Last week, I came across a burr oak (Quercus macrocarpa) in a southwest Ohio city park with leaves on the lower part of the canopy dramatically embellished with a type of “saucer gall.” The galls were the handiwork of the gall wasp, Neuroterus tantulus (“Gall-Wasp” family Cynipidae).
Saucer galls are so-named because of their flat, saucer-like shape. I’ve also heard them described as looking like discs, small flowers, and even octopus suckers.
The galls produce a small depression on the upper leaf surface at the point where each gall attaches to the lower leaf surface. The center spots are light green at first, but eventually turn brown. They are surrounded by a chlorotic halo and multiple halos may coalesce causing leaf discoloration to become apparent when populations are high.
The galls were initiated earlier this season when a female wasp used her ovipositor (ovi=egg) to insert eggs into a nascent leaf bud. She also injected a cocktail of chemicals that turned plant genes on and off at just the right time to direct the development of the saucer galls on developing leaves. Research has shown that the eggs of some gall-makers continue to influence gall formation by exuding gall-forming chemicals.
It's important to note that the chemical influence of gall-making wasps, and perhaps other groups of gall-makers, is limited to “meristematic” cells. These cells are like teenagers; they don’t know what they’re going to be until they grow up. The gall-makers can’t exploit cells that have differentiated into distinguishable plant tissue. In the case of saucer galls, once leaf cells become leaf cells, they cannot be chemically directed to become gall cells.
Each saucer gall contains a single, tiny gall-wasp larva that develops inside a chamber in the center of the gall. As the larvae mature, their galls change colors. Once the larvae complete their development, their galls detach and drop to the ground where the wasps remain until next spring when new females emerge to start the process all over again.
BYGL readers examining the underside of oak leaves this time of the year may also come across two other kinds of galls that are produced by cynipid wasps belonging to the same genus. “Spangle Galls” are so-named because they resemble the spangles sown onto costumes. The descriptively named Jumping Oak Galls … jump.
As with “saucer galls,” the common name “spangle galls” applies to a wide range of gall-making insects including a gall-making midge fly (family Cecidomyiidae). Indeed, if you do a web search using “oak spangle galls,” you may see pictures of galls produced by the midge fly, Cecidomyia poculum (“Gall Midge” family, Cecidomyiidae). Although much larger than those produced by wasps, the midge galls share many features with the oak spangle wasp galls including forming on the lower leaf surfaces of oaks.
Tangent Alert!
There are only 35 “official” common names for galls or gall-makers that have been approved by the Entomological Society of America (ESA). Of these, only 8 apply to wasp galls and 11 apply to midge fly galls.
Beyond that, common names for galls are much like those for plants; they grow out of general usage. The downside is that a common name for a gall, such as “spangle gall,” may be applied to several different species of gall-makers even species belonging to different orders of insects such as wasps and flies. The upside is that we all have the opportunity to create new common names for galls!
For example, in 2020, 2021, and 2022, I posted BYGL Alerts about the fuzzy galls produced by of the gall-wasp, Callirhytis furva (family Cynipidae). Neither the galls nor the gall-making wasps have ESA-approved common names. Thus, online references refer to the galls using a range of common names from “furry oak galls” to "plush galls" to "hairy oak galls." Since none carry any official recognition, one name is as good as another.
However, I believe the galls bear a striking resemblance to a miniature version of Tribbles; the fictional alien species that overran the Enterprise in the 1967 Star Trek episode titled "The Trouble with Tribbles." As a Trekkie, I've taken advantage of the lack of any common name oversight by applying the common name “tribble galls” [see “Tribble Troubles?”, https://bygl.osu.edu/node/2059 ].
Applying the same gall-naming opportunity to “spangle galls,” I propose the galls produced by cynipid gall wasps, specifically those produced by N. umbilicatus, should be called “Oak Sequin Galls.” If you look up the definitions for “spangle” and “sequins,” you’ll find they are one and the same. However, using “sequins” to reference a specific Neuroterus wasp gall would provide some clarity.
Following the same naming opportunity, I may start using “Oak Octopus-Sucker Galls” to refer to the saucer galls produced by the gall wasp, Neuroterus tantulus. Let the name games begin!
Back to the Original Galling Program
Oak sequin galls occur on the underside of oak leaves and have a very similar overall appearance to oak octopus-sucker galls. However, the edges tend to be curled upwards and the center of the galls become thickened as the galls mature.
Other than those general differences, the sequin galls produce the same small depressions on the upper leaf surface that correspond to the gall’s attachment points on the lower leaf surface. The center spots are also surrounded by a chlorotic halo.
As with oak octopus-sucker galls, sequin galls change color as they mature. They also detach from the leaves once the gall-making wasp larvae complete their development and drop to the ground.
The common name, “Jumping Oak Gall”, has long been applied to a single species, N. saltatorius. However, this species appears to be native to the western U.S. which has created some challenges with establishing the true identity of the wasp(s) creating jumping galls in the eastern part of the country including Ohio. Currently, it’s appropriate to use Neuroterus spp. for the gall-making wasp(s) behind the jumping galls found in Ohio until the exact taxonomy can be sorted.
The globular jumping galls are about the size of a sesame seed. They are partially imbedded into the lower leaf surface and blister-like spots appear on the upper leaf that correspond to the points of attachment of these galls. The spots on the upper leaf turn from yellow to brown and heavily galled leaves may drop from the trees.
Like the other two under-leaf galls, jumping oak galls also drop to the ground once the wasp larvae have completed their development within the galls. However, the larvae within the galls can cause the galls to jump around like Mexican jumping beans, thus the common name. They have great entertainment value! It is assumed the jumping behavior helps the galls to drop into cracks in the soil where the larvae can safely pupate inside the gall.
Impacts
It’s rare to see significant numbers of oak saucer and spangle galls on oaks growing in Ohio’s woodlands. However, we occasionally see large numbers of both types of wasp galls on oaks in Ohio landscapes. Still, the numbers tend to rise and fall dramatically from year to year. A year with heavy galling is usually followed by many years with little galling. Also, the galls tend to be confined to individual trees.
The boom-or-bust nature of saucer galls, as well as their highly localized appearance on individual trees, was illustrated to me in the city park where I took many of the photos used in this Alert. Last year, I photographed saucer galls on a burr oak located a short distance in the park from the heavily galled burr oak that I photographed this season. However, the oak that was heavily galled last season has virtually no saucer galls this season.
Saucer and spangle galls seldom appear throughout a landscape tree’s canopy. They tend to be concentrated on leaves near the base of the canopy. Thus, even though the perceived impact may look dramatic, it’s likely that the galls are having no significant impact on the overall health of the tree.
Jumping oak galls share many characteristics with saucer and spangle oak wasp galls. They tend to follow a boom-or-bust population pattern from year to year; however, the boom may be much greater! In the early 2000s, Ohio experienced an outbreak of jumping oak galls on landscape and forest trees in multiple counties in the southern part of the state. Trees turned brown in June and remained affected into the fall. However, such outbreaks are extremely rare.
The boom-or-bust population pattern of the saucer, spangle, and jumping oak galls is driven by environmental factors as well as predators, parasitoids, and perhaps pathogens (the “3-Ps”). As illustrated by the following pictures, wasp gall-makers have enemies.