I used my 98k clean gear at fields of slaughter (without follower/summons at all) in two ways:
1. I used locust and walked around the pack of mobs to see how much % of it dies. I concluded the test around 15 times. It killed ~98% of mobs. 2% survivors were those big tremors.
2. I used locust on a big pack and then jaunt + stalker'd away from it. Number of mobs killed: 5%
That means if you walk more than 1 screen away - the locust stops to work. That goes against the casual logic*, however from programmer's point of view it's simple = conserve server's resources. Yet I report it as a bug, because the user's gameplay should be unhindered in such a way (imho).
If locus worked as it's supposed to, then paragon/legendary farming with pesticilence build would be so amazing....
(*) if I cut myself with a knife and go to kitchen it doesn't mean I will stop bleeding. I will keep on bleeding until my body fixes itself.