Dro spawns in a few particular spots in the scenario, and is invisible. From what I understand he does not path, but stays in the area he spawns at for each run.
So, if you know all the spots he CAN spawn, you can avoid them till you have a sizeable withered army.
The rumor that popping the webbed victims does anything is because many of his spawn spots are right next to a group of webbed victims.