I received several complaints this past week from homeowners and professionals about large numbers of stinging insects buzzing oaks in Ohio landscapes. The most recent involved an oak next to a Columbus playground.
Rough Oak Bulletgalls are at the hub of the buzz. The sturdy galls are produced under the direction of the tiny gall-wasp, Disholcaspis quercusmamma (family Cynipidae). The “rough” part of the 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 galls rise from the stems of oaks in the white oak group, most commonly Bur oak (Quercus macrocarpa), overcup oak (Q. lyrate), white oak (Q. alba), and swamp white oak (Q. bicolor). The galls cause no harm to their oak hosts. However, their stinging "security detail" can be intimidating.
The stinging 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.
Unfortunately, nothing can realistically be done to directly eliminate the security detail partly 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.
Gall Formation: Pulling Genetic Levers
Gall formation under the direction of cynipid wasps involves a series of remarkable events that remain poorly understood. Thus far, no scientist has ever duplicated the process without help from a gall-maker.
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 wasps are so species-specific that the gall-wasp species can often be identified based on the galls alone. The wasps don’t need to be examined.
Of course, the gall-maker can only hijack plant cells that have not yet become part of an organized plant structure. 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. Normally, the cambial meristematic cells differentiate to become xylem (wood) to the inside and phloem to the outside which is how trees increase their girth.
A Complex Life Cycle
Most, if not all, cynipid wasps 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 type of gall produces asexual wasps (no male wasps), and the other type produces sexual wasps (male and female wasps).
The alternation of two different lifestyles, including different methods of reproduction, 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 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.
The bud galls are so inconspicuous that it took me years to finally find one to photograph. They were on my gall-bucket list. These galls give rise to the sexual generation with male and female wasps developing inside the ephemeral bud galls and emerging later in the growing 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 the 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's vascular system is incorporated into the gall structure disrupting vascular xylem flow which limits the volume of water and nutrients moving past the gall.
Paying for Protection
As noted above, rough oak bullet galls include extrafloral nectaries in the gall structure. These are plant organs. The cynipid wasps not only highjack meristematic cambial tissue to grow a home for their offspring, but they also manipulate the oak's genetic code to grow a plant organ in the gall. No matter your perspective on galls, you must admit this is an astounding feat.
As shown in the images below, the extrafloral nectary is located at the tip of the galls. Nectar begins to ooze from the nectary early in the gall development. It continues to ooze across the gall’s surface until the gall-making larvae complete their development.
The gall-nectar may drip onto leaves just like the sticky, sugary honeydew produced by sap-sucking insects like aphids and soft scales. Also, like honeydew, the nectar may become colonized by black sooty molds producing the false diagnosis that the tree is infested with a sap-sucking insect.
Of course, as already noted in this Alert, the reason the galls pump out nectar is to attract stinging and biting insects that protect the immature gall-maker from predators. The sugar bribe (a.k.a. wasp candy) is payment for the protection of the gall maker's helpless offspring as they lounge about in their tiny chambers feasting on nutrient tissue "pizza."
The “security detail” may include baldfaced hornets (Dolichovespula maculata), paper wasps (Polistes spp.), yellowjackets (Vespula spp.), and ants such as large carpenter ants (Camponotus spp.). The images below also include a few other stinging insects.
The bottom line is that trees that are heavily laden with rough oak bulletgalls may literally buzz with wasps as well as flies. 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. Their pugnacious nature is generally confined to driving off the competition.
In fact, many of the pictures used to illustrate this Alert were taken with my camera lens only inches from the stinging subjects. None were taken with a telephoto lens. I’ve never been stung or even buzzed while taking pictures of wasps imbibing nectar oozing from rough oak bulletgalls.
Flying and stinging expends energy and increases the risk of the wasps being killed. It’s simply not in their best interest for the stinging insects to protect a gall-ladened tree for the benefit of multiple insects beyond their own species. Such aggression is generally reserved for defending nests. The payoff for expending energy and risking life and limb to defend a nest is the preservation of their own species, not other species.
Failing to Pay for Protection
On a side note, the price paid if oak gall wasps don’t “pay” for protection with a sugar bribe is demonstrated by Round Oak Bulletgalls produced under the direction of the cynipid wasp, D. quercusglobulus. These galls do not appear to have extrafloral nectaries 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 (family Paridae) as well as downy woodpeckers (Dryobates pubescens, family Picidae). As you can see in the image below, round oak galls can be decimated by bird predation.
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.
