Borrowing on a statement made by Sinclair and Lyon in “Diseases of Trees and Shrubs” about maple tar spot, galls are the most obvious and least damaging of any abnormal growth found on trees. There are somewhere around 800 different types of arthropod galls found on oaks in the U.S. About 700 are produced by tiny wasps (order Hymenoptera) belonging to the family Cynipidae. Of those, only a handful represent a serious threat to the overall health of their oak hosts.
One of the most obvious types of cynipid galls currently rising on oaks in Ohio is the Oak-Apple galls so named for their resemblance to Malus fruit. The galls are a true wonder with some having surface imperfections that resemble those produced by apple pests.
There are over 50 species of gall-wasps that are known to produce oak-apples in North America and there are probably at least 10-15 distinct species of oak-apple gall-wasps found in Ohio. Of course, those numbers are a matter of guesswork because the geographical range of cynipid wasps remains poorly understood and the numbers also imply that all species are known.
A Wonderous Process
I teach six “Gall Laws” in my presentations on the topic. The First Gall Law: Galls are abnormal plant growths produced under the direction of a living gall-maker. They do not arise spontaneously; they are not a response to plant wounding or chemicals that do not involve a gall-maker.
Gall-making wasp females take advantage of undifferentiated meristematic cells to form both a home and food source for their offspring. Meristematic cells are like teenagers; they don’t know what they’re going to be until they grow up. Galls cannot be created from plant cells once they’ve differentiated into their final form; once they’ve “grown-up.”
It’s why leaf galls occur in the spring, but stem galls can arise at any time. Leaf galls are formed from meristematic leaf bud cells before the cells set sail on their way to becoming leaf tissue. The galls can’t develop once the cells reach port as integral parts of functional leaves. The same is true of galls that form on plant reproductive structures. Stem galls that arise from meristematic cambial cells can develop anytime the cambial tissue is active which can be in the spring, summer, or fall.
Research has shown that female wasps launch gall formation by injecting phytohormones along with their eggs to hijack undifferentiated cells. Under the influence of these chemicals, the cells that were originally destined to become flowers, stems, or leaves are set on a new course. The process continues with phytohormones arising from the eggs. Then the larvae remain at the helm to turn plant genes on and off at just the right time to direct the growth of a plant structure we call a plant gall.
Proof that oak-apple galls are constructed from leaf tissue can be seen in the image below. The gall is infected by the same oak anthracnose fungus behind the dark brown to black necrotic symptoms at the tips of the oak leaf.
Thus far, no researcher has ever produced a gall on oaks or other plants without the assistance of a gall-maker. No one has figured out exactly how the phytochemicals turn plant genes on and off to produce such intricate plant structures that are specific to the gall-maker species. Just imagine the plant physiological stories that could be revealed if the process were unraveled.
Oak-Apple Gall Development and Structure
Cutting open plant galls to reveal the internal structure further illustrates the wonders of the gall-making process. The internal structure of oak-apple galls includes a central seed-like chamber housing a single wasp larva. The chamber may be surrounded by succulent tissue, not unlike the flesh of an apple, or you may find delicate white fibers radiating from the larval chamber.
Wasp larvae have chewing mouthparts; so, what do the gall-wasp larvae eat? They don’t eat themselves out of house and home by consuming the gall from the inside out. Instead, the inside of the gall chamber is lined with specialized cells called nutritive tissue which is constantly being replaced as it is consumed by the gall-wasp larva. Imagine lounging in a room with pizzas constantly emerging from the walls.
Plant galls commonly change their appearance as they develop (= mature) which can present an identification challenge. Oak-apple galls range in size at maturity (= they stop expanding) from 1/2 - 2" in diameter. However, it can be difficult to determine at this time of the year whether or not the oak-apple is an inherently small gall or a large gall that’s still developing.
One way to distinguish between different sized oak-apples is to carefully cut open the gall to assess wasp development. Finding a pupa means the gall has reached its full size. Unfortunately, my gall-cutting prowess commonly produces only half of a wasp larva or worse, a macerated larva.
Once the developing oak-apple gall wasp pupates and leaves the gall-building as a newly minted wasp, the gall turns brown, and the true size of the gall is revealed. The third image below shows two different oak-apple galls based on the size of the mature galls.
Unfortunately, arriving at a final identification of the gall-wasp species can be a challenge. Keep in mind that gall-wasp taxonomy is based on morphological features of the adults, not on the structure of their galls. Scientific papers naming gall-wasp species invariably include clear descriptions of morphological features of adult males and females with minimal to no descriptions of their associated galls
Also, there is a general lack of research-based information on oak-apple gall identification. In fact, no rules govern what makes a round gall an “oak-apple” in the first place. The “oak-apple” name carries no taxonomic weight although wasps in the cynipid genus Amphibolips appear to dominate the oak-apple gall-makers.
On the upside, delving into the fascinating but often hidden world of gall-making wasps guarantees you will continually learn something new. The key is to embrace the unknown; to become comfortable with constantly navigating uncharted waters because your final destination is well worth the voyage.
Indeed, oak galls have even captured the attention of luminary naturalist Sir David Attenborough: