These particular galls on oak, what are called the oak spangle gall, are one of the most confusing to determine which specific species the common name is referring to. A frustrating search yields the following possibilities as the species being referred to by "oak spangle gall":
- Hymenoptera: Cynipidae (Gall Wasp family): Phylloteras poculum, known previously as Xystoteras poculum or possibly Cecidomyia poculum (which is even more confusing, more on that below).
- Hymenoptera: Cynipidae (Gall Wasp family): Neuroterus quercusbaccarum or others in that genus
- Diptera: Cecidomyiidae (Gall Midge family): Cecidomyia poculum
The authors of this guide are not convinced that "oak spangle gall" refers to any single species found on oak - in fact, the images one finds in a search for the species mentioned above seem to vary widely. There also seems to be some confusion from gall enthusiasts as to whether or not some of the species mentioned above actually cause the galls in the first place. (Ex. Cecidomyia poculum, if we mean the midge (fly) may have been an inquiline (an organism exploiting the living space of another, for example the gall) of the cynipid wasp responsible for actually causing the galls to form. All of this makes reporting a clear description of the life cycle of this organism very confusing and perhaps impossible without further study. All the more reason to view these galls on oak as just a curiosity that does not require management, especially since damage to the overall health of the tree does not occur. More information about spangle galls and related species can be found from Ohio State University Extension, here: https://bygl.osu.edu/index.php/node/2185 .
The following information is the best that can be gathered at this time, however we hope that someday a gall expert sorts all of this out. Oak spangle galls may be found attached to the undersides of mature host plant leaves in September or October. Concave, button-shaped, 3-4.5 mm in diameter galls may be covered slightly with a whitish bloom. A tiny bump in the center of the concave gall houses the insect larva. (From this bump, an exit hole may be seen where the adult insect emerges.) The galls may also be described as small, flattened disks or suction cups on the undersides of host plant leaves. The upper sides of the leaves may also be covered with yellow-ringed brown spots that can be confused for other leaf issues, such as foliar diseases, especially if the galls on the leaf undersides have dried up and dropped off the leaf.
Information specific to the life cycle of Neuroterus quercusbaccarum is discussed in Hough (1952). They refer to this species as the common oak spangle gall. The sexual generation of this species may be called the "currant gall", causing galls on oak leaves, male catkins, bracts, or young stems. Eggs and galls of that generation are found in April and May. The oak spangle gall is the asexual generation of this species, and may be found on the undersides of oak leaves in June and July, following egg laying. Hough notes that leaves that bear the currant galls in the spring rarely also have the oak spangle galls in the summer. Hough also notes that there is an uneven distribution of common spangle galls on host plant leaves, which is correlated with the stage of development the leaves are in on different trees or even the same individual tree. It was also observed that young oak trees (smaller trees/bushy trees) and the leaves of the lower branches of mature trees are typically more heavily galled by this species (Hough, 1952).
Primarily found on leaf undersides of white oaks, these cup shaped, grayish-white galls are not of concern and usually just a curiosity. The upper sides of the leaves may also be covered with yellow-ringed brown spots that can be confused for other leaf issues, such as foliar diseases, especially if the galls on the leaf undersides have dried up and dropped off the leaf. Smaller (younger), shrubby trees or the lower branches of mature oak are typically more heavily galled than others. Additionally, depending upon which species of insect is responsible for these galls (see discussion above), different species of oak are listed as host plants. Because of the confusion that exists, only white oak is noted above.
If yellow/brown spots are seen on the surface of oak leaves, flip the leaves over to search for galls on the leave undersides corresponding with these spots. Gall formation may be initiated in April/May or June/July but may also not be noticeable until September/October. Again, these time frames vary depending upon the species.
If lower branches/leaves are more severely galled on an individual tree and these branches can be pruned out and destroyed, this may reduce the population of galls on an individual host plant. Only do so if this does not disfigure the tree.
There is some suggestion that inquilines (insects that live in the galls of the oak spangle gall and may be natural enemies of the insect that forms the gall) may exist for this species. Due to the confusion about the true identity of the oak spangle gall, information about natural enemies of this insect is not fully understood at this time. Presumably, natural enemies of the oak spangle gall do exist.
Acephate (NL)
Carbaryl (L)
Dinotefuran (NL)
Emamectin benzoate (L)
Imidacloprid (L)
Pyrethrins + piperonyl butoxide (L)
Rarely requires treatment. The use of chemical management options for oak spangle gall is often not necessary.
Active ingredients that may be applied systemically include: acephate (injection).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
