The “horns” that give the Horned Oak Gall its common name are bursting to the surface in southwest Ohio. The horns make the galls look like a miniature version of a medieval martial arts weapon. The woody stem galls were formed under the direction of the gall-wasp, Callirhytis quercuscornigera (syn. Callirhytis cornigera (Family Cynipidae)).
Cutting galls open prior to the rise of the horns reveals these gall structures were part of the galls all along. They serve a critical function by housing and protecting immature wasps (larvae) in a chamber at the base of the horns.
The larvae have chewing mouthparts and feed on so-called “nutritive tissue” constantly being resupplied by the tree. It would be like lounging about in a room with walls made of an endless supply of pizzas.
A Complicated Life Cycle
The immature wasps spend 33 months within the specialized horn-like structures. However, the horns rise at around 22 – 24 months after eggs were inserted into meristematic stem tissue which marked the initiation of stem gall formation.
The rise of the horns heralds a significant change in larval development. After the horns rise in the spring, the wasp larvae enter a summer diapause which is a type of developmental stasis. They pupate in the fall and emerge as adults from the tips of broken horns the following spring. Thus, the horns popping to the surface the previous spring provide the adult wasps with ready access to the outside world.
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.
The stems galls are called “immature” during wasp development. The outer portion of the galls where the horns are housed is somewhat succulent and can be easily cut open. However, the inner portion of the galls where the stem xylem is heavily incorporated in the gall structure is woody and difficult to cut.
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.
The wasps that develop within the stem galls are all females; there are no males. Reproduction without the need for males is called parthenogenesis. Some of us hope this type of asexual reproduction doesn’t catch on, but that’s just one male’s opinion.
The new parthenogenetic females that emerge from the stem galls are poor fliers. The delicate asexual wasps crawl to leaf buds where they lay eggs to stimulate the production of small, inconspicuous leaf galls that will appear along the leaf veins later in the season.
The immature wasps in the leaf galls require around 3 months to develop. The wasps that emerge are males and females; this is the "sexual generation." The mated females of this generation are relatively good flyers and migrate to twigs to lay eggs and initiate the production of the horned stem galls that arise from meristematic cambial tissue.
The alternation of two different reproductive modes and lifestyles between generations of an animal species is known as "heterogony." However, heterogony with horned oak galls is not synchronous. The gall-maker resides in two locations at the same time on the same tree. Stem galls in various stages of development can be found at the same time as leaf galls.
A Sweet Conundrum
If you look closely at the tips of newly emerged horns, you’ll see tiny glistening droplets oozing from the tips. The liquid is sticky and slightly sweet (personal taste test). Although the inclusion of extrafloral nectaries within the structure of wasp galls is not unusual, scientific studies have not revealed the development of these plant organs within horned oak galls.
However, 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 to release the carbohydrate. Of course, this is speculation on my part.
Another point of speculation is the exact function of the oozing droplets. I’ve been unable to find an explanation in the scientific literature for the purpose of the droplets; however, they may serve as fuel for newly emerging parthenogenetic females.
Of course, it’s not the females developing inside the galls with newly emerging horns that consume the sweet treat. Remember that these females will remain in the larval stage throughout the summer. It’s the parthenogenetic females that are emerging after having spent 33 months in other galls that may take a drink. The carbohydrate-rich droplets may power their crawl to leaf buds where they lay their eggs.
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. Horned oak galls are an exception.
The galls harm oaks by encompassing stems and disrupting the vascular flow. Cutting the galls open will reveal that vascular tissues become disorganized within the gall structure. The portion of the stem beyond the gall may die from being starved for water. The damage seldom kills trees; however, the canopy dieback may destroy the landscape value of heavily galled trees making tree owners wish their galled oak would die.
Horned oak gall wasps have a wide host range with the vast majority of the oak hosts belonging to the “red oak group.” They have been reported on southern red oak (Q. falcata), blackjack oak (Quercus incana), shingle oak (Q. imbricaria), scrub oak (Q. ilicifolia), sand laurel oak (Q. hemisphaerica), turkey oak (Q. laevis), laurel oak (Q. laurifolia), water oak (Q. nigra), pin oak (Q. palustris), willow oak (Q. phellos), red oak (Q. rubra), black oak, (Q. velutina). Galls have also been documented on myrtle oak (Q. myrtifolia) which is a live oak.
It’s often perceived that pin oaks are particularly susceptible to galling and stem dieback in Ohio, but is it a matter of host selection or host impact? I’ve been monitoring three heavily galled mature shingle oaks in Spring Grove Cemetery and Arboretum (Cincinnati, OH) for years. Stem gall density is on par with any heavily galled pin oak that I’ve ever observed. However, stem dieback is rare on all three trees. In fact, it becomes hard to tell that the trees are heavily galled once leaves fully expand.
This is good news because gall management is problematic with no clear method to reverse the galling trend once individual trees become targeted by the gall wasp. Another management challenge is presented by the wasp's complicated life cycle involving asexual and sexual wasps that develop in stem and leaf galls, respectively. Both types of galls and their associated wasps occur at the same time.
The Heterogony Headache. The leaf galls occur every year providing a constant stream of wasps to produce new stem galls. This is why stem gall development is not synchronized. First-season stem galls are found on the same tree at the same time as when 33-month-old galls are releasing their wasps to initiate new leaf galls.
This makes managing this gall-wasp through pruning problematic. Galls of all ages would need to be found, pruned away, and destroyed. Missing a single gall means the eventual release of parthenogenetic female wasps that will produce a new crop of stem galls. Cutting out galled stems may provide some relief on small trees if the trees are not whittled down by continual pruning.
Is it Select-o-Tree or Just Bad Luck? One thing that is clear with horned oak galls: host susceptibility to these gall wasps is highly variable. For example, I've been observing a row of pin oaks in a landscape for several years; they are pictured below. One tree has been a gall-magnet while all of the others have remained gall-free. It's my understanding that the trees originated from the same nursery.
One explanation for the high degree of variability within the pin oaks could be the inherent genetic variability between the trees with some being more susceptible compared to others. It's my understanding these trees aren't clones; they were produced from acorns. Even though they are the same species, they are not genetically identical.
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 produce a gall explosion. Nearby trees escape because the wasps are not genetically "matched" to these host trees.
Other possible explanations involve rampant speculation. 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 makes the tree more attractive compared to the other trees? Of course, it could be all the above, none of the above; or perhaps just bad luck.
The Bottom Line. It is clear that one effective gall-management option is to simply remove trees that for whatever reason have proven to be highly susceptible. Another option is to live with the problem by recognizing that this is not usually a tree killer in Ohio.
In fact, I think these galls provide entertainment value. Cracks in the galls produced by the emerging horns coupled with the horns themselves can make the galls look like some strange creature.
Credit Where Credit's Due
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.