Oak Rough Bulletgalls are produced under the direction of the tiny gall wasp, Disholcaspis quercusmamma (family Cynipidae). The “rough” in the common name refers to the roughened appearance created by tiny hairs covering the surface. The sturdy galls can be round like a musket ball or slightly elongated like a Minie ball, which accounts for the “bullet” part of their common name.
The galls are confined to oaks in the white oak group. The most common hosts are bur oak (Quercus macrocarpa), overcup oak (Q. lyrate), swamp white oak (Q. bicolor), and white oak (Q. alba).
Cynipid wasp galls can only form from undifferentiated (= meristematic) cells. Meristematic cells occur in three places in a tree: the buds, the root tips, and the cambium. Meristematic cells also occur in callus tissue as part of the tree’s response to wounding, but that process deserves another Alert.
Female oak rough bulletgall wasps inject chemicals into the cambium when they lay their eggs (oviposit) that direct the undifferentiated tissue to form a structure that surrounds, protects, and feeds their legless, grub-like offspring. It’s suspected the chemicals are exuded from the eggs and perhaps wasp larvae that also turn plant genes on and off to direct gall growth. It’s one of the most intriguing and poorly understood facets of gall development.
Since the cambial cells are active throughout the summer, stem galls can form anytime during the growing season. This is not true for galls that form from the buds, but more on that later.
Splitting a bulletgall open reveals a single larva per gall resting in a central chamber. The chamber is surrounded by nutritive tissue that is constantly replaced as it's being consumed by the larva using its chewing mouthparts. It would be like sitting in a room with fresh pizzas continually emerging from the walls. I shouldn’t be writing this at lunchtime.
What’s the Buzz About?
Remarkably, not only does the wasp direct oak cells to form a gall that’s specific to the wasp species, but the gall also includes a functional plant organ. It’s called an extrafloral nectary, and it's the pointed structure on top of the gall.
The nectar that oozes from the extrafloral nectary draws in stinging and biting insects. In return for the sweet treat, the “security detail” protects the immature gall-maker from predators.
It’s common for heavily galled trees to be buzzing with baldfaced hornets (Dolichovespula maculata), paper wasps (Polistes spp.), and yellowjackets (Vespula spp.), as well as other impressive stinging insects. Large carpenter ants (Camponotus spp.) may also visit oak rough bulletgalls, adding their impressive mandibles to the defensive arsenal.
It’s easy to imagine the wasps chasing off birds and other predators, with the ants nipping at their heels, to keep the gall-pillagers from grabbing a gall-wasp meat treat. Indeed, I’ve never seen an oak rough bulletgall showing evidence that a bird has pecked out the wasp larva. Of course, various freeloading flies commonly crash the party to lap up the sweet nectar.
As shown in the images below, the nascent galls have functional extrafloral nectaries almost as soon as they break through the bark. Consequently, gall protection starts early in gall development.
Oak rough bulletgalls ooze copious quantities of nectar that drips onto the leaves of heavily galled trees. The sugary nectar can be colonized by black sooty molds if not removed by heavy rainfall.
The nectar on the galls also becomes colonized by black sooty molds. The blackish patina eventually obscures the beautiful coloration. Of course, I like galls, so beauty is in the eye of the beholder.
Failing to Pay for Protection
The closely related cynipid wasp, D. quercusglobulus, produces so-called Round Oak Bulletgalls. The galls demonstrate what happens if oak gall wasps don’t “pay” a sugary bribe for protection. The wasp produces so-called Round Oak Bulletgalls that show no evidence of extrafloral nectaries. The galls do not become covered with black sooty molds, and I’ve never observed stinging insects drawn to the galls.
The literature notes that several species of birds will partake in a gall-wasp larva meat meal, including chickadees, titmice, and tits, as well as downy woodpeckers. The images below show round oak bulletgalls decimated by bird predation.
