How could I have forgotten one of the most conspicuous winter features of dead goldenrod stems: galls of the goldenrod gall fly? The answer: easily, because even though they are dirt common in New York and Pennsylvania (and probably elsewhere), they are not common in central Massachusetts. I had forgotten about them until I stumbled across a place where they were abundant enough for me to notice. So it’s time to talk about old galls.
The goldenrod gall fly, Eurosta solidaginis, produces one generation per year, at least in the northeastern parts of the United States where I’ve seen it. Right now (April), the pupae are living inside of the galls that the larvae produced last summer. The stems were alive and growing when the eggs were laid, and the larvae induced the stem tissue of the host goldenrod to swell, forming the walls of the gall with a hollow chamber inside where the larvae live and feed. When the stems died, the larvae did not – the hardened gall protected them pretty well from physical harm. The galls and flies got cold in the winter, which didn’t bother the galls at all, since they were dead. It didn’t bother the larvae, either, because they have an array of internal antifreezes (complex alcohols) and well-placed water-transport proteins that protect the cells from freezing (some fluids of the larvae might actually freeze, but in locations that don’t matter). At some point, the larvae pupated inside the gall, and waited.
The pupa is inside the reddish brown puparium. It’s surrounded by fungi, and has a groove on it. Is it viable? Only time will tell.
This gall was on a stem that was bent but not fallen in April. The hole shows that something has exited the gall. What was it? The hole is neat, so it’s not the result of birds pecking away at the gall to get to the insect inside, or of predatory beetle larvae boring in to find a meal. Was it an adult fly? Or an adult parasitoid that killed the fly? If we look at the pupa inside the gall, we might be able to tell. If the exit from the puparium is neat, the fly did it. If not, an enemy did it.
Whatever came out of this gall, the gall has been through two winters. That’s a tough gall. If it had fallen to the ground before the fly emerged, decay could have been a problem. I wonder whether the presence of a gall is associated with a tougher stem so that the gall stays above ground long enough for the fly to emerge. Do gall-inhabited stems remain upright more often than gall-free stems? If so, then I also wonder whether the larva stimulates not only the swelling of the gall but also the hardening of the stem?
The exit hole is neat and circular because the larva, before it pupated, bored a tunnel nearly all the way out of the gall. The pupa can’t even move, much less bore its way out, and the adult has weak mouthparts (some would say none at all). The newly emerged fly has to swell its head to push its way through the last thin wall of the tunnel provided by the larva. If all is well, it will find a mate, and then females will find growing stems of Canada goldenrod, tall goldenrod or giant goldenrod on which to lay their eggs.
There are many good sources information about Eurosta solidaginis, so you should have no trouble learning more about them. I will revisit these flies when new galls form this summer.
Warren Abrahamson and Arthur Weis. 1997. Evolutionary Ecology across Three Tropic Levels: Goldenrods, Gallmakers, and Natural Enemies. Princeton University Press.
(If you plug this into Google Scholar, you get a link to all the works that cited this seminal book; some of them involve Solidago)
Rachel, 2010 Young Naturalist Award, American Museum of Natural History