I remember finding out about these when I was a first year Journalism undergrad. I was watching the latest lecture-on-tape of my elective class: “Natural History of Ontario”. *
The instructor** pulled a stem of goldenrod out of a bag and laid it on the desk. We were told the fleshy, spherical structure on the stem was caused by a fly, and that the structure was called a “gall”. The camera operator zoomed in on the pocket knife blade now held firmly against the gall; I watched as the instructor carefully cut through its thick wall, just deep enough to allow him to pry it cleanly apart into two halves. The camera zoomed in closer, and lo: a stout white grub, curled in a protective ball, nestled in a tiny chamber at the centre of the bulbous growth.
This seemed amazing to me at the time…like magic. I had to see if such a thing could be replicated. Sure enough, a trip to a nearby park quickly led to the discovery of galls on dozens of goldenrod stems. My very own pocket knife carefully cut in (not too deeply), and then…TA DAAAA!!!! Magic! Fly larvae out of a round chunk of plant! I had to repeat the procedure a half-dozen times or so to convince myself it wasn’t a fluke.
The galls are formed when larvae of the goldenrod gall fly (Eurosta solidaginis) – deposited in goldenrod (Solidago sp.) stems during spring by the female – chew on the stem tissues, inducing a hormone-mediated response in the plant.
Five Goldenrod galls
The thickened tissue provides both food and protection for the developing larva, which overwinters in its gall.
A single, intact gall.
Not all gall flies survive the winter to adulthood, however. The robust white grubs are a highly sought food source for birds such as Chickadees and Downy Woodpeckers, who bore through the galls with their beaks and pluck the tasty treat from within.
Goldenrod gall raided by a Chickadee. The hole is messy and irregular. A neat, round hole suggests woodpecker feeding.
If you find a gall with a small, single round hole (a woodpecker’s feeding hole would be about the same size as the Chickadees, above, only tidier), the larva managed to avoid predation and chewed its way out of the gall in the spring, emerging as a new adult.
Even after all this time, I still get a little thrill when I find one intact, and carefully split the structure into two neat hemispheres…TA DAAAA!!!! Fly larva, like magic! It’s one of my favourite nature “party tricks” when I’m outdoors with the uninitiated.
* Yes, Journalism. That was my first major. The Natural History course in question was the catalyst for my switch to a Biology major the next year.
** Natural history genius guru of epic proportions. My hero.