Preface: A Gall Tour de Force
Virtually everything we know about horned oak gall development and management is drawn from Eileen Eliason’s (now Buss) Ph.D. thesis research conducted in the late 1990s in partnership with her major advisor, Dr. Dan Potter, Entomology Professor Emeritus, University of Kentucky. Their work is highlighted in the “Selected References” below. It remains a touchstone example of the rigorous research required to unravel the intricate dance between an insect gall-maker and its plant host.
Participants in the OGIA/OSU Extension Diagnostic Walkabout held this past Monday in Spring Grove Cemetery and Arboretum were treated to observing the “horns” popping through the surface of woody stem galls that give the Horned Oak Gall its common name. The stem galls are formed under the direction of the gall-wasp, Callirhytis quercuscornigera (syn. Callirhytis cornigera (Family Cynipidae)).
However, the horns on the stem galls are just the tip of the iceberg with this gall wasp. The life history is complicated, with the wasps alternating between leaf galls and stem galls.
The Tale of Two Galls
Horned oak gall wasps alternate back and forth between woody stem galls and fleshy leaf galls, with the wasps from the stem galls inducing leaf galls and the wasps from the leaf galls inducing stem galls. The gall wasp cannot develop successive generations using the same type of gall.
The fleshy leaf galls house the sexual (sexgen) generation, with males and females emerging from the galls. The woody stem galls house the asexual (agamic) generation, with only females emerging from the galls. The females from the stem galls produce fertile eggs without the services of males. This form of reproduction is called parthenogenesis. Some human males believe this form of reproduction is an evolutionary dead end, but some human females may have a different perspective.
In the animal world, the term heterogony is applied when an animal has alternating lifestyles and reproductive modes from one generation to the next. Recent research suggests that with cynipid gall wasps, heterogony is the rule, not the exception.
A Galling Behind-the-Scenes Sidebar: Gall-makers can only make undifferentiated (meristematic) cells bend to their will to grow a gall. Meristematic cells are like teenagers; they don’t know what they’re going to be until they grow up.
Once the cells “grow up” (differentiate) and become organized into genetically preordained plant tissue, they cannot be changed by the gall-maker. Gall-makers can’t hijack differentiated cells.
Gall-making wasps commandeer meristematic cells using chemicals that are an exact molecular match to plant hormones, or the chemicals are plant hormone analogs, meaning they act like plant hormones. The gall wasps inject the chemicals when they use their ovipositors (ovi = egg) to insert their eggs. The chemicals are also exuded by the eggs and in some cases the resulting immature gall wasp (larva = singular, larvae = plural).
The gall-directing chemicals turn plant genes on and off at just the right time to direct sometimes intricate gall growth. In my opinion, this is the true wonder of gall-makers!
Meristematic cells are found in the buds, root tips, and cambium of trees. Meristematic cells in the buds differentiate once to become leaf, stem, or flower tissue. This commonly occurs once during the growing season, in the spring. It’s why the leaf galls produced by the horned oak gall wasps are initiated in the spring.
Meristematic cells in the cambium and root tips are capable of differentiating throughout the growing season. Thus, galls that form from cambial cells can form anytime during the growing season. Horned oak gall wasps take advantage of this feature.
Back to the Galling Proceedings
The helpful illustration below was published by Eliason and Potter (2001). It will aid in following the complicated life history of the horned oak gall wasp.
The numbers in the text below match the numbers in the life history illustration.
1. Agamic (asexual) females lay eggs in leaf buds in the spring. The resulting leaf galls are simple fleshy structures appearing as elongated bulges along the veins.
2. Each gall houses a single gall wasp larva. It takes around 3 months for larvae to complete their development and pupate.
3. The leaf galls produced the sexual (sexgen) generation with males and females emerging from the galls. Both are excellent flyers.
4. Once mated, the females fly to oak stems where they use their ovipositors to insert eggs into the meristematic cambium along with the aforementioned plant growth-regulating chemicals. This initiates and directs the growth of the woody stem galls. The stem galls support the development of the asexual (agamic) generation, meaning the wasps that will eventually emerge from the galls are all females.
