Oak Rosette Galls are produced under the direction of a small gall wasp, Andricus quercusfrondosus (family Cynipidae). The wasp hijacks buds to direct the oak to produce a home that provides protection and nourishment for its offspring. I’ve included a short, behind-the-scenes summary of what we believe happens during cynipid wasp gall formation.
The rosette galls appear as dense clusters of tiny leaves in leaf axils and stem terminals. I’ve always thought they look a bit like tiny pineapples.
Oak Rosette Wasp Galls on Burr Oak - 1
Oak Rosette Wasp Galls on Burr Oak - 2
Cutting through the rosette will reveal a seed-like structure housing a single wasp larva. Although the larva has chewing mouthparts, it does not simply eat the gall. Instead, specialized tissue called “nutrient tissue” lines the larval chamber and is constantly replenished as it’s consumed by the larva.
Oak Rosette Wasp Gall Structure on Burr Oak
Oak Rosette Wasp Gall Larva in Chamber on Burr Oak
As the wasp larva matures, we say the gall also matures, which is marked by color changes. The rosette leaves turn from green to reddish green to brown and finally black. The galls remain attached to the tree throughout the winter, and the new gall wasps emerge in the spring.
Oak Rosette Wasp Gall on Chinkapin Oak - 1
Oak Rosette Wasp Gall on Chinkapin Oak - 2
The literature notes that the galls may be found on several species of oaks in the white oak group, including white oak (Quercus alba) and swamp white oak (Q. bicolor). The gall hosts pictured in this Alert are chinkapin oak (Q. muehlenbergii) and burr oak (Q. macrocarpa).
The hijacked buds will fail to contribute to the growth and reproduction of affected oaks; however, the total number of galled buds is usually insignificant. As with the vast majority of cynipid galls, oak rosette galls appear to cause no significant harm to the overall health of their oak hosts. Thus, there is no reason to attempt control measures, which is a good thing because there is no data on an effective approach to suppressing the galls.
Oak Rosette Wasp Gall with Unaffected Buds
Summary: What’s Happening Behind the Scenes
This summary is based on scientific evidence, conjecture, and outright guesswork. Plant galls remain one of the most obvious but least understood abnormal plant structures found in nature.
Currently, research evidence suggests that cynipid gall wasps 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 remarkably, the exact genetic levers pulled by the wasp are so species-specific that the wasp species can be identified by its gall without seeing the wasp. That’s what I’m doing in this Alert.
The gall-maker must use plant cells that have not yet become part of an organized plant structure. Galls can’t be formed from plant tissue once the inherited genetic script has been completed, and the plant cells are part of a functional leaf or new stem.
Thus, 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 ring of cambium between the phloem and xylem.
However, only the cambial meristematic cells remain undifferentiated throughout the growing season. This means the cells can help trees deal with injuries by becoming so-called callous tissue to close wounds. It also means that stem galls can form anytime during the growing season, while bud galls can only form in the spring before the meristematic bud tissue differentiates into flowers, leaves, or new stems.
If you peruse all of the scientific papers in the references below, you’ll find galls have not yet given up all of their secrets. That’s a good reason for viewing them with fascination. Churchill’s 1939 quote was not about galls, but it certainly provides a nice description: “A riddle wrapped in a mystery inside an enigma.”
Selected References
Ward, A. K., Sheikh, S. I., & Forbes, A. A. (2020). Diversity, host ranges, and potential drivers of speciation among the inquiline enemies of oak gall wasps (Hymenoptera: Cynipidae). Insect Systematics and Diversity, 4(6), 3.
https://doi.org/10.1093/isd/ixaa017
Schönrogge, K., Harper, L. J., & Lichtenstein, C. P. (2000). The protein content of tissues in cynipid galls (Hymenoptera: Cynipidae): similarities between cynipid galls and seeds. Plant, Cell & Environment, 23(2), 215-222.
https://doi.org/10.1046/j.1365-3040.2000.00543.x
Egan, S. P., Hood, G. R., Martinson, E. O., & Ott, J. R. (2018). Cynipid gall wasps. Current Biology, 28(24), R1370-R1374.
https://www.cell.com/current-biology/fulltext/S0960-9822(18)31357-5
Hearn, J., Blaxter, M., Schönrogge, K., Nieves-Aldrey, J. L., Pujade-Villar, J., Huguet, E., ... & Stone, G. N. (2019). Genomic dissection of an extended phenotype: Oak galling by a cynipid gall wasp. PLoS genetics, 15(11), e1008398.
https://doi.org/10.1371/journal.pgen.1008398
Bellows, E., Heatley, M., Shah, N., Archer, N., Giles, T., & Fray, R. (2024). Comparative transcriptome reprogramming in oak galls containing asexual or sexual generations of gall wasps. Plant Biology, 26(5), 798-810.
https://doi.org/10.1111/plb.13670
Cambier, S., Ginis, O., Moreau, S. J., Gayral, P., Hearn, J., Stone, G. N., ... & Drezen, J. M. (2019). Gall wasp transcriptomes unravel potential effectors involved in molecular dialogues with oak and rose. Frontiers in physiology, 10, 926.
https://doi.org/10.3389/fphys.2019.00926
Martinson, E. O., Werren, J. H., & Egan, S. P. (2022). Tissue‐specific gene expression shows a cynipid wasp repurposes oak host gene networks to create a complex and novel parasite‐specific organ. Molecular Ecology, 31(11), 3228-3240.
https://doi.org/10.1111/mec.16159