Rough Oak Bulletgalls induced by the gall-wasp, Disholcaspis quercusmamma (family Cynipidae) are splitting through the bark and rising on the stems of oaks in the white oak group in southwest Ohio. The galls are most commonly found on bur (Quercus macrocarpa), white oak (Q. alba), and swamp white oak (Q. bicolor).
The common and scientific names associated with the wasp galls are descriptive. The surface of rough oak galls is covered in tiny hairs giving the galls a roughened appearance. They can be round like a musket ball or slightly elongated like a Minie ball. The “quercus” in the specific epithet of the cynipid wasp’s scientific name refers to the host, and “mamma” is taken from the Latin for “breast.”
Gall Formation: Pulling Genetic Levers
Gall formation under the direction of cynipid wasps as well as other gall-making arthropods is one of the most fascinating things you’ll ever run across in Nature. Keep in mind that no scientist has ever duplicated the process.
Wasp gall-makers use chemicals injected with the eggs or exuded from the eggs to turn plant genes on and off at just the right time to direct gall growth. Even more remarkable, the exact genetic levers pulled by the wasp are so species-specific, you can identify the wasp species by its gall.
Of course, the gall-maker must use plant cells that have not yet become part of an organized plant structure. In other words, galls can’t be formed from plant tissue once the plant’s inherited genetic script has been completed and the cells are part of a functional leaf or new stem.
Gall-makers target “undifferentiated” cells that are found in meristematic tissue. The cells are like teenagers; they don’t know what they’ll be until they grow up. Meristematic tissue is found in buds, at the tips of roots, and in the thin cambial ring located between the phloem and xylem. Indeed, the cambial meristematic cells will differentiate to become xylem (wood) to the inside and phloem to the outside which is how trees increase their girth.
A Complex Life Cycle
Although the life cycles for all cynipid wasps have not been established, those that have been studied reveal a significant feature. Cynipid wasps commonly have a complicated life cycle involving two different types of galls that usually arise from different parts of their host plants at different times of the year. One kind of gall gives rise to asexual wasps (no male wasps) and the other kind of gall gives rise to sexual wasps (male and female wasps).
The alternation of two different reproductive modes and lifestyles between generations of a species is known as heterogony. Cynipid wasps aren't the only insects with heterogonic life cycles. Aphids are arguably the most notorious for employing this complex system of development presumably to confuse entomologists.
The life cycle for the rough bulletgall cynipid wasp was published in a paper in 2014. The wasp uses the meristematic cells in the cambium to produce the bullet-like stem galls shown in this Alert and they use meristematic cells in buds to produce a much less apparent bud gall.
The stem galls give rise to the asexual generation in the wasp's life cycle. The galls first look more like fins as they break through the surface of the stems, then they expand to look like “bullets.”
Self-fertile females emerge from the stem galls around the end of September to early October. There are no males. Reproduction without males is called parthenogenesis.
The parthenogenetic females that emerge from the bulletgalls crawl to a dormant leaf bud where they lay a single egg per bud in the meristematic tissue. The eggs hatch in the spring and chemicals exuded by the wasp larvae stimulate the tree to produce small, inconspicuous leaf bud galls.
In fact, the galls are so inconspicuous I’ve never observed and photographed a bud gall. It’s on my gall-bucket list. The bud galls give rise to the sexual generation with male and female wasps developing inside the ephemeral bud galls and emerging later in the season.
The mated females that arise from the bud galls fly or crawl to the most recent twigs where they insert their eggs through the phloem to be in contact with the meristematic cambium. This initiates the formation of the bulletgalls.
Rough oak bulletgalls provide everything that's needed to protect and nourish a single developing wasp larva residing in a chamber located at the center of the gall. The wasp larvae have chewing mouthparts. However, rather than devouring their gall-home, the larvae graze on a continuously recharged supply of food called nutrient tissue that lines its chamber. It’s like living in a home with pizzas continually emerging from the walls!
