Small, round, fuzzy wasp galls on the underside of distorted leaves can draw the attention of concerned tree owners. The galls are produced under the direction of a tiny gall wasp (family Cynipidae) with a big scientific name, Neuroterus quercusverrucarum (syn. N. floccosus).
A Flaky Name
Noted gall expert E. P. Felt labeled the galls “oak flake galls” in his 1917 bulletin, “Key to American Insect Galls,” as well as his book published in 1940 titled “Plant Galls and Gall Makers.” The common names for galls are usually descriptive, such as horned oak gall, oak apple gall, or rough oak bullet gall.
However, Felt never explains why he gave the fuzzy, ball-like galls such a peculiar name. It does not describe any attributes of the galls. They are not thin and flat, nor do they detach (flake off) the leaves. You find them on fallen leaves.
Regardless, “oak flake galls” isn’t a common name approved by the Entomological Society of America (ESA) for the cynipid wasp behind the galls. Since the ESA hasn’t approved a common name, the naming rights are up for grabs. I believe the so-called oak flake galls look like tiny pompoms, as you can see in the image below, thanks to Photoshop.
The ESA requires evidence of the general usage of a common name before approving a proposed name, so I’m building momentum by using the name “pompom galls.” I’m using the same strategy for Oak Tribble Galls [see BYGL Alert https://bygl.osu.edu/node/1876 ].
Pompom Lifestyle
Cynipid wasps typically alternate between two types of galls on their host trees. One type is called “sexgen” galls and produce a sexual generation, with males and females emerging. The other type is called “agamic” and produce an asexual generation with only females emerging. The alternation of two different methods of reproduction between generations of a species is known as heterogony.
The pompom galls are agamic. The sexgen galls have not been identified, although it’s suspected they are also leaf galls. Indeed, they may form on the first flush of leaves early in the growing season. However, many types of oaks have the ability to produce several flushes during the growing season, which is called polycylism.
Polycylism is significant because cynipid gall-wasps can only make undifferentiated (=meristematic) cells bend to their will to produce their gall structures. Once the cells differentiate and become organized into genetically preordained plant tissue, they cannot be changed by the gall-maker. Gall-makers can’t hijack differentiated cells.
Thus, it is suspected that oak pompom galls are initiated by females laying eggs in nascent leaves that are part of a second or perhaps third flush. This is supported by the observation that the heaviest pompom galling occurs on the most recent leaves.
Each gall contains a single chamber housing a developing wasp larva. The galls arise from the lower surface, with their position marked on the upper leaf surface by noticeable smooth blister-like bumps that match the gall’s attachment points. The bumps are light yellow early in the season but turn reddish-brown to brown as the season progresses.
The pompom galls also change colors as the season progresses. They start bright white and gradually progress through various shades of brown. Even though the galls have been with us throughout the growing season, most of my phone calls and emails about the galls typically start arriving once they start turning brown.
Gall formation produces leaf distortion, with the extent of distortion directly related to gall density. This is an important observation because it relates to the overall impact of pompom galls. The first two images below show leaf distortion, but a significant portion of the leaf remains functional for photosynthesis. The second two images show a much greater impact on leaf function.
As noted above, the agamic pompom galls remain attached to shed leaves in the fall. This implies that the females do not leave the galls to pupate in the soil. Instead, they spend the winter as either late-instar larvae or pupae inside the galls attached to the fallen leaves.
The pompom gall wasp’s host range appears to be confined to oaks belonging to the white oak group. I most commonly find them on white oak (Quercus alba), swamp white oak (Q. bicolor), bur oak (Q. macrocarpa), and columnar English oak (Q. robur 'Fastigiata'). Online references report galls also occur on overcup oak (Q. lyrata) and post oak (Q. stellata).
Pompom Impacts
Oak pompom galls can occasionally appear in high densities as illustrated in the images below. Note that leaf spotting and distortions are very apparent.
However, oak pompom galls appear to cause no appreciable harm to the overall health of their tree hosts. Heavy infestations that produce significant leaf distortions are relatively rare.
Otherwise, despite our perception, the pompom galls seldom affect a large enough leaf area, measured across the entire tree canopy, to seriously impede the total quantity of carbohydrates (= “tree food”) produced by photosynthesis.
Another reason the pompom galls do not seriously affect tree health is shared with other cynipid wasps targeting oaks. Gall wasp populations commonly rise and fall dramatically from one year to the next, so their impact over multiple years is inconsistent. Indeed, I observed their “now you see ’em, now you don’t” year-to-year occurrence on the heavily galled trees pictured above. The trees were virtually gall-free the year after I took those pictures.
Although the galls do not cause appreciable harm to their hosts, tree owners may still be concerned when newly planted trees are heavily galled. Unfortunately, there are no published insecticide trials demonstrating effective active ingredients and application timing that reduce pompom gall formation. However, since the females spend the winter in galls attached to fallen leaves, destroying the leaves will eliminate the agamic generation. Thus, raking and destroying the fallen leaves, or using a mulching mower to grind up the leaves, should be an effective management strategy.
Selected References
Gallformers. (August 21, 2026). Neuroterus quercusverrucarum (agamic). https://www.gallformers.org/gall/768
Melika, G. E. O. R. G. E., & Abrahamson, W. G. (1997). Descriptions of four new species of cynipid gall wasps of the genus Neuroterus Hartig (Hymenoptera: Cynipidae) with redescriptions of some known species from the eastern United States. Proceedings-Entomological Society of Washington, 99, 560-584.
https://go.osu.edu/flakegall1997
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