So-called “Oak Flake Galls” are produced under the direction of a tiny wasp with a big scientific name, Neuroterus quercusverrucarum (syn. N. floccosus, family Cynipidae). The small, round, fuzzy wasp galls on the underside of oak leaves are changing color from snowing white to various shades of brown. This coupled with leaf cupping and distortion is drawing the attention of concerned tree owners.
The 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).
The small, round, fuzzy galls are initiated in the spring and arise from the lower leaf surface. Each gall contains a single chamber housing a developing wasp larva. There is one generation per season.
The galls are snowy-white for most of the season but progress through varying shades of brown in late summer. They eventually become dark reddish brown before leaf drop.
The galls are marked on the upper leaf surface by noticeable smooth blister-like bumps that match with the gall’s attachment points on the underside of the leaves. The bumps are light-green and then light-yellow early in the season but turn reddish-brown to brown as the season progresses. This coupled with the twisting and cupping of heavily gall leaves are the two symptoms that typically draw attention to the galls.
A Flaky Name
I have no idea why the fuzzy leaf galls are called “flake galls.” The common name does not describe any of the attributes of the galls. They are not thin and flat, nor do they detach (flake off) the leaves. You find them on fallen leaves.
Noted gall expert, E. P. Felt, appears to be the source for the common name. He labeled the galls “oak flake galls” in his 1917 bulletin as well as his book published in 1940 (see References Cited below). At the time, the scientific name for the cynipid wasp responsible for the galls was N. floccosus with “floccus” being from the Latin for “tuft of wool.” Floccus is a term still used to describe small hairs covering a plant and a mass of fungal hyphae.
However, Felt never explained his reasoning behind the name he gave to the galls. Perhaps he thought the early galls looked like snowflakes and he just dropped the “snow.” Regardless, “oak flake galls” isn’t a common name approved for the cynipid wasp species by the Entomological Society of America (ESA). However, it is used by iNaturalist for the galls.
On the other hand, since there’s no ESA-approved common name, I believe naming rights are up for grabs. Thanks to Photoshop, as you can see in the image below, I think the galls look like polishing pads. Of course, maybe “oak polishing pad galls” garners even less clarity and cachet compared to “flake galls.”
The vast majority of the oak leaf galls induced by cynipid wasps cause no appreciable harm to overall tree health. Part of the reason is that despite our perception, most leaf galls do not cover 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 is the “now you see ’em, now you don’t” year-to-year occurrence of cynipid wasp leaf galls. The wasp populations commonly rise and fall dramatically from one year to the next, so their impact over multiple years is inconsistent.
For example, I’ve been monitoring a group of columnar English oaks in southwest Ohio and have observed that oak flake galls generally conform to the year-to-year population dynamics seen with other leaf galls. The galls were extremely heavy last year compared to previous years and this year.
Thus, oak flake galls, as with most other leaf galls, do not warrant control efforts to protect tree health. This is good given that I can find no published insecticide efficacy trials aimed at reducing gall formation.
Felt, E.P., 1917. Key to American insect galls (No. 200). New York State Museum Bulletin, No. 200, The University of the State of New York, 346 pgs.
Felt, E.P., 1940. Plant Galls and Gall Makers, Comstock Publishing Co, Ithaca, NY. 364 pgs.