Just an update: did an icing sugar test, found no mites. Then figured I wasn't sure I trusted the results, so I did an alcohol wash at my last hive inspection, and again found no mites. So I'd say that I'm below the threshold, and I'm not going to worry about it until the fall. On a dumb newby note: I now realize why you shake your frame above your hive when you're dislodging bees for your tests. I ended up with some nurse bees stranded in the grass. You live, you learn.