As with the vast majority of stem galls on oaks, rough bulletgalls cause no appreciable harm to the overall health of their oak hosts. They may affect the tree’s aesthetics; however, the galls do not penetrate deep into the xylem (wood) to interrupt the vascular flow of water and nutrients to stems and leaves.
A good example of one of the few galls that can cause stem dieback is the so-called Horned Oak Galls produced under the direction of the cynipid wasp, Callirhytis comigera. As the image below shows, the stem vascular system is incorporated into the gall structure causing a disruption in vascular xylem flow which limits the volume of water and nutrients moving past the gall.
Paying for Protection
Another fascinating feature of many plant galls, including rough oak bulletgalls, is the inclusion of extrafloral nectaries (a plant organ) in the gall structure. The nectar oozes across the surface of the galls and can become colonized by black sooty molds. Nectar flow begins almost as soon as the galls emerge from the stems and continue until the gall-making larvae complete their development.
The value in pumping out nectar from extrafloral nectaries is shown in the following images. The gall-nectar attracts a bevy of stinging and biting insects including baldfaced hornets (Dolichovespula maculata), paper wasps (Polistes spp.), yellowjackets (Vespula spp.), and ants such as large carpenter ants (Camponotus spp.).
Presumably, the close attention of this entourage of stinging and biting bodyguards prevents the immature gall-making wasp larvae located within the galls from receiving the unwanted attention of predators and parasitoids. In other words, a little sugar bribe (a.k.a. wasp candy) pays for the protection of the gall maker's helpless offspring as they lounge about in their tiny chambers feasting on nutrient tissue "pizza."
Trees that are heavily ladened with rough oak bulletgalls may buzz with wasps as well as flies, literally. The scene can be intimidating to uninformed homeowners and landscape managers. However, the wasps are too busy acquiring a sugar high to spend time chasing people. They are most aggressive when defending their nests.
Paying for the Lack of Protection
Last year, I came across Round Oak Bulletgalls on a chinkapin oak (Q. muehlenbergii) produced under the direction of the cynipid wasp, D. quercusglobulus. The galls were riddled with holes which I believe were the result of heavy predation presumably by birds.
Although the literature notes that several species of birds will partake in a gall-wasp larva meat meal including chickadees, titmice, and tits (family Paridae), I suspected the primary culprit responsible for the working over the galls was the downy woodpecker (Dryobates pubescens, family Picidae).
Based on personal observations, I’ve concluded that round bulletgalls do not have extrafloral nectaries. I’ve never seen nectar oozing from the galls nor black sooty molds causing the galls to acquire a dusky patina.
Indeed, I did not observe a protection detail in the form of wasps or ants. In other words, rather than paying for protection with a sugar bride, the immature gall-maker was paying for the lack of protection with its life.
I’ve been observing and photographing rough bulletgalls for many years and have never observed a gall that’s been pecked by a bird. Of course, my observations have not been part of a scientific study. Still, unless the predation occurs at night under the cloak of darkness, I can’t imagine a bird brave enough to run the stinging gauntlet for a morsel of gall-maker meat.
Selected References
Melika, G., J. Pujade-Villar, D. Bellido, and G.S. López. 2001. The current state of knowledge of heterogony in Cynipidae (Hymenoptera, Cynipoidea). Sessió Conjunta d'Entomologia, pp.87-107.
Stone, G. N., K. Schönrogge, R.J. Atkinson, D. Bellido, and J. Pujade-Villar. 2002. The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual review of entomology, 47(1), 633-668.
McEwen, C., S. Digweed, J.A. Nicholls, and W. Cranshaw. 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), pp.294-310.
Nicholls, J.A., G. Melika, and G.N. Stone. 2017. Sweet tetra-trophic interactions: multiple evolution of nectar secretion, a defensive extended phenotype in Cynipid gall wasps. The American Naturalist, 189(1), pp.67-77.
Weaver, A.K., G.R. Hood, M. Foster, and S.P. Egan. 2020. Trade‐off between fecundity and survival generates stabilizing selection on gall size. Ecology and evolution, 10(18), pp.10207-10218.