Participants in an Ohio Certified Volunteer Naturalist (OCVN) Field Day walk-about this past Saturday at the Cincinnati Nature Center were treated to observing the “horns” that give the Horned Oak Gall its common name popping through the surface of a gall. The image below shows the group's gallenthusiasm for witnessing this fascinating gall-event.

The woody oak 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-maker wasp. The wasps also produce leaf galls, and that is where things get a bit complicated [get coffee].

Golden Gall Rules

Here are a few rules that every gall-making wasp (or midge fly) must follow; no exceptions.

RULE #1: 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. They inject the chemicals when they insert their eggs. The chemicals are also exuded by the eggs and in some cases the resulting immature gall-maker (larva).

RULE #2: Meristematic cells are found in the buds, root tips, and cambium of trees. The meristematic cells in the buds differentiate in the spring. This means that the formation of galls that develop from buds, such as leaf and flower buds, starts in the spring before the cells differentiate. The galls can’t form later in the growing season.

However, 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. This feature is taken advantage of by horned oak gall wasps.

The Complicated Lives of Horned Oak Gall Wasps [drink more coffee]

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 wasps, heterogony is the rule, not the exception.

Horned oak gall wasps alternate back and forth between large, obvious woody stem galls and tiny 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 wasps that arise from the two different types of galls also have different types of reproduction. The wasps that develop in the leaf galls reproduce sexually; males and females emerge from the galls. This is the sexual generation.

The leaf galls are simple fleshy structures appearing as elongated bulges along the veins. Each gall houses a single immature wasp (larva) and it takes around 3 months for the larvae of the sexual generation to complete their development and emerge as adults.

The females from the leaf galls are good fliers. They fly to oak stems where they use their ovipositors (ovi = egg) to insert eggs into the cambium along with the aforementioned plant growth regulating chemicals. This initiates and directs the development of the stem galls. I’ve always thought horned oak galls look like a medieval mace weapon.

The wasps that develop in the stem galls are all females; there are no males. This is the asexual generation. Reproduction by insects and other animals without the services of males is called parthenogenesis.

It takes around 33 months for the wasps to complete their development in the stem galls. The galls are called “immature” during wasp development.

Cutting open the immature 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 wasp larvae in a chamber at the base of the horns.

The 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 horns rise to the surface at around 22 – 24 months after the initiation of the stem gall. Once the horns break the gall surface, the wasp larvae enter a summer diapause which is a type of developmental stasis.

The size of the stem galls is correlated with the number of wasp larvae developing within the galls. Small galls may only contain one or two larvae whereas others may support over 100 new adults. Since each horn houses a single wasp larva, you can determine the number of new wasps that will eventually emerge from the gall by counting the number of horns.

The wasp larvae wake from their diapause stasis in the fall to pupate and new adults emerge from the tips of broken horns the following spring. Thus, the horns popping to the surface this spring will provide the female wasps with easy access to the outside world next spring.

Unlike the sexual generation females, 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 wasps emerge. The club-like spent galls are extremely woody and they darken and become cracked as they dry out.

A Sweet Conundrum

If you look closely at the tips of newly emerged horns this spring, you may see tiny glistening droplets oozing from the tips. The liquid is sticky and slightly sweet (personal taste test).

The exact function of the sweet liquid is not known. Some oak galls include extrafloral nectaries that attract stinging and biting insects such as wasps and ants that serve as the security detail protecting the developing wasp larvae. However, I’ve never observed ill-tempered stinging insects gathering on horned oak galls and scientific studies have not revealed the development of these plant organs within horned oak galls.

A study published in 1988 showed that horns accumulate starches. Starch molecules attract water, so the droplets may result from starch molecules accumulating water to a point where the tips of the horns rupture releasing a sweet slurry. I’m speculating, but this may be the way the horns break open so the females can exit the galls next spring.

Perhaps the oozing droplets also serve as an energy drink for the newly emerging parthenogenetic females. The woody stem galls are not synchronized. It’s common for galls with dripping horns to be located near galls with the tips of the horns broken open to release the females. The females may sip the sweet liquid to power their crawl to the leaf buds where they lay their eggs.

However, I must stress that we simply do not know the source and function of the droplets of sweet liquid oozing from the tips of the newly emerged horns. I’ve simply provided a few possibilities based on rampant speculation on my part.

Gall Impact and Management

The vast majority of the insect and mite galls found on oaks cause little to no harm to the overall health of their host trees. However, horned oak galls are a possible exception.

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.

Unfortunately, gall management is problematic. We have no insecticidal tools that have been shown through 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.

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 on 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.

One thing that has become clear is the amount of galling among trees of the same species planted near one another can be highly variable. It is not unusual to observe one tree that’s heavily galled while other trees 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. So, once again, we’re left with speculation.

Wild 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 Speculation #2: Another possibility plays on the theme that gall growth and development require a tightly choreographic dance between the gall-maker and its host. Is there a “founder effect” with 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 Speculation #3: A final possibility involves chemical communication. Are the 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.

Regardless of the reason, one effective gall-management option is to remove trees that have proven to be highly susceptible. 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 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.

Finally, it’s important not to apply basal pruning too quickly in response to horned oak galls. I’ve been photographing the shingle oak (Quercus imbricaria) shown below for years. The tree is located in southwest Ohio and despite heavy galling, there’s been no significant branch dieback. The extent of the galling is only revealed when leaves drop in the fall.

Another interesting observation is that the number of new stem galls has been steadily declining over the past few years. The tree has never been treated with an insecticide.

I have no explanation for the lack of stem dieback or the reversal in stem gall development. There are too many viable possibilities to speculate. Perhaps it’s just good luck.

Standing on the Shoulders of Giants

Virtually everything we know about horned oak gall development and management comes from the Ph.D. thesis research conducted in the late 1990s by Eileen Eliason (now Buss) in partnership with her major advisor, Dr. Dan Potter, Entomology, University of Kentucky. Their work is highlighted in the “Selected References” below and remains a touchstone example of the rigorous research required to unravel the intricate dance between an insect gall-maker and its plant host.

Ron Wilson