Uloborus glomosus parasitism?

Today I visited the shrub where I first observed U. glomosus a few weeks ago, and noticed that one of the strings of egg sacs had a pale sac with a hole in it. Now that I've examined the picture, I think my impression at the time was correct: it's empty. I didn't see any spiderlings, and besides, odds are that a sac on the end of the string would hatch first. So I suspected a predator—probably a wasp—had made that hole.

A few blocks down the street I chanced upon the scene below. As I approached, the U. glomosus specimen shown in the pictures crawled up the egg sac strand. I didn't know whether that movement was in response to my approach, or to the little wasp, but as you can see, she resumed her resting position, and the wasp kept going about its business. So it appears that the spider wasn't aware of the wasp.

A quick look at a few research papers and at BugGuide tells me that chalcid wasps are known parasites of Uloborus, so it seems likely that we're looking at one, although my cell phone shots make it impossible to determine the species.