Last Wednesday Erika and I strolled in the Carleton College Arboretum. Our goal was to find a Canada Goldenrod gall. Erika found one almost immediately.
The thick, hard gall is produced by an insect grub, the offspring of the Goldenrod Gall Fly. The presence of the larval insect causes goldenrod cells to produce the gall, a growth much like a cancerous tumor. Inside the gall is a chamber in which the larva overwinter. You can see the chamber and the grub in the next photo. In addition to the gall, the grub. winthin its body, produces glycerol, which works like antifreeze.
Grubs are used as bait during the winter when fishermen can not find worms. The larvae are also preyed upon by Downy Woodpeckers. Because the woodpeckers prefer large galls (they have bigger grubs), areas with lots of woodpeckers tend to have smaller galls. However, a wasp preys upon Goldenrod Gall Fly larvae by laying eggs in the gall. The larvae are eaten by the young wasps. In areas with lots of wasps, goldenrod galls tend to be large (harder for the wasps to penetrate). Areas with both wasps and Downy Woodpeckers support galls of intermediate size. (Most of this information is from the Hilton Pond Center for Piedmont Natural History.)