The “horns” that give the horned oak gall its common name are rising to the surface in southwest Ohio. The woody stem galls were formed under the direction of the gall-wasp, Callirhytis cornigera (Family Cynipidae) to feed and house immature grub-like wasps.

The immature wasps spend 33 months developing at the base of specialized horn-like structures. Cutting the galls open will reveal these “horns” embedded within the galls.

As the wasps approach maturity, the horns rise to the surface giving the new wasps access to the outside world. However, the woody stem galls don’t go away. They eventually dry out to look like some sort of medieval weapon.

The wasps that develop within the woody stem galls are all females; there are no males. Reproduction without the need for males is called parthenogenesis. Unfortunately, this reproductive strategy doesn't appear to be an evolutionary dead-end, but that’s just one male’s opinion.

The new parthenogenetic females 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 journey of the asexual wasps may be fueled by a sugary treat provided by their former horn abode. If you look closely at the tips of newly emerged horns, you’ll see tiny glistening droplets of nectar. At least, I believe it’s nectar; it’s sticky and sweet (personal taste-test).

The inclusion of extrafloral nectaries within the structure of wasp galls is not unusual. However, most galls ooze their sugary treat later in the season with the apparent goal of attracting stinging or biting gall-defenders.

A good example is oak rough bulletgalls that are produced under the direction of the cynipid wasp Disholcaspis quercusmamma (see Extrafloral Nectaries, Myrmecophiles, and Other Trivial Pursuits, BYGL Alert April 26, 2021). It’s too early in the season for the nectar oozing from horns of horned oak galls to attract gall-protectors from like bald-faced hornets (Dolichovespula maculata) and yellowjackets (Vespula spp.); they’re still in the early stages of nest construction. So, it’s speculated that the sugary treat may provide fuel for the newly emerged asexual horned oak gall wasps to help them with their journey.

The immature horned oak gall wasps that develop in the leaf galls require around 3 months to complete their development. The wasps that emerge are both 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 stem galls that arise from meristematic cambial tissue.

Gall Impact

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 can harm oaks if they encompass stems and disrupt 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.

Horned oak gall wasps appear to be confined to members of the “red oak group.” Oaks that are commonly affected include black (Q. velutina), blackjack (Q. marilandica), pin (Q. palustris), shingle oaks (Q. imbricaria), Shumard (Q. shumardii), water (Q. nigra), and willow oaks (Q. phellos).

It’s generally perceived that pin oaks are particularly susceptible to galling and stem dieback. However, is it a matter of host selection or host impact? I and the horticulture professionals at Spring Grove Cemetery and Arboretum (Cincinnati, OH) have been monitoring three heavily galled mature shingle oaks 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.

Don’t Be Deceived

In the grand scheme of things, there is a strong argument that tree galls should be considered an integral part of forest ecosystems. For example, the ecological niche occupied by the horned oak gall wasp and its associated woody stem gall is important to the survival of over 30 other arthropods.

These gall-crashers are known as inquilines and they live in and feed on horned oak galls. They have nothing to do with gall development; they just take advantage of the work of the gall-making cynipid wasp. The beautiful little clearwing moth (family Sesiidae) known as the oak gall borer (Synanthedon decipiens) develops inside horned oak galls.

A Word from Management

The Heterogony Headache. 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.

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 the row of pin oaks pictured below for several years. 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.

You Can't Spray the Problem Away. Unfortunately, there is no scientific insecticide efficacy data available to guide the use of topical contact insecticides or systemic neonicotinoids for suppressing horned oak gall development. Anecdotal accounts of suppressing gall formation using canopy sprays or soil drenches/tree injections with neonicotinoids should be viewed with skepticism. An observed drop in stem galls after an insecticide application may be due to some other factor.

Nature Doesn't Play Favorites. Research has shown that these native gall wasps have several natural enemies that are capable of significantly affecting wasp population densities. The legless, grub-like wasp larvae residing within the thin-walled leaf galls are particularly alluring to parasitoids. Wasp numbers may also be affected by environmental events. A driving rain would be disastrous for the delicate wasps trying to make their way to emerging leaves. These natural challenges translate into wide swings in horned oak gall-wasp populations.

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 a tree killer. In fact, I think these galls add ornamental value, but I may be biased.

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 remains a touchstone example of the rigorous research required to unravel the intricate dance between an insect gall-maker and its plant host.