So last night I was gonna post that it seems wisps don't despawn, as that node I had encased was still surrounded by them. Login today and they're all gone, so I don't know if chunk unload kilsl them or if simple time does (I didn't rush out to the node instantly to check)
Then a bit later, there's 4 or 5 more wisps in the cage, but not the 30 or so from before. I've done zero thaumcraft all day, and the goggles say flux is moderate. A neighbor node (which is slightly larger) also says moderate but seems to have no wisps.