The physical emergence of the "horns" that give horned oak galls their common name is such a short-lived event I've only witnessed it one other time in 26 years. I've commonly observed fully extended horns, but seeing them in the process of breaking through the gall surface is a rare treat and it's happening right now in southwest Ohio.
Horned oak galls are formed under the direction of the gall-wasp, Callirhytis cornigera (Family Cynipidae). 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, these galls are an exception. Vascular tissue becomes disorganized within the gall structure which disrupts vascular flow. The portion of the stem beyond the gall is often starved of water and dies. This doesn't kill trees, but canopy dieback may be so severe you wish the tree would die.
Galls produced by this wasp may be found on pin, black, blackjack, Shumard, water, and willow oaks. However, pin oaks are particularly susceptible. Unfortunately, gall management is problematic with no clear method to reverse the galling trend once trees become targeted by the gall-wasp.
What Lies Beneath
It's common to think of horned oak galls as the dark brown, fractured, woody structures that look like some sort of medieval weapon. However, these are the old, spent galls. They may linger for years marring the tree's aesthetic value.
Both the spent galls and those that are now maturing serve as a home and feeding ground for a wide range of creatures other than the gall-making wasp. They are collectively known as "inquilines." These include over 30 arthropods such as the beautiful little clearwing moth (family Sesiidae) known as the Oak Gall Borer (Synanthedon decipiens). However, the spent galls will never be repopulated by the cynipid wasp originally responsible for producing the galls.
The Stem Galls:
The galls that are currently sprouting horns appear succulent, green to greenish-gray, and have smooth surfaces. Growth of these galls were first stimulated by the female wasps when they injected chemicals along with eggs into cambial tissue. This directed plant genes in the undifferentiated cambial cells to be turned on and off at just the right times to start forming the stem gall. Both the eggs and hatched wasp larvae continued to exude chemicals to further direct gall growth.
The stem galls are communal and may house anywhere from 1 to 160 wasp larvae. The size of the galls depends on the number of eggs laid by the females with larger galls supporting a larger number of larvae. The new wasps emerge from the horns, so you can determine the number of new wasps that developed inside the gall by counting the number of horns.
The immature wasps spent around 33 months developing inside specialized structures imbedded within these galls with the galls gradually enlarging each season. Cutting the maturing galls open last season would have revealed the wasp homes including the cone-like structures that rising to the surface this spring.
As the immature wasps reach pupation, the horns rise from within the gall to break through the surface. This provides the new wasps easy access to the outside world.
This remarkable event is very time-sensitive and only lasts a few days. Horns began to emerge in Cincinnati at 270 accumulated Growing Degree Days (GDD) with snowdrift crabapples in full bloom; a possible phenological indicator. Of course, another predictor is the emergence of new leaves on host oaks. These will be targets for the next phase in this gall-wasps life history.
The exposed horns exude sugary liquid from the tips. It is speculated this liquid may nourish newly emerging wasps, or it may serve to attract ants and other insects that protect the galls until all wasps have emerged.
The Leaf Galls
The wasps that emerge from the horns on the stem galls are agamic; they are all females. This form of reproduction, where females do not require fertilization by males to produce fertile eggs is called parthenogenesis.
These wasps are poor flyers. They crawl to developing leaves to lay eggs and initiate the production of a second type of gall; tiny, inconspicuous leaf galls that develop on the leaf veins.
The immature wasps in the leaf galls require around 3 months to develop and emerge. Both male and female wasps emerge from the leaf galls; 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. This alternation of two different reproductive modes and life-styles between generations is known as "heterogamy."
A Word from Management
Management strategies for the horned oak gall wasp must take into account the two locations where the gall-maker resides. Adding to the complexity is that nothing is synchronized. Leaf galls occur every year providing a constant stream of wasps to produce new stem galls. This makes managing this gall-wasp by pruning and destroying stem galls an annual, multi-season process. However, this approach may provide some relief on small trees if the trees are whittled down by continual pruning.
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 the application or some other factor.
Research has shown that these native gall wasps have a number of natural enemies that are capable of 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 hitting the delicate wasps trying to make their way to emerging leaves would be disastrous for the wasps.
One thing that is clear: host susceptibility to these gall-wasps is highly variable. For example, I've been observing a row of pin oaks in a landscape for a number of 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 based on the inherent genetic variability between the trees with the one tree being genetically more susceptible compared to the 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.
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 make the tree more attractive compared to the other trees?
Is there a "founder effect" with wasps that are genetically best suited for utilizing the tree being selected over time; successive generations of their progeny thrive and multiply to produce a gall explosion. Nearby trees escape because the wasps are not genetically matched to the trees.
Of course, it could be all the above, none of the above; or perhaps just the luck of the draw. However, it is clear that one effective method to avoid heavy galling is to remove trees that have become heavily selected by the wasps, or live with the problem because the galls are not a tree killer. In fact, I think these galls add ornamental value, but I may be biased.
Regardless, more research is required to fully unravel the integrate dance between this gall-making wasp and its oak hosts. The same is true for the vast majority of gall-making insects as well as other arthropods. However, thanks to research conducted by the University of Kentucky, Entomology, we have a considerable head start with horned oak gall.
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 (Buss) in partnership with her major advisor, Dr. Dan Potter, Entomology, University of Kentucky. Indeed, both provided such a life-altering summary of their research at our first Bluegrass-Buckeye BYGLive!; I attribute my gall-mania to their strong influence.
Here are some citations for further reading:
- Eliason EA and DA Potter. 2000. Biology of Callirhytis cornigera (Hymenoptera: Cynipidae) and the arthropod community inhabiting its galls. Environ Ent 29: 551-559
- Eliason EA and DA Potter. 2000. Impact of whole-canopy and systemic insecticidal treatments on Callirhytis cornigera (Hymenoptera: Cynipidae) and associated parasitoids on pin oak. J Econ Ent 93: 165-171.
- Eliason, EA and DA Potter. 2001. Biology and management of the horned oak gall wasp on pin oak. J Arboriculture 27: 92-100.