host. Of course, this applies to the vast majority of the galls found on oaks and other plants.
As noted above, “tentacled oak acorn gall” is a made-up common name. This means there’s room for other proposed common names for this gall as demonstrated below.
Oak Petiole Gall (A. quercuspetiolicola, formerly A. concolorans)
As the common name implies, this gall typically arises from the leaf petiole. However, it may also appear on the central leaf vein or major lateral veins.
Gall formation typically results in deformed leaves. Heavy galling can potentially affect the overall health of small trees by reducing the total area for photosynthesis.
Clustered Oak Midrib Gall (A. dimorphus, formerly Cynips dimorphus)
The “cluster” in the common name refers to an aggregation of individual teardrop-shaped galls that arise from a hijacked bud. Each gall houses a single wasp larvae. “Oak Grape Galls” is an alternate common name that sometimes appears in online references and is based on the galls looking like a cluster of grapes.
The galls change colors from tan to deep red as they “mature.” This is the first gall in this gall gallery that detaches at the end of the season and drops to the ground where the gall-maker spends the winter. The galls typically detach with the initiation of fall leaf senescence.
Oak Lobed Stem Gall (A. quercusstrobilanus, formerly Cynips strobilana)
This is another wasp gall that’s composed of a collection of individual galls arising from a leaf bud. The "strobilanus" in the specific epithet of the scientific name is derived from the Greek “strobilo” which means "cone.” An alternate common name for this gall that occasionally appears in the literature is the “Pine Cone Oak Gall.”
The wedge-like galls are tightly packed together, and the entire gall structure may appear ball-like ranging in size from less than 1” to almost 2” in diameter. Each individual gall houses a single wasp larvae.
Occasionally, multiple buds are hijacked along a stem giving rise to a more elongated gall structure. The true nature of the structure isn’t revealed until the clustered individual “kernels” are broken off to show multiple points of attachment.
The hard, corky galls progress through a range of colors as they mature from pink or purplish-red, to yellow, and finally dark brown. Although the galls may remain attached well into the winter, this is another oak gall that eventually detaches from the host and drops to the ground.
Oak Lobed Leaf Gall (A. nigricens, formerly Cynips nigricens)
The overall structural arrangement of these galls is similar to the lobed stem galls with a collection of individual galls clustered together. However, the leaf galls are so distorted and jammed together that it’s sometimes hard to discern the individual galls in the cluster.
I’ve always thought that collectively, the cluster of gnarled galls look like a wad of chewed bubble gum (oak bubble gum gall?). The individual kernel-like galls arise from a main leaf vein. Each gall houses a single wasp larva.
The color of the galls tends to remain within the range of browns; however, they may occasionally appear pinkish-red. I’ve found these galls on fallen leaves, but I don’t know if that’s normal or if the individual galls commonly detach from the leaves before leaf drop in the fall.
Oak Lobed Petiole Gall (Andricus sp.)
I’ve never been able to identify this gall. I’m confident that it’s a cynipid wasp gall and I’m placing it in the Andricus genus based on the structural features as well as its location on the oak host. Of course, there’s a probability the wasp has not yet been identified by science.
Although the clusters of galls look similar to the oak lobed stem galls, the Devil’s in the details when it comes to gall identification. Of course, even a cursory look should reveal they are not the same.
Aside from their bright red coloration, the galls have a roughened top and are also much more regularly shaped compared to the stem galls. Another key difference is that the galls arise from the petiole rather than a bud on the stem or a leaf vein.
Bonus Content: A Brief Visit to the Gall Factory
The following is a slightly modified excerpt from my posting las week on Rough Oak Bullet galls produced under the direction of the cynipid wasp, Disholcaspis quercusmamma. So, if you read that posting about this tiny wasp with a giant name, you don’t need to read further. Otherwise, the following is why I find plant galls to be so fascinating.
Wasp gall-makers use chemicals injected with the eggs or exuded from the eggs to turn plant genes on and off at just the right time to direct gall growth. Even more remarkable, the exact genetic levers pulled by the wasp are so species-specific that the wasp species can be identified by its gall without seeing the wasp. That’s what I’m doing in this Alert.
Of course, the gall-maker must use plant cells that have not yet become part of an organized plant structure. In other words, galls can’t be formed from plant tissue once the plant’s inherited genetic script has been completed and the cells are part of a functional leaf or new stem.
Gall-makers target “undifferentiated” cells that are found in meristematic tissue. The cells are like teenagers; they don’t know what they’ll be until they grow up. Meristematic tissue is found in buds, at the tips of roots, and in the thin cambial ring located between the phloem and xylem. Indeed, the cambial meristematic cells will differentiate to become xylem (wood) to the inside and phloem to the outside which is how trees increase their girth.