Oak Rough Bulletgalls are at the hub of a buzz about wasps and flies swarming oaks belonging to the white oak group, particularly bur oak (Quercus macrocarpa), overcup oak (Q. lyrate), white oak (Q. alba), and swamp white oak (Quercus bicolor). The sturdy galls are produced under the direction of the tiny gall-wasp, Disholcaspis quercusmamma (family Cynipidae).
The “rough” part of their common name stems from their roughened appearance, created by tiny hairs covering the surface. The 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 insects are drawn to nectar oozing from extrafloral nectaries that are part of the gall structure. In return for the sweet treat, the stinging insects protect the immature gall-maker inside the galls from predators.
The “security detail” commonly includes baldfaced hornets (Dolichovespula maculata), paper wasps (Polistes spp.), and yellowjackets (Vespula spp.), as well as other impressive stinging insects. It’s easy to imagine the wasps chasing off birds and other predators intent on 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.
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.
Large carpenter ants (Camponotus spp.) may also visit oak rough bulletgalls, adding their impressive mandibles to the defensive arsenal. Of course, various free-loading flies commonly crash the party to lap up the sweet nectar.
The oak rough bulletgalls ooze copious quantities of nectar that 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.
The closely related cynipid wasp, D. quercusglobulus, demonstrates 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.
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. They do not disrupt the vascular flow within the phloem or xylem.
There’s nothing that can realistically 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.
More Information
Cynipid wasps have complex reproductive lifestyles that involve sexual and asexual generations. The alternation of two different reproduction methods between generations of a species is known as heterogony. Oak rough bulletgalls give rise to the asexual generation of D. quercusmamma. Bud galls produce the sexual generation.
You can read more about this wasp as well as learn the behind-the-scenes details about gall formation by reading the BYGL Alert, “Rough Oak Bulletgalls are Creating a Buzz,” posted on September 14, last year. Just click this link:
https://bygl.osu.edu/index.php/node/2427
Selected References
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
