Oh, the Gall of it All
QUESTION
The client submitted a photo of a strange growth on his oak tree and asked to know what it is and if he should do something about it.
DIAGNOSIS / RECOMMENDATION
It was identified as the wool sower gall, which is a growth caused by a small wasp, Callirhytis seminator. There is no reason to do anything, as these will not cause a problem.
ADDITIONAL INFORMATION
Many (but not all) galls on plants are caused by insects. These often present as very unusual growths. In the case of the wool sower gall maker, the galls are white, spongy, fibrous masses with yellow, seed-like capsules throughout the gall. A gall wasp larva develops within each capsule (1). The wasp itself is very small (about 1/8 inch long) (2).
The wool sower gall is specific to white oaks for the most part and only occurs in the spring. When the gall is pulled apart, inside are small seed-like structures inside of which the gall wasp grubs develop (the wool sower gall is also called the oak seed gall). Gall wasps have alternation of generations in which one generation develops in one type of gall (leaf gall) and their offspring develop into another type of gall (stem gall). Wasps of each alternate generation are slightly different in size and the galls of each generation are enormously different from the parent galls. Fortunately, wool sower galls are hardly ever abundant enough to cause real harm to white oaks (2).
Oaks in particular seem to have more than their share of interesting galls (1, 3). However, controlling wasp galls is difficult and even systemic insecticides rarely give satisfactory control. Pruning out heavily galled portions of a plant and destroying them is sometimes feasible and may help reduce populations of the gall insects. The good news is that most galls are harmless to the plant. Overall, it is best to accept galls as curiosities of nature (1)
RESOURCE LINKS
(1) https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ENTO-145/ENTO-422.pdf
(2) https://content.ces.ncsu.edu/wool-sower-gall-wasp
(3) https://content.ces.ncsu.edu/galls-on-oaks
(All resource links live and accessed March 23, 2022)
By: Betsy Brown, Bedford Extension Master Gardener Volunteer; Help Desk Coordinator, and Marla Swanson, Help Desk Co-Coordinator