5. The stem galls may occur singly, or multiple galls may arise close together. The image below shows multiple, closely-packed galls on the stem.
Cutting open the galls will reveal organized horn-like structures embedded within a disorganized matrix of woody tissue. The embedded “horns” serve a critical function by housing and protecting female gall wasp larvae in a chamber at the base of the horns.
The gall wasp larvae have chewing mouthparts and feed on so-called “nutritive tissue,” which is constantly being resupplied by the tree. It’s like lounging about in a room with walls made of an endless supply of pizzas.
The size of the stem galls is correlated with the number of female gall wasp larvae developing within the galls. Small galls may only contain one or two larvae. Since each horn houses a single gall wasp larva, you can determine the number of new female gall wasps that will eventually emerge from the gall by counting the number of horns.
For example, the cluster of galls shown in the first image below only supports one gall wasp per gall. The second image shows a large horned oak gall supporting the development of numerous new gall wasps. I’ve always thought that heavily populated horned oak galls look like a medieval mace weapon.
The horns rise to the surface on 22- to 24-month-old stem galls. Once the horns break the gall surface, the gall wasp larvae enter a summer diapause, which is a type of developmental stasis.
The larvae wake from their diapause stasis in the fall to pupate, and new females emerge from the tips of broken horns the following spring. Thus, the horns popping to the surface this spring will provide the female gall wasps with access to the outside world next spring. The total time that the new wasps spent developing in the woody stem galls is around 33 months.
A sweet, nectar-like liquid oozes from the tip of the horns as they rise through the gall surface. The purpose of the liquid is not well understood.
Some cynipid gall wasp galls include extrafloral nectaries in their structure that ooze nectar to attract a “security detail” to protect the gall-maker from predators. This is shown in the pictures below of an Oak Rough Bullet Gall produced under the direction of the cynipid gall wasp, Disholcaspis quercusmamma. The nectar attracts some serious protection!
However, I’ve never observed insects imbibing on the sugary fluid seeping from horned oak stems galls. Trees with oak rough bullet galls are commonly buzzing with insects, while trees with horned oak galls are free of such protective attention.
It’s also been speculated that sticky liquid provides a sweet treat for newly emerged horned oak gall wasp females. However, the timing is off. The gall wasps housed within the current newly emerging horns will not emerge until next spring. On the other hand, stem galls are not synchronized. You will find immature galls devoid of horns occurring at the same time as 22-24-month-old and 33-month-old galls. Perhaps the liquid provides a shot of energy for females emerging from older galls.
Unlike the sexual generation females that emerge from the leaf galls, the asexual generation females that emerge from the stem galls are very poor fliers. They typically crawl from their galls to leaf buds where they insert eggs and chemicals to initiate the leaf galls.
The stem galls are considered “spent” once the gall wasps emerge. The spent galls are extremely woody, and they darken and become cracked as they dry out.
The club-like woody galls may have been useful to our ancestors.
Hosts with the Most
Horned oak galls are confined to members of the red oak group. In Ohio, I have most commonly found the stems galls on Shingle Oak (Quercus imbricaria), Pin Oak (Q. palustris), and Scarlet Oak (Q. coccinea), and occasionally on Northern Red Oak (Q. rubra). Various published host lists exclude scarlet oaks; however, the oversight may reflect inaccurate tree identification rather than gall wasp host preference. I’d welcome input through pictures from BYGL readers.
One thing that has become clear is that the amount of galling among trees of the same species planted near one another can vary significantly. It is not unusual to observe one tree that’s heavily galled while other oaks of the same species growing nearby are much less affected or even free of galls. There has been no research on horned oak galls that explains this common observation. As with much of what we think we know about galls, we’re left with speculation.
Wild-Haired Speculation #1: Perhaps the high degree of variability in the development of galls is associated with the inherent genetic variability within an oak species. In other words, some trees are genetically more resistant, while others are more susceptible.
Wild-Haired Speculation #2: Another possibility plays on the theme that gall growth and development requires a tightly choreographed dance between the gall-maker and its host. Is there a “founder effect” with gall wasps that are genetically best suited for utilizing a particular tree being selected over time? The successive generations of their progeny would then thrive and multiply to eventually produce a gall explosion.
