Rough oak bulletgalls are rising from oak stems in Ohio accompanied by their entourage of bodyguards. The galls are found on oaks in the white oak group with burr (Quercus macrocarpa) and swamp white oak (Q. bicolor) most commonly affected. The "mature" galls are commonly covered in black sooty mold but more about that later.
The galls are produced under the direction of the gall wasp Disholcaspis quercusmamma (family Cynipidae). The wasp has a complicated life cycle involving two different types of oak galls that give rise to asexual and sexual wasps at different times of the year. The alternation of two different reproductive modes and lifestyles between generations of a species is known as heterogamy.
Heterogamy may be a familiar term to alert BYGL readers. The gall wasp Callirhytis cornigera which produces horned oak stem galls, as well as tiny leaf galls, is another heterogamous gall-maker. I described its unusual life cycle in a BYGL Alert posted earlier this season (see "Horns are Popping," by clicking this hotlink: https://bygl.osu.edu/index.php/node/1777 )
A Complex Heterogamous Life Cycle
The life cycle of the rough oak bulletgall wasps involves the current bulletgalls as well as leaf galls that develop in the spring. The bulletgalls will give rise to self-fertile females around the end of September to early October. This is the asexual generation; there are no males. Reproduction without females needing to mate with males is called parthenogenesis.
The emergence of the females is heralded by a small hole in the bulletgall. The "spent" bulletgalls shrivel and darken in color and most will remain attached to continue providing gall-interest next season.
The parthenogenetic females that emerge from the bulletgalls crawl to a dormant leaf bud where they lay a single egg per bud. The eggs hatch in the spring and chemicals exuded by the wasp larvae stimulate the tree to produce small, inconspicuous leaf galls. The leaf galls give rise to the sexual generation. Both male and female wasps develop inside the leaf galls and adults emerge later in the season.
The mated females that arise from the leaf galls fly or crawl to the most recent twigs where they insert their eggs through the phloem to be in contact with the undifferentiated (meristematic) cambium. The thin cambial layer is located between the xylem and the phloem. The wasp requires the services of meristematic cells to grow their bulletgalls.
Rough oak bulletgalls provide everything that's needed to protect and nourish the single developing wasp larva residing in a chamber located at the center of the gall. The wasp larva has chewing mouthparts, but rather than devouring its gall-home, the larva grazes 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!
Rough oak bullet galls cause no appreciable harm to the overall health of their oak hosts. The galls do not penetrate deep into the xylem (wood) to interrupt the vascular flow of water and nutrients to stems and leaves. This is unlike some stem galls, most notably horned oak galls, that disrupt vascular xylem flow causing stem dieback.
The oviposition damage produced by Brood X (10) 17-year periodical cicadas earlier this season can muddy the impact waters. Stem dieback caused by the cicadas may be mistakenly blamed on the rough oak bulletgalls.
Paying for Protection
One of the most fascinating features 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.
The first time I observed blackened bulletgalls, I thought the sooty molds had colonized sugary honeydew dripping onto the galls from the backend of phloem-sucking insects such as aphids or soft scales. Of course, there was not such infestation. The sugar source was from the galls themselves.
The nectar exuded from the extrafloral nectaries attracts a variety of stinging insects including baldfaced hornets (Dolichovespula maculata), paper wasps (Polistes spp.), and yellowjackets (Vespula spp.) as well as biting insects such as carpenter ants (Camponotus spp.).
Paper wasps, baldfaced hornets, and yellowjackets are Jekyll and Hyde wasps. They are predaceous beneficial insects during most of the growing season chowing down on soft-bodied insects such as caterpillars and sawfly larvae. However, they will not ignore high-carbohydrate sweet treats.
This is particularly true of yellowjackets and baldfaced hornets in the fall when they switch from high-protein diets to high carbohydrate diets. They become a nuisance as they compete with our own high-carb consumption in the form of soda, donuts, and certain adult beverages.
The wasp's high-carb hankering draws them to the nectar (a.k.a. wasp candy) oozing from the extrafloral nectaries incorporated in the rough oak bulletgall structure. Of course, ants sporting powerful biting mandibles will also show up to the sugar party along with flies.
Presumably, the close attention of stinging and biting insects 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 pays for the protection of the gall maker's helpless offspring as they lounge about in their tiny chambers feasting on nutrient tissue "pizza."
On a final note, trees that are heavily ladened with rough oak bullet galls, as well as other wasp galls that ooze nectar, may literally buzz with wasp activity. 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.
I got very close to the subjects pictured in this Alert without getting stung. Of course, I also didn’t press my luck; there are limits.