Ron Wilson

Ron Wilson

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Oak Apple Gall - Buggy Joe

An overly generous conference moderator once introduced a talk I was about to give titled, “Tree Galls:A Management Conundrum,” as being presented by a “gall expert.” I looked around to see if they’d made a last-minute speaker substitution.

An expert is somebody who is more than 50 miles from home, has no responsibility for implementing the advice he gives, and shows slides.” – a pre-Zoom quote attributed to political gadfly Edwin Meese

My presentation included six “Gall Laws” relative to plant galls produced by insects. The First Gall Law was: 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.

A Wonderous Process

I commonly use galls produced by wasps belonging to the family Cynipidae to illustrate the First Gall Law. Gall-making wasp females take advantage of undifferentiatedmeristematiccells 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 form in the spring, but stem galls can arise at any time. Leaf galls begin to be 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. Stem galls that arise from meristematic cambial cells can develop anytime the cambial tissue is active; 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.

An expert is a man who tells you a simple thing in a confused way in such a fashion as to make you think the confusion is your own fault. – William Castle, American filmmaker

Cutting open plant galls to reveal the internal structure further illustrates the wonders of the gall-making process. For example, so-called “oak-apple” galls are produced by cynipid wasps hijacking buds. They are so named because they resemble apples. The galls are a true wonder with some having surface imperfections that resemble those produced by apple pests. Oak-apple galls are currently appearing on their namesake hosts in Ohio.

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 appearing elsewhere on the oak leaf.

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.

Always Doubt “Without a Doubt”

The Fourth Gall Law that I presented in my gall presentation was: Gall structures and locations on the plant are so species-specific; the species of the gall-maker can be identified by the gall structure alone without the need to see the gall-maker itself. I believe this gall law remains largely true; however, the identification of an oak-apple wasp species based entirely on gall structure should never be prefaced “without a doubt.”

An expert is a person who avoids the small errors while sweeping on to the grand fallacy.” - Steven Weinberg, Nobel laureate in Physics

I once believed that I was on solid ground with identifying oak-apple gall culprits to species. Eventually, I recognized the grand fallacy of my overconfident self-assurance. While I may have been occasionally correct, I’ve been increasingly confronted with oak-apple galls that “fit” with descriptions of the handiwork of multiple cynipid wasp species meaning that narrowing down a wasp ID based on gall structure is often a matter of guesswork.

Expert: a man who makes three correct guesses consecutively.” – Laurence J. Peter, educator best known for the formulation of the “Peter Principle.”

There are over 50 species of gall-wasps that are known to produce oak-apple galls in North America and there are probably at least 10-15 distinct species of oak-apple gall-wasps found in Ohio. Of course, these numbers are also a matter of guesswork because the geographical range of cynipid wasps remains poorly understood and these numbers also imply that all species are known. 

Oak-apple galls range in sizeat maturityfrom 1/2 - 2" in diameter. Their range in size presents another ID challenge. The only way to know whether or not the oak-apple is an inherently small gall or a large gall that’s still developing is tocarefullycut open the gall to assess wasp development. Unfortunately, my gall-cutting prowess commonly produces only half of a wasp larva or worse, a macerated larva.


Another ID challenge is the 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. While most so-called oak-apple galls are produced by cynipid wasps belonging to the generaAmphibolipsandAndricus, I occasionally run across online references to “oak-apples” being produced by wasps belong to other cynipid genera.

Keep in mind that wasp taxonomy is based on adults, not larvae within a gall. 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

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.

Isaid that an expert was a fella who was afraid to learn anything new because then he wouldn't be an expert anymore.” – Harry S. Truman, 33rd president of the United States

The bottom line is that wasp gall identification is in a constant state of flux meaning that what we think we know today will probably change tomorrow. Of course, that’s science.

An expert is one who knows more and more about less and less.” – Nicholas M. Butler, Nobel Peace Prize laureate

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