Wild-Haired Speculation #3: A final possibility involves chemical communication. Are the gall wasps communicating through chemical signals that translate into "this tree is good eats," causing females to remain on the tree? Could the galls themselves exude volatiles that make the tree more attractive compared to the other trees?
Of course, heavy galling could be associated with all the above, none of the above, or just bad luck. Science has yet to provide an answer.
Galling Impacts and Management Challenges
We have no insecticidal tools that have been shown through published, non-biased research to be a viable approach to eliminating galling by the horned oak gall wasp. In fact, published insecticide efficacy trials targeting other gall-making wasps are almost nonexistent.
Thankfully, the vast majority of insect and mite galls found on oaks cause little to no harm to the overall health of their host trees. Horned oak galls are a possible exception; however, the impact varies among hosts.
Occasionally, the gall growth fully encompasses stems and disrupts the vascular flow. The portion of the stem beyond the gall may become starved of water and die. I’ve never observed the damage becoming so severe that it has killed trees. However, the canopy dieback may destroy the landscape value of heavily galled trees, making tree owners wish their galled oak would die.
While pruning to remove the stem galls may seem a viable option, it’s not a realistic option on large trees. It’s also a challenge with small trees because gall development is not synchronous. Leaf and stem galls are commonly found on the same tree at the same time.
This means the effectiveness of managing horned oak galls by pruning out stem galls is undercut by the leaf galls releasing new wasps to initiate a new crop of stem galls. Of course, constantly removing stem galls will eventually exhaust the wasp supply, but by then the tree may be whittled down to a single telephone pole-like stem.
However, one effective gall-management option involves a severe form of pruning known as “basal pruning.” In other words, trees that have proven to be highly susceptible are removed.
The image below shows a row of pin oaks (Quercus palustris) in southwest Ohio. I took the picture in 2013 and have been re-visiting the oaks over the years. The tree in the foreground has been heavily galled since I started observing the trees. The others remained gall-free until recently, with the development of a sparse collection of galls, but nothing nearly as dramatic as the number of galls found on the gall-magnet.
The image below shows that the gall-magnet has been removed and replaced with a honeylocust (Gleditsia triacanthos). The removal and replacement illustrate two approaches to gall management. Removal of the unsightly gall-magnet reduces the localized horned oak gall wasp population, and the replacement tree is outside of the wasp’s host range, which speaks to the value of plant diversity.
On the other hand, some trees appear to handle heavy galling with no appreciable impact. The images below show a shingle oak in Spring Grove Cemetery and Arboretum that I’ve been monitoring for several years. The first image shows the tree has been heavily galled; however, the second image shows no apparent dieback or canopy thinning. Indeed, over the past few years, the number of galls has been declining. The tree has never been treated, and I have no explanation for the galling reversal.
Selected References
Eliason, E.A. and Potter, D.A., 2000. Biology of Callirhytis cornigera (Hymenoptera: Cynipidae) and the arthropod community inhabiting its galls. Environmental entomology, 29(3), pp.551-559.
Eliason, E.A. and Potter, D.A., 2000. Budburst phenology, plant vigor, and host genotype effects on the leaf-galling generation of Callirhytis cornigera (Hymenoptera: Cynipidae) on pin oak. Environmental Entomology, 29(6), pp.1199-1207.
Eliason, E.A. and Potter, D.A., 2000. Impact of whole-canopy and systemic insecticidal treatments on Callirhytis cornigera (Hymenoptera: Cynipidae) and associated parasitoids on pin oak. Journal of Economic Entomology, 93(1), pp.165-171.
Eliason, E.A. and Potter, D.A., 2001. Biology and management of the horned oak gall wasp on pin oak. Journal of Arboriculture, 27(2), pp.92-101.
Melika, G. and Buss, E.A., 2002. Description of the sexual generation of Callirhytis quercuscornigera and a new inquiline (Hymenoptera: Cynipidae). Florida Entomologist, 85(4), pp.625-631.
Taft, J.B. and Bissing, D.R., 1988. Developmental anatomy of the horned oak gall induced by Callirhytis cornigera on Quercus palustris (pin oak). American Journal of Botany, 75(1), pp.26-36.
