There are a number of similar galls on goldenrod that I’m not sure how to tell apart. They look like dense bunches of leaves at the top of the stem, and sometimes they are so abundant that they might seem to be the way goldenrod normally grows. Many of these galls are caused by Rhopalomyia solidaginis, a midge (Cecidomyiidae), but other midges, as well as fruit flies (Tephritidae) in the genus Oedaspis, cause similar growths. On March 20 I encountered a bunch of these galls and I decided to collect some and see what emerged.
Five days later, I looked in the bag containing the gall pictured above, and saw this little jumping spider:
It had evidently overwintered in the shelter provided by this dense mass of leaves. I’ve found jumping spiders hiding in all kinds of places, including hollow stems and inside the “black knot” fungi that grow on cherry twigs. I posted some shots of this one on BugGuide.Net, and Lyn Atherton identified it as a Hentzia species, apparently H. mitrata. I kept it for a while to see if I could raise it to maturity–jumping spiders can change dramatically with each successive molt, and I thought it would be neat to come up with a series like this one. I put it in the jar with the walnut ants and added a few other insects for it to eat, but it never seemed to eat anything. It just built itself a little shelter in a groove in the walnut and cowered in there while the tiny ants milled about all around it.
So ultimately I let it go. Anyway, I was talking about goldenrod galls here. So, on April 11 I looked in the bag containing these smaller galls:
…and seven tiny green wasps had emerged. There was such a variety in size that at first I thought there might be more than one kind, but they turned out to all be torymids, ranging from a 2.1 mm male to a 3.2 mm female. One of the distinctive things about torymids is the long ovipositor of the female, which allows her to insert eggs deep within galls (or fruits, in some purely vegetarian species).
Here’s another blurry-but-interesting-shot-taken-through-a-plastic-bag, showing the sexual dimorphism in this species.
So, I got to meet some interesting bugs by collecting these galls, but I’m no closer to sorting out which ones are caused by which flies. It looks like if I want to make any progress, I should collect them in the fall: this Rhopalomyia solidaginis specimen emerged from its gall on October 1.
Update, 4/19/2011: As of today, a total of 15 torymids have emerged from the smaller galls, plus eight pteromalids like this one (thanks to Ross Hill for the family identification):