A Complex Life Cycle
Cynipid wasps alternate between two types of galls on their host trees. One type is called “sexgen” galls and produce a sexual generation, with males and females emerging. The other type is called “agamic” and produce an asexual generation with only females emerging. The alternation of two different methods of reproduction between generations of a species is known as heterogony.
The oak bulletgalls are agamic and house the developing asexual generation. The females that emerge contain unfertilized eggs, a method of reproduction called parthenogenesis. The females chew their way out of the stem galls in the fall and lay their eggs in dormant buds.
The egg-laying activity directs the buds to form small, seed-like sexgen galls in the spring that house the developing sexual generation. The male and female wasps complete their development in the early summer, and the mated females move to stems to lay their eggs into stem cambium, starting the life cycle over again.
Bulletgall Impact
Bulletgalls may affect tree aesthetics; however, they cause no real harm to the overall health of their oak hosts. They are attached to the surface of the stems, arising from the meristematic tissue comprising the stem cambium, as seen in the first image below. The galls do not disrupt the vascular flow within the phloem or xylem as with horned oak gall in the second image.
However, large numbers of wasps, such as baldfaced hornets, yellowjackets, and paper wasps, which are drawn to the nectar-oozing stem galls, may be perceived as a threat. Keep in mind that despite their belligerent reputations, the wasps are seldom aggressive away from their nests. There’s simply no benefit in wasting energy chasing after people far from the wasp’s home base.
As proof, most of the images used to illustrate this Alert showing wasps on oak rough bulletgalls were taken with the lens within a foot, or even closer, of the subjects. I have never been chased away by the security detail, although some do notice me, as shown in the image below. However, most often, it was me who accidentally chased away the wasps as my camera lens got too close.
It’s important to view the wasps realistically because there’s nothing that can be done to directly eliminate the security detail protecting the oak rough bulletgalls because the stingers may include important pollinators such as honey bees (Apis mellifera) or other beneficial insects. Thus, insecticide applications are not an option.
However, fear and anxiety may be reduced through education. After all, exactly what’s going on is one of the most fascinating stories you’ll ever come across in Nature.
Selected References
Guiguet, A., Tooker, J. F., Deans, A. R., Mikó, I., Ning, G., Schwéger, S., & Hines, H. M. (2023). Comparative anatomy of venom glands suggests a role of maternal secretions in gall induction by cynipid wasps (Hymenoptera: Cynipidae). Insect Systematics and Diversity, 7(5), 3.
https://academic.oup.com/isd/article/7/5/3/7310975
McEwen, C., Digweed, S., Nicholls, J. A., & Cranshaw, W. (2014). Description and biology of the sexual generation of Disholcaspis quercusmamma (Walsh and Riley) (Hymenoptera: Cynipidae), with notes on associated parasitoids. Proceedings of the Entomological Society of Washington, 116(3), 294-310.
https://doi.org/10.4289/0013-8797.116.3.294
Nicholls, J. A., Melika, G., & Stone, G. N. (2017). Sweet tetra-trophic interactions: multiple evolution of nectar secretion, a defensive extended phenotype in Cynipid gall wasps. The American Naturalist, 189(1), 67-77.
https://www.journals.uchicago.edu/doi/abs/10.1086/689399
Pujade-Villar, J., Bellido, D., López, G. S., & Melika, G. (2001). Current state of knowledge of heterogony in Cynipidae (Hymenoptera, Cynipoidea). Sessió Conjunta d'Entomologia, 87-107.
https://raco.cat/index.php/SessioEnto/article/view/335
Stone, G. N., Schönrogge, K., Atkinson, R. J., Bellido, D., & Pujade-Villar, J. (2002). The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual review of entomology, 47(1), 633-668.
https://doi.org/10.1146/annurev.ento.47.091201.145247
Weaver, A. K., Hood, G. R., Foster, M., & Egan, S. P. (2020). Trade‐off between fecundity and survival generates stabilizing selection on gall size. Ecology and evolution, 10(18), 10207-10218.
https://doi.org/10.1002/ece3.6682
