Cotton Balls on Oak Trees

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White, rounded, woolly-looking galls are appearing on oaks throughout Ohio.  They are the Wool Sower Gall and are produced under the direction of the gall wasp, Callirhytis seminator (family Cynipidae).


The sower galls are at first almost pure white, around 1 – 2” in diameter, and look like cotton balls hanging on the trees.  As the galls mature, they develop slightly raised points with the tips ranging in color from red to brown to black.  Opening the galls will reveal a collection of green seed-like structures, each containing a single wasp larva. 


Wool Sower Gall


Eventually, the galls change colors and texture turning light brown and developing a hard surface like they’ve been varnished.  This change signals that the small wasps that developed inside the seed-like structures are emerging to initiate a different type of gall.


wool Sower Gall


A family trait of cynipid wasps is to produce two different types of galls that alternate between generations.  Wool sower galls are produced from hijacked leaf buds.  The wasps that emerge from the sower galls fly to stems where they produce small, insignificant stem galls.  The wasps from the stem galls fly back to the buds in the spring to produce bud galls.


Wool sower galls develop on members of the white oak group with white oak (Quercus alba) being the most common host.  The galls may also occasionally occur on swamp white oak (Q. bicolor) and chestnut oak (Q. montana).


As with the vast majority of leaf, bud, and stem galls on oaks, the wool sower gall wasp causes no harm to the overall health of their oak hosts.  Thus, controls aren’t necessary. 


Indeed, cynipid gall-wasp populations tend to rise and fall dramatically from year to year meaning that while oak sower galls are common this season, they will likely become uncommon next season.  So, just like enjoying other fleeting events like solar eclipses and that provincial school up north winning a national football championship, we should simply enjoy the galls while they last.