Goldenrod Gall Flies
Goldenrod is very common on our property, with several large patches of hundreds of plants each. And, if one examines the goldenrod stems, a lot of them will have these nearly spherical galls about an inch in diameter:
The grub has practically no identifying features, but a lot of gall-formers will overwinter inside of their galls to emerge in the spring. So I decided to wait until this spring and try to find out what would come out. This past April we all went out and found a couple of galls that were still intact and did not have emergence holes. These were put into a jar, and then we waited until something came out.
This is a Goldenrod Gall Fly, Eurosta solidaginis.
Goldenrod gall flies are not particularly strong fliers, and this one didn’t show much inclination to fly off while I was taking pictures. It is quite possible that they don’t eat as adults.
We actually had two of them emerge, one from each gall. But by the time we noticed, one of them was dead. It was noticeably smaller than the one that was still alive (having probably come out of the smaller of the two galls), so it just didn’t have the internal resources to go on as long.
They mainly just pop out of their galls after the new goldenrods have started to grow, walk around until they find a mate, and then the female climbs up a stem and lays an egg at the growing tip. When the egg hatches, the larva digs into the stem, and irritates the plant so that it grows a hard, round tumor around it. The larva is then pretty well shielded from predators, and all it has to do is suck on the plant juices that weep from the inside of the gall. By the fall, the grub is pretty big, and once the goldenrod stalk dies it pupates in the gall. Around here, the stalk promptly gets mashed down by our heavy snowfalls, so the gall ends up buried safely under the snow until spring. There is a bit of a dangerous time for the gall flies in the spring, though. After the snow melts, there are some birds (particularly woodpeckers and chickadees) that know there is a succulent little morsel in the middle of these galls. So, they will peck a hole in it and eat the pupa. When we were looking for galls to hatch out, about a third of the ones we found had been pecked open.
It’s not just birds that eat the gall flies, they also have a number of parasitoids that will infest the galls, eat the fly larva, and take over the gall themselves. Which is why we were none too sure of what exactly was going to come out of the ones we collected here.
Also, the round galls aren’t the only kind of goldenrod galls – goldenrod also has elliptical stem galls that are made by a moth larva; “bunch” galls, leaf-cluster galls, blister galls, and flower galls that are made by several kinds of midges; another kind of “bunch” gall that is made by another moth; and of course several different kinds of wasps (and at least one beetle) that parasitize the insects that make each of these kinds of galls. So, a goldenrod plant can be quite the menagerie.
Goldenrod seems to be particularly subject to galls (or, at least, galls that are obvious to humans), probably because of certain characteristics that it has:
1. It smells and tastes pretty rank, and most grazing animals don’t eat it. This reduces the chance that a gall-former will end up getting eaten by something that wasn’t even aware of the gall’s existence.
2. It is a perennial plant, so after overwintering in a gall, it is easy to find fresh goldenrod to lay eggs on. The gall-formers therefore don’t have to be very mobile.
3. It forms extensive patches, so there is a lot of opportunity for gall-formers to build up a large population, and the plant is large and robust enough that the galls don’t tend to make the plant die back.
4. It is a tallish, but not too tall, plant, so the galls that form end up right around hand-level (or eye-level if one is younger than about 6 years old). This makes the galls easy for humans to spot.