Pretty much anywhere you find goldenrod, you can see goldenrod ball galls. These round formations on the stem of the goldenrod plant are winter homes for the larvae of the Goldenrod Gall Fly (Eurosta solidaginis). By this time of the winter, many of the galls have holes excavated by chickadees or woodpeckers, who find the larvae a tasty winter treat. For more on ball galls, see How Galling!
While not quite as common as ball galls, at least around here, goldenrod bunch galls can often be found in patches of goldenrod as well. They are easily identified. They look like flowers with many woody petals set at the tip of the goldenrod stem. Bunch galls are caused by a midge, Rhopalomyla solidaginis, and are associated with a particular species of goldenrod, Solidago canadensis.
The midge lays its egg in a leaf bud. After the larva hatches, the stem stops growing longer, even though the goldenrod continues to produce leaves. This results in a tight, flower-like cluster of foliage at the top of the goldenrod’s main stalk. An assortment of insects, including spiders and other midge species, may make the bunch gall home.