What caused the hole in this goldenrod ball gall?
A hungry downy woodpecker! The goldenrod ball gall was providing food and shelter to a cream-colored plump larva of a goldenrod gall fly. A neat conical hole is the sign that a downy woodpecker (hairy woodpeckers are too big to perch on the goldenrod stalks) retrieved a nutritious snack. Research suggests that downy woodpeckers tap on the galls to locate the fly’s emergence tunnel (which the gall fly excavated in the fall) so they will have less gall to excavate.
Black-capped chickadees are also predators of goldenrod ball gall fly larvae. Since chickadees don’t have the powerful beak of the downy woodpecker, they grab and pull apart the gall instead, leaving larger and messier holes. Other holes you may observe on goldenrod ball galls are the emergence holes of the goldenrod gall flies (around 2mm wide) or their wasp or beetle parasites (around 1mm wide).
Likely a chickadee-excavated gall on left (in which a spider has taken up residence) and adult goldenrod ball gall fly exit hole on the right.