Goldenrod Galls: Late Goldenrod (Solidago altissima with two subspecies) and Giant Goldenrod (S. gigantea) are subject to three different types of stem galls caused by tiny insects that lay their eggs on the plants The larva from the hatched egg then eats its way into the stem. In two cases (ball gall and spindle gall) the plant then responds to this event by rapidly increasing cell growth around the intrusion, enveloping the larva in a woody protective sheathing that not only keeps the larva safe, but they have a ready-made food supply for the remainder of the summer and a home to overwinter in and emerge in spring as a new adult, unless a woodpecker finds them. In the case of the rosette gall (3rd photo below), the plant creates a dense growth of what looks like small leaves at the top of the plant after a larva hatches at the top of the stem. This rosette is caused by the plant stopping stem growth without stopping leaf production. In the case of the spindle gall (below- 2nd photo), the eggs are laid on an autumn leaf where they overwinter. In spring the hatched larva migrate from the dead leaf to a nearby stem, bore in for a home and the plant responds as indicated above. S. canadensis (two subspecies) is not known to be seriously affected by the galls.
John Muir wrote in 1911: Each species seems to know what kind of plant will respond to the irritation or stimulus of the puncture it makes and the eggs it lays, in forming a growth that not only answers for a nest and home but also provides food for the young." From his essay Mt. Hoffman and Lake Tenaya.
Below: 1st photo - The "ball" or "apple" gall. 2nd photo - The "spindle" gall. 3rd photo - The " numerous rosette" galls on Giant Goldenrod.
The insects that cause these galls are as follows: Ball gall - Goldenrod Gall Fly, Eurosta solidaginis; Spindle Gall - Goldenrod Gall Moth, Gnorimoschema gallaesolidaginis; Rosette Gall - Goldenrod Gall Midge, Rhopalomyia solidaginis. This gall only occurs on Late Goldenrod. Below: 1st photo - A larva of the Gall fly in the sectioned gall. Note the gall has a dense woody structure. 2nd photo - An example of a stem affected by all of them.