The good folks at my local nursery identified the infestation as SCALE. The bug is very tiny, or at least, my eyes haven't found them yet. I don't know if they are doing any real damage but, I bought an organic treatment that suffocates the bugs by coating the plant (and bugs) with an oil. The product says that it is safe on fruit and vegetables up to the day before harvest, so the hops should be okay by the time I pick them in a few weeks. Once I see that the bugs are gone, I'll start hosing the plants down just to be on the safe side.